The ABCs of AHC - How to Measure, Flush, and Adjust all in one place (2 Viewers)

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That would be the 50 A AHC main fuse in the engine room fuse box. The fusible links generally protect the main wires/components from starting a fire in case of a short circuit. The 50 A AHC fuse is for protecting the wiring, AHC relay and AHC motor in case of a short. I would examine those three areas carefully with the aim of finding anything suspect.

One thing doesn't quite match though. Even if the fuse blows and the pump stops, the suspension shouldn't drop to low. Is this the case, that as soon as the fuse blows, the suspension lowers to the low position?
 
Has anyone had an issue with the AHC fusible link (Part No.: 90982-08285) failing? About 8 months ago, my AHC system dropped to low and the "OFF" light started flashing. I inspected the fusible link and it failed. I ordered a new one, replaced it, and all was good until a few days ago when the issue occurred again. I ordered and replaced the fusible link and drove maybe 200 yards and the system dropped to low again with the "OFF" light flashing. It's possible that I missed it when looking through this thread but did not see any comments on the fusible link and what is likely to cause it to fail. My initial thought is that as with any fuse that fails, too much current is passing through it which would indicate the pump is having to work too hard which would point to the pressures being too high. The problem is I can't get the system to work to check graduations or pressures so I'm not sure what my next step is. For context the vehicle is a 2005 LX470. I have changed the AHC fluid twice during my ownership, purchased in 2010. The LX has 222,000 miles. Any words of wisdom on next steps?
IDK no if issue covered in thread. Perhaps a search of thread for "FUSE", would be helpful!

Could a pump pull to many AMPs, and blow a fuse. Short answer YES! But I've not seen this personally, on a 100 series.

I have seen a fuse blow, from a motor being over-worked. But it was actually, a child electric riding cart. When a 220LB adult would ride, fuse would blow. With Child on under 100LB, it would never blow. So yes a motor can be over taxed, resulting in pulling to many AMPs, blowing a fuse.

AHC pressure switch, runs pump up to a predetermine pressure. If pump running to long or not shutting down, it would get hot may even overheat. Would this cause to many AMPs to be pulled. IDK! But wroth looking into.

Accumulator, aids in raising from L to N and N to H reducing time required to lift. A failed accumulator would increase pump run time, when changing height higher. Did failure happen while changing height.

How much pressure is read in tech stream at fronts, rear & accumulator has more to do with weight distribution once predetermined pressure reached. To much pressure, means to much weight carried by AHC system and not enough on rear coils springs and or frt T-bars. I don't think this would tax pump if pressures high. It does tend to reduce life of globes.

If you brought to me:
Checking for any DTC (codes) confirmed or pending, would be first step.

I'd ask, When you flushed (twice) before:
What did you set front pressure at then?
What was rear pressure?
What was accumulator pressure?
What changes in pressure were noted and grads form flush adj. to flush adj.
What were heights (axle to fender).
Are height sensors still at and always been at factory setting?
Any weight over factory, and or added since last flush & adjust?
I'd check each height sensor visually.
I'd look at T-bars and how much threads or expose, too get idea of where you had adjusted to.

I'd also ask about:
Weather condition, wet roads condition or just after washing did a failure happen.
WATER entry into cabin or even on wiring under vehicle getting wet may be a clue and concern!

I'd also look for rodent damage, aftermarket wiring and wire damage.

From there. I'd work through FSM diagnostic tree.

Hope that helps!
 
That would be the 50 A AHC main fuse in the engine room fuse box. The fusible links generally protect the main wires/components from starting a fire in case of a short circuit. The 50 A AHC fuse is for protecting the wiring, AHC relay and AHC motor in case of a short. I would examine those three areas carefully with the aim of finding anything suspect.

One thing doesn't quite match though. Even if the fuse blows and the pump stops, the suspension shouldn't drop to low. Is this the case, that as soon as the fuse blows, the suspension lowers to the low position?

Yes, as soon as the fuse blows the car drops to Low.
 
Yes, as soon as the fuse blows the car drops to Low.
Hmmm - What can the reason be.... Could one of the level valves be stuck and not closing properly, so that the pump has to run continuously to keep the car up?
Do both front and rear drop? Does the pump run for a long time when you put in a new fuse? It's supposed to tun only until the car is at N level, plus a few secs to top up the accumulator.
 
