The ABCs of AHC - How to Measure, Flush, and Adjust all in one place (6 Viewers)

[boobird]

When ‘cross-levelling’ complete, then we think about correct heights. We switch “ON” the engine and also make sure that the AHC is switched “ON” and operating. The AHC Electronic Control Unit (ECU) will now receive information from the Height Control Sensors at Front Left, Front Right and Rear.

@IndroCruise

So front and rear heights are measured with the vehicle turned ON? Does the vehicle height change when the ignition is turned ON?

 

[IndroCruise]

When ‘cross-levelling’ complete, then we think about correct heights. We switch “ON” the engine and also make sure that the AHC is switched “ON” and operating. The AHC Electronic Control Unit (ECU) will now receive information from the Height Control Sensors at Front Left, Front Right and Rear.

@IndroCruise

So front and rear heights are measured with the vehicle turned ON? Does the vehicle height change when the ignition is turned ON?

Assuming that the AHC system is operating normally and all components are in good order, then the vehicle height only changes if so directed by the Height Control Sensors and the AHC Electronic Control Unit (ECU).

If, for example, the engine has been stopped immediately after a large suspension deflection and before the suspension has completed automatic self-levelling as directed by the Height Control Sensors and the ECU, then movement might be seen when the engine and AHC system are switched “ON” and the vehicle completes self-levelling.

Further examples which may involve height changes when the AHC system is re-started when the engine and system are switched “ON” might include:

• height recovery after clearing a fault condition in the AHC system which has caused the ECU to prohibit AHC system operation,
• height recovery after sinking due to a significant leak of AHC fluid from the system, or, internal leak due to a fault in a control valve or check valve within the system.

Ordinarily, the vehicle height would not be expected to change simply because the ignition is turned “ON”.

Here is an old link providing some succinct guidance on ‘cross levelling’ and height adjustment which may help. A Search of IH8MUD will reveal other references.

 

[silver02accord]

Apologies – I can see that my wording has caused some confusion so I will try again to be more clear.

No – the torsion bar adjusters are not “used to set the correct height (19. 75 front and 20.50)”.

The torsion adjusters are used to do only two very separate things in a vehicle with AHC suspension:

1. Cross-levelling
2. Adjusting front AHC pressure

The aim of so-called ‘Cross-levelling’ in Step 1 using the torsion bar adjusters is only to get the same load carried by RHS torsion bar and LHS torsion bar. It is not to set the height of the vehicle. The principle here is that torsion bars behave like springs. To get the same load, we want the same deflection or movement on both the RHS side of the vehicle and the LHS of the vehicle, without any involvement of the AHC system. This is why this is done with engine and AHC turned “OFF” in this Step. We assume that the RHS torsion bar and LHS torsion bar are the same product, same specification and not defective. Then if the front hub-to-fender distance on the RHS and LHS are the same, we can conclude that the RHS torsion bar and the LHS torsion bar are carrying the same load. It only important in Step 1 that the RHS front hub-to-fender distance and the LHS front hub-to-fender distance are equal. The actual distances do not matter in this Step. Do not worry about 19. 75 front and 20.50 rear in this Step – it is not relevant in Step 1. Those numbers are the specifications which are only relevant when the engine and AHC are switched “ON” and running. That comes later, not in Step 1. Achieving “19. 75 front and 20.50 rear” is never done with the torsion bar adjusters.

It is important to have the torsion bars carrying the same loads because

• the Right and Left sides of the vehicle must be in balance so that the vehicle does not behave differently when turning left and right, and,
• so that the operating heights can be set correctly in the later Steps.

When ‘cross-levelling’ complete, then we think about correct heights. We switch “ON” the engine and also make sure that the AHC is switched “ON” and operating. The AHC Electronic Control Unit (ECU) will now receive information from the Height Control Sensors at Front Left, Front Right and Rear. These Sensors will determine the actual heights. Probably the heights will not be correct. We now wish “to set the correct height (19. 75 front and 20.50 rear)” on the RHS and LHS of the vehicle. We definitely do not do this with the torsion bar adjusters because that would change the loads carried by the torsion bars and make them unequal. That would destroy the effort already made in Step 1. So we “set the correct heights (19. 75 front and 20.50 rear)” using the adjusters at the Height Control Sensors, starting at the Front LHS and RHS and then at the Rear.

