Temp sensor disconnected skips some AHC data (1 Viewer)

[MarkTR]

Just picked up my '05 470 and the first thing I did was to measure fender lip to hub centers and then raise the front end where it should belong. At 190K miles, I expected some settling. I am in my 70's and find the same thing is happening to me.

I used YotaMD's cheat sheet and my old Panasonic Tough Book with TechStreem. That TB is a slow heavy brick.

I found a few things interesting.
First- if I disconnect the AHC temperature connector I get no data for the accumulator pressure----and the rear pressure is way low. I repeated this about five times today and still the same. Went under and checked the accumulator connector and all is well. I also had data yesterday with the sensor connected so I figured this was the issue.

So I reconnected the temperature sensor and the accumulator came online and the rear pressure went from 2.9 to 6.9.

I am low on the Left Rear (20") so I am thinking this is the cause for the 6.9 pressure--- so time for new rear springs. If I want stock is the OEMs the only way to go?



Has anyone else had no data for the accumulator pressure and wonky rear pressure with the temp sensor disconnected?

 

[ramangain]

Your front pressures are low, so you may want to raise the front end via torsion bars.

Remember, the goal is to get the AHC pressures in spec. In my opinion, ALL other data can have a decent delta if the AHC pressures are in spec. The exception is the graduation test, which provides evidence of failing globe(s).

 

[MarkTR]

Your front pressures are low, so you may want to raise the front end via torsion bars.

Remember, the goal is to get the AHC pressures in spec. In my opinion, ALL other data can have a decent delta if the AHC pressures are in spec. The exception is the graduation test, which provides evidence of failing globe(s).

Ramangain--

OK - I admit that I may have all this backward- but I thought (and experienced) that by turning the TB bolt clockwise will raise the front end and thus lower the Globs pressures ---the extra twist induced in the TB to raise the height causes the TBs taking on more of the load and in turn lowers the load shared by the Globes--- thus the pressure goes down.

Has it been your experience that raising the front end by turning the TB bolts clockwise to raise the height --should result in higher pressures?



I had to turn the TBs clockwise to raise the front to get near to the recommended 19.75" lip to the center of the hub. When I started this measurement was about 19" which I assume was sagging down of the TB over the last 16 years.

As I raised the level by turning the TB clockwise--- I saw the front pressure go down as the front end went up-- from 6.0 down to 5.5 then down to 5.1.


I have no idea how I can bring the pressure back up in the front without going lower than the 19.75". I would have to back down to 19" or lower to get to 6.9 MPa.



Sounds like I need exploer why at 19" I was at only 6.0 and at 19.50" my pressures went down from 6.0 to 5.1

 

[ramangain]

My bad! I got the concept backwards. You are correct. I cannot tap my head and rub my tummy at the same time.

 

[ramangain]

For the record my rig sits below 19.75" up front for correct AHC pressure for (as far as I know) healthy globes. Fine for me.

 

[MarkTR]

I had assumed that the rear pressures I was seeing with the sensor disconnected (-2.6 MPa)-was the wonky value.

Now I am wondering if the 6.9 MPa with sensor connected--was the wonky pressure value.

I looked through other forum members' data outputs and I noticed many did not show accumulator values- so this is not a glitch and I should trust the sensor disconnected data as the more accurate. Seems the consensus is to disconnect the sensor to achieve the best values.



So is my 2.6 MPa the real number? And if it is- then all of my values are very low.

I thought my mismatched Right to Left level numbers (L19.5 R20) indicated a worn Left spring ---so the higher rear number made sense since the Globes had to take more of the load.

The question now is --why such low pressures at stock height. What does this indicate?

I consistently measured 11 tick marks going H to L at the reservoir.

 

[ramangain]

The question now is --why such low pressures at stock height. What does this indicate?

I consistently measured 11 tick marks going H to L at the reservoir.

My advice is to not lose sight of the forest among the trees. Lower the front to get pressures in spec and send it.

 

[MarkTR]

My advice is to not lose sight of the forest among the trees. Lower the front to get pressures in spec and send it.

I still would like to know why the pressures are so low at stock heights-it must mean something.