Mmmm – exceeding 50 Amps seems like a LOT of current! AHC Pump overload? Possible but seems less likely as the cause of breaking the 50 Amp fuse-link. There are too many other protections which should operate first, as part of the AHC/TEMS systems as set out in the “DIAGNOSTIC” section of the Factory Service Manual.

As mentioned by @uHu and @2001LC, a repetitive fault of this nature suggests a short circuit somewhere which enables such a large current flow. This seems more likely than an overload.

May I add the following to the suggestions already made in earlier posts in this thread:
  1. Check the AHC Motor Relay for correct operation and no short circuits -- failure of this relay is unusual but not impossible -- it is located in the Junction Box in the Engine Bay (not to be confused with the AHC Main Relay which is located on/near the Suspension ECU under the dashboard). ,
  2. Check for short circuits in all parts of the harness and all connectors between the 50 Amp fuse-link and the AHC Motor. For example, it would not be the first time that problems have arisen in 100 series vehicles below the Junction Box in the Engine Bay,
  3. Suggest always look for simple things first,
  4. After the “simple things” it may be necessary to look closely at relevant parts of the wiring harness, searching for road debris or sticks-and-stones damage and/or damage caused by heat from hot exhaust leaks and/or the effects of salt and water ingress into supposedly water-tight connectors, and/or test for short circuits, etc, etc, etc. See for example the stories and pictures relating to various circuit problems in the following threads:
MY06LX EWD needed badly - https://forum.ih8mud.com/threads/my06lx-ewd-needed-badly.1067265/#post-11750050 (whole thread including some early observations by @PADDO),

The ABCs of AHC - How to Measure, Flush, and Adjust all in one place - https://forum.ih8mud.com/threads/the-abcs-of-ahc-how-to-measure-flush-and-adjust-all-in-one-place.1211999/page-13#post-14467215 (this post and onwards),

The ABCs of AHC - How to Measure, Flush, and Adjust all in one place - https://forum.ih8mud.com/threads/the-abcs-of-ahc-how-to-measure-flush-and-adjust-all-in-one-place.1211999/page-22#post-15206568 (this post)

The point is that the fact that the vehicle drops to LO may be a distraction.

If there are short circuits then the Suspension ECU may be picking up mixed/incorrect signals and then the AHC/TEMS system will act in strange ways. The Suspension ECU may be adopting a false “fail safe mode” and placing the suspension in LO height.

As @uHu highlights, non-operation of the AHC Pump is not by itself enough to send the vehicle into LO height. If it was, then the vehicle would drop to LO every time the engine was switched off. The normally closed Levelling Valves in good condition in the Control Valve Assembly allow the vehicle to hold its height. A leaky Levelling Valve could cause the vehicle to drop slowly to LO (not quickly) -- but this would be an unusual fault, especially in a not-so-old vehicle. It just seems more likely (to me at least) that the Suspension ECU is picking up false signals as a result of a short circuit somewhere.

As a longtime Member of the Forum you probably are aware of all of the following bundle of references in relation to the AHC/TEMS systems but here they are again in case they are useful to anyone who comes this way ….

These are sections of the actual LC100 Factory Service Manual issued by Toyota. Where it is fitted, LC100 model shares the same AHC and TEMS systems as on LX470. I have not seen an on-line LX470 resource.

General Description of the AHC/TEMS systems with explanations and diagrams of components – definitely worth another read to refresh how these systems are meant to work:

LC100 Workshop Manual - https://lc100e.github.io/,
then scroll down through the index panel on the LHS of the opening page:
+ New Car Features
+ CHASSIS
+ Suspension
+ Active Height Control and Skyhook TEMS

Scrolling to “Repair Manual” and “DIAGNOSTICS” reveals relevant diagnostic and test information, etc. For example:

LC100 Workshop Manual - https://lc100e.github.io/
+ Repair Manual
+ DIAGNOSTICS
+ ACTIVE HEIGHT CONTROL SUSPENSION & SKYHOOK TEMS

Scrolling to Electrical Wiring Diagram and visually tracing the circuits helps with understandings of how things work:

LC100 Workshop Manual - https://lc100e.github.io/
+ Electrical Wiring Diagram
+ OVERALL ELECTRICAL WIRING DIAGRAM (choose LHD or RHD)
+ Active Height Control Suspension
+ Choose relevant year -- note: there can be some differences in EWD's in different years for LC100 and LX470