After “setting the correct heights (19. 75 front and 20.50 rear)” using only the adjusters at the Height Control Sensors (not torsion bar adjusters), then it is time to consider Front and Rear AHC pressures.

The front AHC pressure is adjusted using the torsion bar adjusters. Note that this will not change the operating heights because the heights are set by the Height Control Sensors when the engine and AHC are running and these already have been adjusted.

In your case, you mention that you have “high 7s low 8 (Mpa) at front”. This may be different when you have set the correct heights as previously discussed – so the AHC pressures should be measured again before going further. If the pressures really are so high even at the corrects heights (maybe due to other loads, 'armour', or equipment on the vehicle?), then you need to get down to the lower part of the FSM-specified range for front pressure of 6.4Mpa to 7.4Mpa. Usually, one full turn on both torsion bar adjusters in the same direction makes a change of about 0.2Mpa, so you will need to do at least 5 turns, probably more like 8 turns.

After such a big change, you should again re-check the ‘cross-levelling’ (with engine and AHC “OFF”) and the correct operating heights (with the engine and AHC “ON”), in case something has been disturbed.

Hope this is more clear.

View attachment 2377887

After engine off and AHC "off" I adjusted torsion bars this way and that and got the fronts to be equal at 19.375 each and the rear is at 20.5. I wasn't prepared quite yet to adjust the AHC sensors(need to read up here a bit and have proper tools with me). I turned each bolt 6 full rotations and then I ran several L to N tests and got the Front pressure at 6.9 and rear pressure at 6.3. FR height is -2.8 and FL height is -3.2.

It sounds like I just need to adjust the front sensors at this time as the rear seems to be spot on? Will I just move the slider adjuster by 0.375 of an inch(measured 19.375, spec 19.75) on each side to "raise" the height to 19.75 front?

 

[LndXrsr]

It’s definitely not a 1:1 ratio. Some on here have suggested a 2:1 ratio (½” sensor movement changes suspension height 1”), but even that’s probably an oversimplification. You’ll have to try some small movements and see what you get. Even better, adjust the heim joint part of the sensor bolt and count the turns so you can easily reverse or advance things further depending on your results.

 

[IndroCruise]

After engine off and AHC "off" I adjusted torsion bars this way and that and got the fronts to be equal at 19.375 each and the rear is at 20.5. I wasn't prepared quite yet to adjust the AHC sensors(need to read up here a bit and have proper tools with me). I turned each bolt 6 full rotations and then I ran several L to N tests and got the Front pressure at 6.9 and rear pressure at 6.3. FR height is -2.8 and FL height is -3.2.

It sounds like I just need to adjust the front sensors at this time as the rear seems to be spot on? Will I just move the slider adjuster by 0.375 of an inch(measured 19.375, spec 19.75) on each side to "raise" the height to 19.75 front?

Looks like good progress!! Some would say, “Near enough, good enough, don’t touch anything!” If you are quite satisfied with the ‘ride quality’ at this point, then that might be good advice.

You could do a little more “fine tuning” if you wish. This would involve a few iterations of adjustments to get things exactly where you want them.

If you increase the physically measured functional (operating) front hub-to-fender height by another 0.4 inches from current 19.375 inches to target 19.750 inches using the Height Control Sensor adjusters, you can expect some increase in front AHC pressure. Why? Because increasing height means that torsion bars are relaxed a bit and carrying a bit less load and front AHC is carrying a bit more load and front AHC pressure will rise. This is not a problem – it is just a reminder of how load sharing works in the AHC system.

So, thinking ahead, if the operating height is to be raised, then as part of the process this pressure increase will need to be compensated later by more load transferred to the torsion bars in order to reduce the front AHC pressure back into the target range.