I will have to go below 19" to get above 6 MPa-- how low is acceptable? Is a bit over 6 MPa good enough?

 

[MarkTR]

For the record my rig sits lower below 19.75" up front for correct AHC pressure for (as far as I know) healthy globes. Fine for me.

How low did you have to go?

 

[suprarx7nut]

I still would like to know why the pressures are so low at stock heights-it must mean something.

I will have to go below 19" to get above 6 MPa-- how low is acceptable? Is a bit over 6 MPa good enough?

A couple clarifications:

1. Pressures shown in this thread in your screenshots: Are those all from L to N with plenty of time given for Techstream to update? It might take a solid 60 seconds after movement has stopped. Early screenshots can show partial pressure (artificially low). That 2.X value seems suspicious... dare I say, even wrong.

Look like your front torsion bars might be over-tight. Back them off to get that front pressure up a little. Pressure too low means AHC is working less. Good for reliability, bad for comfort.

2. Don't move height sensors to adjust pressure up front. To alter front pressure, turn torsion bars. Ride height should remain perfectly static. Set it to what you want (anywhere near your cheat sheet values is fine) and keep them there.

3. Pressures. In spec is best (duh). Aim for 6.9 Front and 6.1 Rear. Anywhere in spec is fine. Your 6.9 in the rear would be fine with me, unless you plan on having a really dynamic load. Add 30mm spacers to pull that rear pressure down into spec if you feel the need.

 

[MarkTR]

A couple clarifications:

1. Pressures shown in this thread in your screenshots: Are those all from L to N with plenty of time given for Techstream to update? It might take a solid 60 seconds after movement has stopped. Early screenshots can show partial pressure (artificially low). That 2.X value seems suspicious... dare I say, even wrong.

Look like your front torsion bars might be over-tight. Back them off to get that front pressure up a little. Pressure too low means AHC is working less. Good for reliability, bad for comfort.

2. Don't move height sensors to adjust pressure up front. To alter front pressure, turn torsion bars. Ride height should remain perfectly static. Set it to what you want (anywhere near your cheat sheet values is fine) and keep them there.

3. Pressures. In spec is best (duh). Aim for 6.9 Front and 6.1 Rear. Anywhere in spec is fine. Your 6.9 in the rear would be fine with me, unless you plan on having a really dynamic load. Add 30mm spacers to pull that rear pressure down into spec if you feel the need.

All data taken-L-N with plenty of time.

I had to crank the TBs up to get to static 19.5"-- but I am starting to think I really only should have matched the higher side and not gotten both up to 19.5".

I have not touched the sensors. I have only used the TB to set the static height at 19.5"on both sides. Perhaps my mistake.

Unfortunately, my 6.9 MPa at the rear was when the temperature sensor was connected. Disconnected it reads only 2.6 MPa!



Here is the piece I think I am missing --when you say "To alter front pressure, turn torsion bars. Ride height should remain perfectly static". I think you mean is that when I am cranking up the TBs I am not really changing the dynamic height because that is set by the height sensors which are currently set at some mechanically set 0.0 position--I am only affecting the load that the TBs and the Globes are sharing at arriving at the 0.0 sensor set points

So if one side is low- it is only because a TB or spring is weak and along with the Globe pressure, the system cannot bring it to the sensor 0.0 point height point. Cranking up the TB adds the extra lift needed to be level but the sensors will always be the real controller of the height.

Too much time working on Air-cooled Porsche suspensions where the TB controls the stiffness and the height- no height sensors.


If I am on the right track, I need to turn the TB's CC to hit the 6.5 or higher Globe pressures --and the height will be wherever the height sensors are currently set at 0.0



Very open to correction

 

[suprarx7nut]

All data taken-L-N with plenty of time.

I had to crank the TBs up to get to static 19.5"-- but I am starting to think I really only should have matched the higher side and not gotten both up to 19.5".

I have not touched the sensors. I have only used the TB to set the static height at 19.5"on both sides. Perhaps my mistake.

Unfortunately, my 6.9 MPa at the rear was when the temperature sensor was connected. Disconnected it reads only 2.6 MPa!