The coloured EWD is better -- found at Post #420 at the thread linked below, posted by @usedname and relevant to 2006 LX470 (also should be good for 2005 LX470):

The ABCs of AHC - How to Measure, Flush, and Adjust all in one place - https://forum.ih8mud.com/threads/the-abcs-of-ahc-how-to-measure-flush-and-adjust-all-in-one-place.1211999/page-21#post-14909889

Scrolling to “Repair Manual” reveals settings, repair information etc. For example:

LC100 Workshop Manual - https://lc100e.github.io/
+ Repair Manual
+ SUSPENSION AND AXLE
+ ACTIVE HEIGHT CONTROL SYSTEM
+ ON-VEHICLE INSPECTION
 

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  • AHC Motor Relay Circuit - m_di_0243.pdf
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  • FSM pages IN-27 to IN-37 - m_in_0027.pdf
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  • FSM pages IN-38 to IN-42 - m_in_0038.pdf
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Just wanted to share a few points after getting my system dialed in a little better:

After swapping out my globes, (replacing AHC globes/accumulators - https://forum.ih8mud.com/threads/replacing-ahc-globes-accumulators.716383/post-15227827) I started tuning. As expected, the pressures were high, but not too bad. Low 7s for the front and right around 7.0 for the rear.

Front: Front was a little low on the left, so with a few twists of the TB was able to get it aligned with the right. Overall needed about 5 turns on the left and 2 on the right and was able to get it in the mid 6s for the front with right around 19.75 wheel to fender measurement.

Rear: For the rear I ended up with 30mm spacers (note: get the Slee spacers) and that brought mine down roughly 0.5MPa. I didn’t see the 1MPa drop that others have mentioned. Even though I'm sure my 250k springs might be tired, still doesn't seem like new OEM springs are really going to change things that much here? I’m planning to stay mostly stock so I think the king springs would be overkill. Nevertheless, I’m happy with where I ended up, I will take 6.4 MPa.

The Ride: Man, I know others have already figured this out, but it is just SO lovely. It isn’t like magic or anything, but it is just so much difference with new globes and dialed in pressures. Again, thanks to all those for all the research here, esp YodaMD with the very approachable sheet and video which makes this far less intimidating. For anyone debating if they should do this after globes, the answer is an easy yes. Adjusting the TBs and adding the spacer was just as impactful as the new globes themselves.

My only comment (question?): The pressures are so sensitive to the flatness of the surface you are on! I tried this in roughly 4 different “flat” places over a few weeks and it always felt like the pressure would vary quite a bit. My last time I just found the flattest place I could and went up and down 6 times. Even then, the pressures ranged ~0.5MPa from high to low.

After a whole bunch of measurements this seemed to be a roughly average reading. I think I am going to call this good!

IMG_2936.JPG
 
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I finally tackled the height control bouncy suspension with a fluid flush and four new accumulators/globes. The transformation is incredible! Like riding on a cloud. Night and day difference. It's like a new vehicle. The old accumulators/globes were original as far as I can tell, which makes them 25 years old and over 80,000 miles. So they were definitely due for replacement.
 
I replaced my rear springs (2006 with 238k) and added 30mm spacers and it got me a whopping 0.2-0.3ish MPa rear pressure gains... I was a little disappointed.

Current specs after crossleveling and torsion bar pressure adjustment:

FR Height Control Sensor: -0.3 inch
FL Height Control Sensor: 0.0 inch
RR Height Control Sensor: -0.2 inch
Front Pressure Sensor: 6.7 MPa
Rear Pressure Sensor: 6.6 MPa
Accumulator Press Sensor: 10.6 MPa

Do I mess with the FR height sensor and RR height sensor to get closer to 0.0 or just let it ride? It's ever so slightly low on the hub-to-fender measurement.
 
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I replaced my rear springs (2006 with 238k) and added 30mm spacers and it got me a whopping 0.2-0.3ish MPa rear pressure gains... I was a little disappointed.

You bought all new OEM rear springs? Did you notice anything like unloaded spring height differences between the old and new?

6.6 isn't that bad. This means you were at ~6.9 before you did anything? That is really good for 238k! I'll also say mine changed almost every time I checked it. You might want to try some other "flat" spaces just to see if it moves around more.

For the fronts: In my case, the sensor height measurements matched the tape measurements. So looking at your sensor measurements makes me think you need to crank your FR TB a few more turns. If they really don't match up (which is rare, but could happen), yes you would need to adjust the sensors themselves.
 