You have indicated some understandable apprehension about Height Control Sensor adjustments. The advice is – read widely and follow a careful, safe, methodical approach without distractions. These very detailed notes are intended to provide some comfort.

Note this very recent quote from PADDO: “The vast majority of undefined, random height/leveling issues are the result of some sort of sensor or harness defect - wear and tear, broken/maladjusted mechanisms, internal corrosion or a harness issue. Without monitoring the sensors feedback (all three) and generally assessing the system it’s not possible to diagnose 100%. But I bet it’s a bad sensor”.

The point I take from this message is that “100 series” vehicles with AHC are 13 to 20+ years old, so we had better learn to love the Height Control Sensors and their sometimes indeterminant issues and consequences.

SAFETY WARNING: ALWAYS TURN “OFF” THE ENGINE AND AHC WHEN HEAD AND HANDS ARE IN THE WHEEL ARCH OR ANY PERSON IS UNDER VEHICLE WHILE MAKING ANY AHC ADJUSTMENTS. (Later, the engine and AHC will have to be turned back “ON” to make any new hub-to-fender measurements),

Before doing anything:

• On a level surface, with front wheels straight, turn the engine “ON”, recheck and write down the Left and Right front hub-to-centre distances one more time, just to be sure,
• Turn the front wheels so that you can see in behind each Upper Control Arm and do these preparatory things:
  • Clearly mark the positions of the sliding adjusters on the Left and Right Height Control Sensors so you know exactly their current positions and have a reference point for any adjustments,
  • Take a picture of both the Left and Right front Height Height Control Sensors, similar to my pictures in my previous post, so that you have a record of the positions before adjustment of both the hein bolt (meaning the double-ended threaded adjusting bolt), and, the slider adjuster. This is always handy if for some reason you want to retrace your steps,
  • Treat the Height Control Sensor adjuster linkages with PB Blaster or other penetrating agent and let that soak in for few hours – gently clean off any accumulated rust, dirt, debris from the linkages with a wire brush if necessary,
  • There is no need to remove front wheels but it may help to raise the AHC to “HI” if it helps to gain access to do the above things – but ALWAYS take any actual hub-to-fender measurements ONLY at “N” (and never “HI” nor “LO” – because the operating performance of AHC and the damping system is designed around the “N” setting),
  • If there are any grounds to be suspicious of the condition or performance of the Height Control Sensors or if there simply are concerns about their old age, remove them completely, electrically test them as set out in the FSM or at least by the short cut suggested by uHu,
  • remove the plastic cover and internally inspect closely, decide whether condition warrants replacement, or if all is satisfactory take photographs before and after a gentle internal clean, reassemble and refit to the vehicle.

Now, think about how much adjustment is required at the Height Control Sensor adjusters to gain 0.4 inches in the hub-to-fender measurements. As LndXrsr already has indicated, the geometry of the front suspension and all the Height Control Sensor linkages mean that the required adjustment will be quite small.

If you wish to proceed, the following pathway is offered:

1. Turn the each front wheel in turn so that you have easy access to its Height Control Sensor Adjuster,
2. The suspension can be left in “HI” for the moment if that aids access (but MUST be returned to “N” for ANY hub-to-fender measurements),
3. AGAIN, THE SAFETY WARNING: ALWAYS TURN “OFF” THE ENGINE AND AHC WHEN HEAD AND HANDS ARE IN THE WHEEL ARCH OR ANY PERSON IS UNDER VEHICLE WHILE MAKING ANY AHC ADJUSTMENTS. (Later, the engine and AHC will have to be turned back “ON” to make any new hub-to-fender measurements),
4. Start by moving the slider upwards by say 0.25 inch on both the Right and Left sides. Take some care but this does not have to be exact -- keep in mind that that very fine adjustment, up or down, can be done in a subsequent step with the threaded hein connector. Alternatively, you may prefer to use only this threaded connector – if so, simply release the upper and lower locknuts and turn the body of the bolt to obtain a shortening effect. This will cause the vehicle to raise when the AHC system is powered up.
5. Withdraw hands, head and body, start engine, AHC “ON”, steering straight ahead, AHC returned to “N”, measure front hub-to-render distances,
6. Repeat Steps 1 to 5 until satisfied that front hub-to-centre distances are as close as possible to the targeted 19.75 inches,
7. Re-check the front hub-to-fender measurements after moving the suspension to “LO” and then back to “N”. (Expect some natural differences in the various measurements as bushes, ball-joints etc won’t be perfect and may cause some variances),
8. Re-check ‘cross-level’ and if necessary correct with torsion bar adjusters (NOT Height Control Sensor adjusters),
9. Now with front hub-to-fender distances adjusted and correct, it is time adjust front AHC pressures with equal turns on torsion bars until they are back in the FSM range. Note that there is little point in adjusting front AHC pressures until front hub-to-fender distances are correct, because heights directly affect pressures,
10. Check rear hub-to-fender distances and rear AHC pressures and make changes with spacers or spring replacements if necessary,
11. When checking front and rear pressures after adjustments, also check the Height Sensor readings on Techstream or other reader – should be close to zero, if wildly different then there are other problems which need to be pursued,
12. Consider whether AHC fluid has been replaced recently or should happen now,
13. Finally, with AHC pressures and heights correct as prescribed by FSM, conduct “HI/LO Volume Test” and observe difference in graduations at AHC tank, as an indicator of Gas Chamber (‘globe’) health (14 = ‘globes’ still relatively new; approaching 7 = time to plan ‘globe’ replacements),
14. Time for a drive!!

If not already seen, you may find some of the following AHC-related posts to be helpful:

Early posts by PADDO compiled by PabloCruise:

FSM Diagnostics concerning AHC posted by IndroCruise:

 

[silver02accord]

Looks like good progress!! Some would say, “Near enough, good enough, don’t touch anything!” If you are quite satisfied with the ‘ride quality’ at this point, then that might be good advice.

You could do a little more “fine tuning” if you wish. This would involve a few iterations of adjustments to get things exactly where you want them.

If you increase the physically measured functional (operating) front hub-to-fender height by another 0.4 inches from current 19.375 inches to target 19.750 inches using the Height Control Sensor adjusters, you can expect some increase in front AHC pressure. Why? Because increasing height means that torsion bars are relaxed a bit and carrying a bit less load and front AHC is carrying a bit more load and front AHC pressure will rise. This is not a problem – it is just a reminder of how load sharing works in the AHC system.

So, thinking ahead, if the operating height is to be raised, then as part of the process this pressure increase will need to be compensated later by more load transferred to the torsion bars in order to reduce the front AHC pressure back into the target range.

You have indicated some understandable apprehension about Height Control Sensor adjustments. The advice is – read widely and follow a careful, safe, methodical approach without distractions. These very detailed notes are intended to provide some comfort.

Note this very recent quote from PADDO: “The vast majority of undefined, random height/leveling issues are the result of some sort of sensor or harness defect - wear and tear, broken/maladjusted mechanisms, internal corrosion or a harness issue. Without monitoring the sensors feedback (all three) and generally assessing the system it’s not possible to diagnose 100%. But I bet it’s a bad sensor”.

The point I take from this message is that “100 series” vehicles with AHC are 13 to 20+ years old, so we had better learn to love the Height Control Sensors and their sometimes indeterminant issues and consequences.