Here is the piece I think I am missing --when you say "To alter front pressure, turn torsion bars. Ride height should remain perfectly static". I think you mean is that when I am cranking up the TBs I am not really changing the dynamic height because that is set by the height sensors which are currently set at some mechanically set 0.0 position--I am only affecting the load that the TBs and the Globes are sharing at arriving at the 0.0 sensor set points

So if one side is low- it is only because a TB or spring is weak and along with the Globe pressure, the system cannot bring it to the sensor 0.0 point height point. Cranking up the TB adds the extra lift needed to be level but the sensors will always be the real controller of the height.

Too much time working on Air-cooled Porsche suspensions where the TB controls the stiffness and the height- no height sensors.


If I am on the right track, I need to turn the TB's CC to hit the 6.5 or higher Globe pressures --and the height will be wherever the height sensors are currently set at 0.0



Very open to correction

Yup, you got it. Sensors control height. Tbs control side to side lean and overall front (and to an extent rear) pressure.

The 2.6 reading is still weird. Not sure what's going on there, but otherwise you seem to be pretty close to spec! For a real recent purchase it's looking real good.

 

[IndroCruise]

Just picked up my '05 470 and the first thing I did was to measure fender lip to hub centers and then raise the front end where it should belong. At 190K miles, I expected some settling. I am in my 70's and find the same thing is happening to me.

I used YotaMD's cheat sheet and my old Panasonic Tough Book with TechStreem. That TB is a slow heavy brick.

I found a few things interesting.
First- if I disconnect the AHC temperature connector I get no data for the accumulator pressure----and the rear pressure is way low. I repeated this about five times today and still the same. Went under and checked the accumulator connector and all is well. I also had data yesterday with the sensor connected so I figured this was the issue.

So I reconnected the temperature sensor and the accumulator came online and the rear pressure went from 2.9 to 6.9.

I am low on the Left Rear (20") so I am thinking this is the cause for the 6.9 pressure--- so time for new rear springs. If I want stock is the OEMs the only way to go?



Has anyone else had no data for the accumulator pressure and wonky rear pressure with the temp sensor disconnected?

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One of the COVID problems in a country where lock-downs are still favoured is that there is too much time for writing long posts -- so apologies for that!!

Probably you have picked up the details from posts by @ramangain and @suprarx7nut -- the following may be superfluous and/or there may be some duplication but just to add some further clarifications in case that helps:

• There is only one pressure sensor in the AHC/TEMS system – it is attached to the machined block which separates the AHC Pump and the AHC Motor,

• The ECU does not cause readings from the Pressure Sensor to be displayed on Techstream in real time,

• Instead, the maximum pressures appear sequentially for Rear AHC, Front AHC and Height Control Accumulator after a raise, and importantly, after the AHC Pump has completed the sequence including recharging the Height Control Accumulator,

• The only relevant time to read AHC pressures for comparison with FSM specifications and IH8MUD experiences is after moving from “N” to “LO” then back to “N” per FSM procedure, AND, after allowing the AHC Electronic Control Unit (ECU) and AHC Pump to complete the raise-and-recharge sequence,

• Theoretically, in a new healthy system in perfect condition this should take about 15 seconds after the green AHC dashboard light has stopped blinking and after the suspension has settled in the new position – but allow around 60 seconds as @suprarx7nut suggests to allow for age, wear and imperfections and then be sure that the AHC Pump has stopped ie is silent. (At this point, you may see a slight ‘quiver’ in the fluid in the AHC Tank on closure of the solenoid valve at the forward end Height Control Accumulator. You may also be able hear this closure or feel it with your hand on the pipes at this Accumulator near its solenoid valve).

• It is best to run and read say three tests or enough to discard outliers. Spurious and inconsistent readings do occur, usually due to some imperfection in which a previous result has not been overwritten by the new result in the ECU memory

• AHC pressure readings done in any other way are spurious and certainly are not comparable with the FSM specifications and IH8MUD experience,

• The FSM is quite specific about disconnection of the Temperature Sensor when reading Rear AHC pressures. Some IH8MUD reports indicate large differences when this is done, while some (including me) do not see much difference. The reasons for such variances in experience are unclear,

• A zero pressure reading for the Height Control Accumulator also is common when the Temperature Sensor is disconnected. The reason for this is unclear, at least to me,

• As you have noted, increasing hub-to-fender height means that less vehicle weight is carried by less-deflected springs and torsion bars (F = kX, so less X means less F) and correspondingly more vehicle weight is carried by the AHC system. In turn, this means higher Front and Rear AHC pressures,

• The only means of adjusting AHC pressures is the indirect method of adjusting the share of weight carried by the torsion bars and springs – using torsion bar adjusters at the Front (and sometimes re-indexing the torsion bars when no more adjustment is available), and, by increasing the pre-load on the Rear coil springs with the spacers or by replacing the springs,

• ‘Ride Height’ adjustment is not accomplished using the torsion bar adjusters on a vehicle fitted with Active Height Control (AHC). Some change may be visible at the time of actually turning the torsion bar adjusters but this will be over-ridden by the AHC system after start-up and driving away,

• The fundamental design feature of the AHC system is to maintain the selected height (“LO” or “N” or “HI”) within the FSM-specified load limits and FSM-specified speed limits – provided the system is correctly adjusted and in the absence of fault conditions. Torsion bar adjustment does not override AHC operation,

• Torsion bar adjusters have only two purposes on an AHC-equipped vehicle:

1. ‘cross-levelling’ the Front of the vehicle – meaning correcting any ‘lean’ and equalising the Front Left and Front Right ‘static heights’ per FSM, thereby equalising Left and Right torsion bar loads (assuming Left and Right mechanical components are in good order and Right and Left wheels/tyres are the same overall diameters and at same pressure),
2. Adjusting Front AHC pressures – meaning adjusting the combined weight share carried by the Front torsion bars,

• ‘Ride Height’ adjustment is accomplished only by adjusting the Height Control Sensor Adjusters – in effect moving these adjusters so that the point at which a zero or near-zero reading appears on Techstream corresponds to a different hub-to-fender height at “N” height setting. (Note that the Techstream reading is in inches or millimetres per Techstream settings; the actual output of the Height Control Sensors is a signal in volts sent to the ECU, about 2.25 volts at “N” height setting),

• Obviously, changing the ‘ride height’ with Height Control Sensor adjusters will cause a change in AHC pressures. Then Front AHC pressures would then need to be adjusted using the torsion bar adjusters; Rear AHC pressures are not adjustable without varying springs in some way – spacers or new springs, or accepting the height which provides acceptable Rear AHC pressure, or varying the load in some way which provides acceptable Rear AHC pressures,

• To avoid going in circles of endless frustration, in any AHC review or adjustment process it is important to follow the sequence of steps set out in the “Cheat Sheet” prepared by @suprarx7nut. The sequence is just as important as the steps. No point in adjusting anything until front ‘cross-level’ has been done; no point in adjusting AHC pressures until ‘ride height’ is where you want it; no point in checking overall ‘globe’ condition via “HI” and “LO” fluid level differences at AHC tank until AHC pressures are in the correct range (and as an aside, no point in adjusting Front end geometry and alignment until ‘ride height’ is where you want it),

• I am not sure what to make of a persistently low Rear AHC pressure of 2.6 Mpa if that is what is occurring. If it remains persistent and all else has been done correctly, then probably I would become suspicious of the Rear Height Control Sensor, I confess mainly out of naked prejudice rather than reason. The Height Control Sensors are frequent nuisances in AHC matters. The ECU only records a Diagnostic Trouble Code (DTC) in relation to these Sensors in very specific circumstances defined in the FSM – but absence of a DTC provides no assurance that all is well with Height Control Sensors. For example, a Sensor can give a low voltage reading but no DTC if the voltage is still within the prescribed range. It may be worth checking this Sensor. However, this is pure speculation on my part – at the moment I cannot offer a reason connecting this Sensor to the Low Rear AHC pressure,