SAFETY WARNING: ALWAYS TURN “OFF” THE ENGINE AND AHC WHEN HEAD AND HANDS ARE IN THE WHEEL ARCH OR ANY PERSON IS UNDER VEHICLE WHILE MAKING ANY AHC ADJUSTMENTS. (Later, the engine and AHC will have to be turned back “ON” to make any new hub-to-fender measurements),

Before doing anything:

• On a level surface, with front wheels straight, turn the engine “ON”, recheck and write down the Left and Right front hub-to-centre distances one more time, just to be sure,
• Turn the front wheels so that you can see in behind each Upper Control Arm and do these preparatory things:
  • Clearly mark the positions of the sliding adjusters on the Left and Right Height Control Sensors so you know exactly their current positions and have a reference point for any adjustments,
  • Take a picture of both the Left and Right front Height Height Control Sensors, similar to my pictures in my previous post, so that you have a record of the positions before adjustment of both the hein bolt (meaning the double-ended threaded adjusting bolt), and, the slider adjuster. This is always handy if for some reason you want to retrace your steps,
  • Treat the Height Control Sensor adjuster linkages with PB Blaster or other penetrating agent and let that soak in for few hours – gently clean off any accumulated rust, dirt, debris from the linkages with a wire brush if necessary,
  • There is no need to remove front wheels but it may help to raise the AHC to “HI” if it helps to gain access to do the above things – but ALWAYS take any actual hub-to-fender measurements ONLY at “N” (and never “HI” nor “LO” – because the operating performance of AHC and the damping system is designed around the “N” setting),
  • If there are any grounds to be suspicious of the condition or performance of the Height Control Sensors or if there simply are concerns about their old age, remove them completely, electrically test them as set out in the FSM or at least by the short cut suggested by uHu,
  • remove the plastic cover and internally inspect closely, decide whether condition warrants replacement, or if all is satisfactory take photographs before and after a gentle internal clean, reassemble and refit to the vehicle.

Now, think about how much adjustment is required at the Height Control Sensor adjusters to gain 0.4 inches in the hub-to-fender measurements. As LndXrsr already has indicated, the geometry of the front suspension and all the Height Control Sensor linkages mean that the required adjustment will be quite small.

If you wish to proceed, the following pathway is offered:

1. Turn the each front wheel in turn so that you have easy access to its Height Control Sensor Adjuster,
2. The suspension can be left in “HI” for the moment if that aids access (but MUST be returned to “N” for ANY hub-to-fender measurements),
3. AGAIN, THE SAFETY WARNING: ALWAYS TURN “OFF” THE ENGINE AND AHC WHEN HEAD AND HANDS ARE IN THE WHEEL ARCH OR ANY PERSON IS UNDER VEHICLE WHILE MAKING ANY AHC ADJUSTMENTS. (Later, the engine and AHC will have to be turned back “ON” to make any new hub-to-fender measurements),
4. Start by moving the slider upwards by say 0.25 inch on both the Right and Left sides. Take some care but this does not have to be exact -- keep in mind that that very fine adjustment, up or down, can be done in a subsequent step with the threaded hein connector. Alternatively, you may prefer to use only this threaded connector – if so, simply release the upper and lower locknuts and turn the body of the bolt to obtain a shortening effect. This will cause the vehicle to raise when the AHC system is powered up.
5. Withdraw hands, head and body, start engine, AHC “ON”, steering straight ahead, AHC returned to “N”, measure front hub-to-render distances,
6. Repeat Steps 1 to 5 until satisfied that front hub-to-centre distances are as close as possible to the targeted 19.75 inches,
7. Re-check the front hub-to-fender measurements after moving the suspension to “LO” and then back to “N”. (Expect some natural differences in the various measurements as bushes, ball-joints etc won’t be perfect and may cause some variances),
8. Re-check ‘cross-level’ and if necessary correct with torsion bar adjusters (NOT Height Control Sensor adjusters),
9. Now with front hub-to-fender distances adjusted and correct, it is time adjust front AHC pressures with equal turns on torsion bars until they are back in the FSM range. Note that there is little point in adjusting front AHC pressures until front hub-to-fender distances are correct, because heights directly affect pressures,
10. Check rear hub-to-fender distances and rear AHC pressures and make changes with spacers or spring replacements if necessary,
11. When checking front and rear pressures after adjustments, also check the Height Sensor readings on Techstream or other reader – should be close to zero, if wildly different then there are other problems which need to be pursued,
12. Consider whether AHC fluid has been replaced recently or should happen now,
13. Finally, with AHC pressures and heights correct as prescribed by FSM, conduct “HI/LO Volume Test” and observe difference in graduations at AHC tank, as an indicator of Gas Chamber (‘globe’) health (14 = ‘globes’ still relatively new; approaching 7 = time to plan ‘globe’ replacements),
14. Time for a drive!!