• The difference of half-inch at Rear seems incidental to the main game. Possibly it may indicate worn components somewhere at the Rear OR the Front. If the chassis is perfect and all suspension and running components are perfect and correctly adjusted, then it would be geometrically impossible to have the Front cross-level without the Rear also being cross-level as well. (Incidentally, this is the reason that there is only one Rear Height Control Sensor),

• Forgive me if I am preaching to the wise -- but consider a stiff rectangular object in one plane without any twist or warp, such as a stiff book or piece of metal plate. Place the front end on a level surface with the rear end raised. The heights at both sides of the raised end will be same. Press one rear corner to cause it to be lower. The only way this can happen is when the diagonally opposite front corner is raised,

• However, nothing is perfect in this world and Rear cross-level differences ARE observed on LC100/LX470 vehicles with AHC. There are a range of possibilities – from imperfect chassis manufacturing or subsequent wear and tear causing a slight chassis twist, to (more likely) worn suspension mountings and bushing affecting the way a vehicle ‘sits’ on a level surface, or more simply, slight differences at the 10 attachment points and bushes where the body is attached to the chassis. So there has to be some tolerance. A good wheel alignment shop should be able measure and to tell you the cause in your particular case,

• As a suggestion, start with wheel/tyre diameters and pressures all round, then inspection of Rear spring seats (cannot see how a poor Rear spring would not affect the diagonally opposite Front height), inspection of Rear AHC shock absorber mountings and bushes, inspection of bushes on Rear upper and lower control arms etc. Then do the same at the Front. It is entirely possible that worn Front shock absorber bushes or Front control arm bushes or ball-joints have been compensated by cross-levelling with the torsion bar adjusters to equalise hub-to-fender static heights – if so, this will mean that the Front fender-to-floor measurements would be slightly different side-to-side – and in turn that will reflect in some differences at the Rear if the chassis is perfect and the body fitment is consistent all round. Obviously, there can be a combination of such causes. In all this, it becomes doubtful whether ‘pursuit of perfection’ is worthwhile unless there are other suspension problems to fix or wear-and-tear items to replace. Your experience on finer Porsche suspensions may be more insightful on this issue!!

 

[baltistyle]

The way I dealt with this logic was to replace the springs and in the back of the vehicle with stock parts. I think this is often overlooked, that we have very old springs in these heavy beasts.

 

[ramangain]

How low did you have to go?

Oh, I think it settled between 19.25" and 19.5"

 

[ramangain]

If I am on the right track, I need to turn the TB's CC to hit the 6.5 or higher Globe pressures --and the height will be wherever the height sensors are currently set at 0.0



Very open to correction

No correction needed, you just put the proverbial "money shot" into words.

 

[MarkTR]

One of the COVID problems in a country where lock-downs are still favoured is that there is too much time for writing long posts -- so apologies for that!!

Probably you have picked up the details from posts by @ramangain and @suprarx7nut -- the following may be superfluous and/or there may be some duplication but just to add some further clarifications in case that helps:

• There is only one pressure sensor in the AHC/TEMS system – it is attached to the machined block which separates the AHC Pump and the AHC Motor,

• The ECU does not cause readings from the Pressure Sensor to be displayed on Techstream in real time,

• Instead, the maximum pressures appear sequentially for Rear AHC, Front AHC and Height Control Accumulator after a raise, and importantly, after the AHC Pump has completed the sequence including recharging the Height Control Accumulator,

• The only relevant time to read AHC pressures for comparison with FSM specifications and IH8MUD experiences is after moving from “N” to “LO” then back to “N” per FSM procedure, AND, after allowing the AHC Electronic Control Unit (ECU) and AHC Pump to complete the raise-and-recharge sequence,

• Theoretically, in a new healthy system in perfect condition this should take about 15 seconds after the green AHC dashboard light has stopped blinking and after the suspension has settled in the new position – but allow around 60 seconds as @suprarx7nut suggests to allow for age, wear and imperfections and then be sure that the AHC Pump has stopped ie is silent. (At this point, you may see a slight ‘quiver’ in the fluid in the AHC Tank on closure of the solenoid valve at the forward end Height Control Accumulator. You may also be able hear this closure or feel it with your hand on the pipes at this Accumulator near its solenoid valve).

• It is best to run and read say three tests or enough to discard outliers. Spurious and inconsistent readings do occur, usually due to some imperfection in which a previous result has not been overwritten by the new result in the ECU memory

• AHC pressure readings done in any other way are spurious and certainly are not comparable with the FSM specifications and IH8MUD experience,

• The FSM is quite specific about disconnection of the Temperature Sensor when reading Rear AHC pressures. Some IH8MUD reports indicate large differences when this is done, while some (including me) do not see much difference. The reasons for such variances in experience are unclear,

• A zero pressure reading for the Height Control Accumulator also is common when the Temperature Sensor is disconnected. The reason for this is unclear, at least to me,

• As you have noted, increasing hub-to-fender height means that less vehicle weight is carried by less-deflected springs and torsion bars (F = kX, so less X means less F) and correspondingly more vehicle weight is carried by the AHC system. In turn, this means higher Front and Rear AHC pressures,

• The only means of adjusting AHC pressures is the indirect method of adjusting the share of weight carried by the torsion bars and springs – using torsion bar adjusters at the Front (and sometimes re-indexing the torsion bars when no more adjustment is available), and, by increasing the pre-load on the Rear coil springs with the spacers or by replacing the springs,

• ‘Ride Height’ adjustment is not accomplished using the torsion bar adjusters on a vehicle fitted with Active Height Control (AHC). Some change may be visible at the time of actually turning the torsion bar adjusters but this will be over-ridden by the AHC system after start-up and driving away,

• The fundamental design feature of the AHC system is to maintain the selected height (“LO” or “N” or “HI”) within the FSM-specified load limits and FSM-specified speed limits – provided the system is correctly adjusted and in the absence of fault conditions. Torsion bar adjustment does not override AHC operation,

• Torsion bar adjusters have only two purposes on an AHC-equipped vehicle:

1. ‘cross-levelling’ the Front of the vehicle – meaning correcting any ‘lean’ and equalising the Front Left and Front Right ‘static heights’ per FSM, thereby equalising Left and Right torsion bar loads (assuming Left and Right mechanical components are in good order and Right and Left wheels/tyres are the same overall diameters and at same pressure),
2. Adjusting Front AHC pressures – meaning adjusting the combined weight share carried by the Front torsion bars,

• ‘Ride Height’ adjustment is accomplished only by adjusting the Height Control Sensor Adjusters – in effect moving these adjusters so that the point at which a zero or near-zero reading appears on Techstream corresponds to a different hub-to-fender height at “N” height setting. (Note that the Techstream reading is in inches or millimetres per Techstream settings; the actual output of the Height Control Sensors is a signal in volts sent to the ECU, about 2.25 volts at “N” height setting),

• Obviously, changing the ‘ride height’ with Height Control Sensor adjusters will cause a change in AHC pressures. Then Front AHC pressures would then need to be adjusted using the torsion bar adjusters; Rear AHC pressures are not adjustable without varying springs in some way – spacers or new springs, or accepting the height which provides acceptable Rear AHC pressure, or varying the load in some way which provides acceptable Rear AHC pressures,

• To avoid going in circles of endless frustration, in any AHC review or adjustment process it is important to follow the sequence of steps set out in the “Cheat Sheet” prepared by @suprarx7nut. The sequence is just as important as the steps. No point in adjusting anything until front ‘cross-level’ has been done; no point in adjusting AHC pressures until ‘ride height’ is where you want it; no point in checking overall ‘globe’ condition via “HI” and “LO” fluid level differences at AHC tank until AHC pressures are in the correct range (and as an aside, no point in adjusting Front end geometry and alignment until ‘ride height’ is where you want it),

• I am not sure what to make of a persistently low Rear AHC pressure of 2.6 Mpa if that is what is occurring. If it remains persistent and all else has been done correctly, then probably I would become suspicious of the Rear Height Control Sensor, I confess mainly out of naked prejudice rather than reason. The Height Control Sensors are frequent nuisances in AHC matters. The ECU only records a Diagnostic Trouble Code (DTC) in relation to these Sensors in very specific circumstances defined in the FSM – but absence of a DTC provides no assurance that all is well with Height Control Sensors. For example, a Sensor can give a low voltage reading but no DTC if the voltage is still within the prescribed range. It may be worth checking this Sensor. However, this is pure speculation on my part – at the moment I cannot offer a reason connecting this Sensor to the Low Rear AHC pressure,