If not already seen, you may find some of the following AHC-related posts to be helpful:

Early posts by PADDO compiled by PabloCruise:

FSM Diagnostics concerning AHC posted by IndroCruise:

Absolutely amazing detail. Truly appreciate your help and opinions. I am going to flush fluid today then I'll try the sensor adjust per your recs. I am close on oem spec and happy with ride quality but would be nice to nail it too.

Any idea what the life/mileage of the shocks would be? I think I am in original but not sure (good maintenance records but didn't see them replaced).

 

[boobird]

@IndroCruise

I'm still in a bit of a confused state..

My front heights are perfect 19.75 Front Left&Right
My AHC presures are perfect 6.9 Front 6.1 back

My height sensors however are reading FL= -0.7 inch and FR = 0.6 inches

What to do?

 

[ramangain]

Let it be until you need to re-adjust. Then you can chase down and fiddle with the height sensors.

I run a ~1 inch delta between the front height sensors. Everything runs and works fine. I'll adjust it (like you may be tempted to right now) when "the lean" starts to come back.

 

[silver02accord]

@IndroCruise

I'm still in a bit of a confused state..

My front heights are perfect 19.75 Front Left&Right
My AHC presures are perfect 6.9 Front 6.1 back

My height sensors however are reading FL= -0.7 inch and FR = 0.6 inches

What to do?

seems perfect to me? rear at 20.5?

 

[uHu]

My front heights are perfect 19.75 Front Left&Right
My AHC presures are perfect 6.9 Front 6.1 back

My height sensors however are reading FL= -0.7 inch and FR = 0.6 inches
What to do?

That is a delta of 1.3". Not too bad, but you can (should?) adjust the two sensors so that the delta is < 0.3". If you adjust only one of the sensors, you'll end up with the front higher or lower, depending on which one you adjust to match the other.

 

[boobird]

So if i drop the RH sensor (lengthen the heim joint), wouldn't this drop the RH from 19.75"? Then would I have to tighten the RH TB (clockwise) to increase the right side back up to 19.75?

BTW, when i moved the joint up its slider, the FR sensor height reading went from 0.6 to -0.2 in... is this correct behavior?

 

[uHu]

So if i drop the RH sensor (lengthen the heim joint), wouldn't this drop the RH from 19.75"? Then would I have to tighten the RH TB (clockwise) to increase the right side back up to 19.75?

BTW, when i moved the joint up its slider, the FR sensor height reading went from 0.6 to -0.2 in... is this correct behavior?

The sensors do nothing to L/R lean. There is an open hydraulic connection between L & R fronts (same for the rears) so that you always have the same pressure and "AHC lifting" on both sides. The connection is closed when you drive in curves. The front height is set by the average between L & R sensor value, but the system will (partly) stop functioning if the difference is too big.

For adjusting height up front, you have to extend the link for lowering the height. That means extending the link for making the number for the sensor value higher. Higher number will make the ahc ecu react by lowering the car to make it read zero again (i.e. the make the average close to zero). So, the sensor that has a too high value gets a longer linkage, and the sensor with a too low value (negative) has to get a shorter link.

(At the rear you move the link down in the slot to lower it).

As long as the AHC is set to N (meaning normal driving), the only aim of the AHC part of the system is to keep the sensor values at zero. So, if the sensor value reads above 0, the pressure will be reduced, and if the sensor value is under 0, the pressure at that end is increased. There is a threshold value for the ahc to effect changes, so that the pump will not run continuously.

 

[uHu]

Deleted dbl post. (Where did the delete button go?)