• The difference of half-inch at Rear seems incidental to the main game. Possibly it may indicate worn components somewhere at the Rear OR the Front. If the chassis is perfect and all suspension and running components are perfect and correctly adjusted, then it would be geometrically impossible to have the Front cross-level without the Rear also being cross-level as well. (Incidentally, this is the reason that there is only one Rear Height Control Sensor),

• Forgive me if I am preaching to the wise -- but consider a stiff rectangular object in one plane without any twist or warp, such as a stiff book or piece of metal plate. Place the front end on a level surface with the rear end raised. The heights at both sides of the raised end will be same. Press one rear corner to cause it to be lower. The only way this can happen is when the diagonally opposite front corner is raised,

• However, nothing is perfect in this world and Rear cross-level differences ARE observed on LC100/LX470 vehicles with AHC. There are a range of possibilities – from imperfect chassis manufacturing or subsequent wear and tear causing a slight chassis twist, to (more likely) worn suspension mountings and bushing affecting the way a vehicle ‘sits’ on a level surface, or more simply, slight differences at the 10 attachment points and bushes where the body is attached to the chassis. So there has to be some tolerance. A good wheel alignment shop should be able measure and to tell you the cause in your particular case,

• As a suggestion, start with wheel/tyre diameters and pressures all round, then inspection of Rear spring seats (cannot see how a poor Rear spring would not affect the diagonally opposite Front height), inspection of Rear AHC shock absorber mountings and bushes, inspection of bushes on Rear upper and lower control arms etc. Then do the same at the Front. It is entirely possible that worn Front shock absorber bushes or Front control arm bushes or ball-joints have been compensated by cross-levelling with the torsion bar adjusters to equalise hub-to-fender static heights – if so, this will mean that the Front fender-to-floor measurements would be slightly different side-to-side – and in turn that will reflect in some differences at the Rear if the chassis is perfect and the body fitment is consistent all round. Obviously, there can be a combination of such causes. In all this, it becomes doubtful whether ‘pursuit of perfection’ is worthwhile unless there are other suspension problems to fix or wear-and-tear items to replace. Your experience on finer Porsche suspensions may be more insightful on this issue!!

Wow--Very appreciative that you took the time to put all of the issues I was trying to get straight in my head in logical bullet points.-even after reading many long threads on this calibration/setup process. I keep forgetting all of the finer points that I read which leads me to not fully appreciate the big picture.

Most important is: The static height and the dynamic ride height can be two different lip-to-hub center dimensions depending upon where the height sensors are mechanically set.

You set the static lip to hub centers equal to make sure both TB's are equally sharing the front load when the truck is level side-to-side. The 19.75" optimum distance is a suggested value that often achieves the correct sharing of the front load between the TB's and the AHC pressure system-but is not set in concrete.

What is important is setting the TBs to where the truck is statically level and that both the TB's and the AHC share the load where the system pressure is in the 6.5 MPa region.




A few bullets made me think if my logic is still fuzzy. I thought when the TBs are cranked/twisted up (CW-increasing pre-load) lengthening the static lip to hub center distance --the TB's start to carry more of the load resulting in the Globes carrying/sharing less of the load- and the pressure should therefore go lower. This bullet seems to indicate the opposite. Am I reading your bullet wrong?

"As you have noted, increasing hub-to-fender height means that less vehicle weight is carried by less-deflected springs and torsion bars (F = kX, so less X means less F) and correspondingly more vehicle weight is carried by the AHC system. In turn, this means higher Front and Rear AHC pressures,"

And this

"No point in adjusting anything until front ‘cross-level’ has been done; no point in adjusting AHC pressures until ‘ride height’ is where you want it; no point in checking overall ‘globe’ condition via “HI” and “LO” fluid level differences at AHC tank until AHC pressures are in the correct range..."