 

[boobird]

The sensors do nothing to L/R lean. There is an open hydraulic connection between L & R fronts (same for the rears) so that you always have the same pressure and "AHC lifting" on both sides. The connection is closed when you drive in curves. The front height is set by the average between L & R sensor value, but the system will (partly) stop functioning if the difference is too big.

For adjusting height up front, you have to extend the link for lowering the height. That means extending the link for making the number for the sensor value higher. Higher number will make the ahc ecu react by lowering the car to make it read zero again (i.e. the make the average close to zero). So, the sensor that has a too high value gets a longer linkage, and the sensor with a too low value (negative) has to get a shorter link.

(At the rear you move the link down in the slot to lower it).

As long as the AHC is set to N (meaning normal driving), the only aim of the AHC part of the system is to keep the sensor values at zero. So, if the sensor value reads above 0, the pressure will be reduced, and if the sensor value is under 0, the pressure at that end is increased. There is a threshold value for the ahc to effect changes, so that the pump will not run continuously.

Thanks uHu for explaining the height sensors.

With perfect front heights 19.75, perfect front pressure 6.9 / rear 6.1, but imbalanced sensor heights (FL= -0.7 inch and FR = 0.6 inch), it rode really rough.

After adjusting the links the sensor heights are now both -0.1 inch. It rides a lot better (I have no idea why).

 

[ramangain]

Did you re-level with TB adjustments too?

 

[boobird]

Did you re-level with TB adjustments too?

Not yet.

After moving the RH link up its slider, the right side is now 20.25". I think I have to rotate the right TB 4x CCW.
After moving the LH link down its slider, the left side is still 19.75.

After adjusting the height links, I think I also need to check my front pressures again.

I have a very flimsy understanding everything

 

[IndroCruise]

So if i drop the RH sensor (lengthen the heim joint), wouldn't this drop the RH from 19.75"? Then would I have to tighten the RH TB (clockwise) to increase the right side back up to 19.75?

BTW, when i moved the joint up its slider, the FR sensor height reading went from 0.6 to -0.2 in... is this correct behavior?

To add to the replies from uHu and also plagiarise part of an old post from PADDO, the AHC/TEMS system does not and cannot hydraulically adjust individual corners. There is a front levelling circuit and a rear levelling circuit and the same pressures are applied to both Left and Right shock actuators in their respective circuit so the front raises or lowers evenly, as also happens at the rear. Individual torsion bar adjustments are what induces or corrects side to side front lean (and lower/raise pressures). Individual corner hydraulic height adjustments would require a four channel system with four levelling valves. The 100/470 platform is a two channel system with two levelling valves.

From far away and with little knowledge of the vehicle and the condition of its systems, it is hard to say what is/are the actual cause(s) of inconsistencies between Right and Left Height Control Sensor readings as read by Techstream or other scanners -- but the age and condition of the Height Control Sensors themselves would not be beyond suspicion. Different effects, Right versus Left, of linkage wear, internal wear, moisture ingress, corrosion, electrical resistance, different voltage supply from the harness, are all possibilities. If there are these kinds of problems, then there will be erratic results, including the possibility of the ECU prohibiting some AHC/TEMS functions if conflicting signals are received and resulting in a harsh ride. You may be inclined to remove the Sensors, clean, examine and test them following the FSM procedure or just follow the simpler method of comparing resistances and also check the supply voltage from the harness. Obviously, good Sensors and harness connections to a reliable ECU are essential -- otherwise it will be a case of "garbage in, garbage out" in terms of signals and responses. It is assumed that cross levelling is correct and has been maintained using torsion bar adjusters if necessary, and that the posture of the vehicle does not change when the vehicle is turned "OFF".

The actual cause of different Right and Left Front Height Control Sensor scanner readings could be ignored or reduced, at least temporarily, by tweaking the Height Control Sensor adjusters to bring the scanner readings closer together and preferably closer to zero. That would cause a small corresponding overall physical height change at the front and also a corresponding small AHC front pressure change but as already explained, this cannot cause different physical heights nor different AHC pressures at the Right and Left sides of the vehicle. Any AHC front pressure change could be adjusted at the torsion bars in the usual way -- and the cross level also re-checked to make sure it is not lost -- and adjusted at torsion bars if necessary.

In other words, you may be able to "tune" reasonable settings in which heights, pressures and scanner readings are within or close to tolerance, even if there is some compromise.

The ultimate test is settings which give satisfaction with 'ride quality' -- hope it goes well!

 

[IndroCruise]

Absolutely amazing detail. Truly appreciate your help and opinions. I am going to flush fluid today then I'll try the sensor adjust per your recs. I am close on oem spec and happy with ride quality but would be nice to nail it too.

Any idea what the life/mileage of the shocks would be? I think I am in original but not sure (good maintenance records but didn't see them replaced).

Life of the AHC 'shock absorbers' is long -- they are really little more than hydraulic struts which raise/lower the vehicle. It is the Gas Chambers (a.k.a. 'globes') and the Damping Force Actuators (to which the 'globes' are attached) working together with the 'shock absorbers' which are responsible for damping under the control of the ECU -- provided that no fault conditions or prohibitions have occurred which cause the ECU to prevent or limit some AHC/TEMS functions.

Excessive AHC pressures sustained over a long period can challenge the seals in the 'shock absorbers' and leaks then can occur. It is said that leaks often can be corrected if excessive AHC pressures are corrected.

In my case, sweaty front 'shock absorbers' arose from prolonged excessive front AHC pressures which went uncorrected after I added a full steel ARB Deluxe Bar. I was assured that the torsion bars had been adjusted and that all was good but I did not personally check AHC pressures for a year, maybe longer – a stupid case of neglect on my part, although partial excuses were great difficulty with Techstream which was eventually overcome with an alternative scanner, and less than helpful local dealerships. When the vehicle was at 141,900 kilometres (~88,000 miles) and 10 years old, I took the very conservative course and replaced the sweaty front ‘shock absorbers’ for reliability on planned long trips (thousands of kilometres) in remote parts of Australia. With AHC pressures corrected, probably they could have been made to last for many years but I preferred peace of mind. ‘Globes’ were still good but eventually four OEM replacements were installed at 183,231 kilometres (~114,000 miles) at 8 graduations at the AHC tank – again an early, conservative decision in preparation for yet another long, remote trip.

Rear ‘shock absorbers’ are original and still dry at 205,000 kilometres (~127,000 miles) and there are no reasons nor plans for replacements.

I think you would find that other IH8MUD Members around the world have achieved much, much longer ‘shock absorber’ lives and ‘globe’ lives than my conservative approach.

 

[LndXrsr]

I'm up to 194k on the original AHC "shocks" and globes. No leaks or weeping, still have 8 graduations from L to H. Will need globes at some point, but don't see any limit to the lifespan of the "shocks."

Do the maintenance, keep pressures in spec, and the system will treat you well.

 

[silver02accord]

I'm up to 194k on the original AHC "shocks" and globes. No leaks or weeping, still have 8 graduations from L to H. Will need globes at some point, but don't see any limit to the lifespan of the "shocks."

Do the maintenance, keep pressures in spec, and the system will treat you well.

Awesome, thank you both for your input. I don't see any leaks from the shocks for now and I am happy with ride quality. I am at 211k miles, 2006 LX.

I wanted to further fine tune today. I measured with ahc off, engine off and there was a slight discrepancy between left and right heights which I corrected with respective turns of torsion bars. I remeasured at N and was spot on. Then I did 2 tiny turns on the heim(sp?) and got both front heights to 19.625 so calling that good. With these tweaks I remeasured pressures and got 6.7 front, 6.2 rear and 10.3 for the accumulator. The FR height is -2.4mm and the FL height is -4.6mm which if I understand it is in oem spec.

AHC fluid should arrive tomorrow so after flush ill recheck it all. Thank you for your help!!! The AHC(and TEMS/damper) is the wife and mine favorite part of the truck thus far.