To decide if you like the Ride Height I assume that you measure the lip-to-hub center after you have cycled from L-N a few times and the truck is running so the AHC is operating. If you like the height where the sensors have raised the truck- you are good to go.

What if the Ride Height is not level L to R (but the static was)- do you start playing with the TBs to get things level and the pressures where you want them while the truck is running-or is this a case where you play with the sensors to get the Ride height level?



I have no idea why I measure 2.6 MPa with the sensor disconnected and 6.9 MPa with it connected. I cycled L-H many times and got the same 2.9. As you pointed out, connected and disconnected pressures should be closer together than these two values.


I am going to crank down the TBs while the truck is running after a few L-H cycles and see where my pressures go. Then I will repeat at a different flat surface.


Thanks to everyone for their input so far-- I know long time forum members have been through many many past threads on this topic---and even though we nubies read them and should have absorbed the process- it is great when old members pitch in real-time once again to provide help

 

[MarkTR]

OK- Cranked the front end back down and I am going to stop here. I think the numbers are looking good. Ride height is only .25" low in the left front and .5 low in the Right rear-- which makes sense with my rigid box as IndroCruise pointed out.

If I raise the left up with the sensor I assume the right rear would go down. But I am fine where I am at-- a functional AHC.

I have no idea what the Rear 2.6 MPa was all about- but it never showed up this time. A high but acceptable Rear 7.0MPa.

Two improvements-- raise and lower times greatly reduced-- and I now have 13-14 gradations in the reservoir going from H-L.

At 190K this is not bad-- only servicing through the years with fluid flushes.

Thanks to everyone-- hopefully I will retain some of this to help the next. After years of my P38 Range Rover this is like working on a new machine.

Now on to spark plugs which have about 70K on them and the gaps are wide.

 

[ramangain]

Nailed it.

The next thing to dive into is if you always get an accumulator pressure of zero. If you do, you need more ahc fluid to recycle as you bleed out the accumulator. I posted the steps I take in another thread to do this as efficiently as possible.

Best approach is to find multiple methods and use the common denominators. Lowest risk.

 

[MarkTR]

Nailed it.

The next thing to dive into is if you always get an accumulator pressure of zero. If you do, you need more ahc fluid to recycle as you bleed out the accumulator. I posted the steps I take in another thread to do this as efficiently as possible.

Thanks ramangain.

When I hook up the AHC fluid temperature sensor I get about 10.5 or so MPa. I think the blank value is a glitch when I disconnect the sensor to run the calibrations and setup. I will look up your procedure and add it to my growing PDF file--things to baseline.

So far things are falling my way with this truck- expected many more issues. My '02 P38 RR had me in contorted positions wrenching on it day one.

I bought four bottles of AHC fluid intending to perform a flush. Curious, I pulled out some of the existing AHC fluid and it was clear. It looks like the system was flushed at the dealer soon before he traded it in. This is a one-owner truck that was serviced by Lexus at all major milestones. It also looks like he declined a few major repairs suggested by the dealer. Leaking rack, and CV boots. Both, after clean-up, look minor normal for 190K miles. Monitoring.

I found this detailed history by pulling the Lexus history on the Lexus site to check it against the CarFax (which tells you almost nothing). I then went through all of the services and checked them against what should be serviced at each milestone service. He only declined to repair a few Red Flags near the end of his ownership-- and passed them on to me.

This has given me some confidence that the owner actually paid the big bucks to service per Lexus recommended schedule and had the fluids routinely flushed and drivetrain greased. Still baselining the truck- taking no chances- but feeling pretty lucky after checking the history of other 470's I was considering.

The declined repairs are issues you will never know just reading the CarFax.- when it says "serviced at the dealer".

I think I got very lucky-- a one-owner dealer milestone serviced Nashville truck that has no rust.

The under carriage has now been steam cleaned and painted with chassis black. Now ordering Waxwool to undercoat. Up here in the mountains, salt and chemicals are the norm. I do not want to be the owner who rusts out this truck--undercarriage rust is why I am selling one beloved truck and buying this one as a replacement.