I replaced my rear springs (2006 with 238k) and added 30mm spacers and it got me a whopping 0.2-0.3ish MPa rear pressure gains... I was a little disappointed.

Current specs after crossleveling and torsion bar pressure adjustment:

FR Height Control Sensor: -0.3 inch
FL Height Control Sensor: 0.0 inch
RR Height Control Sensor: -0.2 inch
Front Pressure Sensor: 6.7 MPa
Rear Pressure Sensor: 6.6 MPa
Accumulator Press Sensor: 10.6 MPa

Do I mess with the FR height sensor and RR height sensor to get closer to 0.0 or just let it ride? It's ever so slightly low on the hub-to-fender measurement.

If new OEM AHC springs, then AHC pressures are function of the weight of the vehicle and the 'ride height' of the vehicle at the "N" height setting -- more weight than stock and/or more height than stock means more AHC pressure. If Rear AHC pressure is not sufficiently corrected by a spacers (which causes pre-loading of the springs), then more spring rate is necessary, such as with King KTRS-79 springs.

You already understand the Height Control Sensor readings -- but to set it out again for anybody else who comes this way ....

Suggest re-check front cross-level on level ground -- “level” means a levelled concrete surface like a garage, or within a built parking station, or similar -- not in the street nor in an open parking lot -- these are cambered or sloped to drain and are neither flat nor level by design (or lack of design). Then look for tape-measurements hub-to-fender which are equal within FSM-specified tolerance of +/- 10 mm (0.39 inches), adjusting the equalisation if necessary using ONLY the torsion bar adjusters with engine OFF per FSM, and NOT using the Height Control Sensor adjusters. At this point, the actual hub-to-fender measurements are interesting but unimportant -- it is the equal heights within tolerance which are important. Vehicle ‘ride height adjustment’ is a separate step which comes later -- on an AHC-equipped vehicle ‘ride height adjustment’ is NOT done using the torsion bar adjusters.

A check of Front cross-levelling is the starting point for any adjustments on the AHC system.

By the way, the hub-to-fender measurements are not the FSM-specified method of vehicle height measurement but are an easy-to-use, widely recognised and reliable approximation. The FSM method is more accurate but is very fiddly and inconvenient.

With the Front of the vehicle properly cross-levelled, and with vehicle in “N” height setting and still on level ground, check Height Control Sensor readings Front Left, Front Right and the single Rear sensor on Techstream (or other suitable reader), looking for zero within FSM-specified tolerance of +/- 5 mm (0.2 inches).

If necessary, adjust the Sensors with engine OFF by using the slider adjustment to obtain as close to zero as possible -- can use the threaded heim arrangement on the Front sensors (not fitted on the Rear sensor) but these are very fiddly so suggest use slider first (as also mentioned in FSM).

Suggest be wary of “pursuit of perfection” in these measurements and adjustments. There can be some variation, especially if the ground is not level, or due to age and wear of suspension components and bushings, and/or small variations in manufacture of the vehicle (although Toyota/Lexus manufacturing tolerances are quite tight), and/or body or chassis damage in an accident or very hard use, etc, etc -- which can cause the vehicle to settle a little differently from time to time -- this is the purpose of the “tolerances”.

Suggest be aware that Height Control Sensors cannot be used to adjust side-to-side vehicle height differences on the AHC/TEMS system on LX470 (or LC100 where fitted). Attempting to do so is guaranteed to lead to frustration and re-work.

This is because the Gate Valves in the Control Valve Assembly are “normally open” when the vehicle is stationary or not cornering at speed. This means the pipework connecting the Front Right and Front Left sides of the suspension equalise the system pressures on Front Right and Front Left sides (ditto at the Rear) when the vehicle is stationary or not cornering at speed.

If the Height Control Sensor readings are different, then then the ECU works out an average and commands an up/down signal to get the sum displacement to zero -- if one sensor says +2 and the other says -2 then the sum displacement is 0 -- the ECU causes the vehicle adopt a vehicle height related to the average of the readings.

If the differences in Height Control Sensor readings readings are too far different, then the ECU may be unable to resolve the differences and may put the vehicle in ‘fail safe mode’ – fixed height, fixed damping, no automatic AHC or TEMS response, rough ride. Sometimes this is indicative of a failed sensor -- especially if the fixed height is strangely high or low.

‘Ride height adjustment’ is achieved by setting the vehicle at the desired height at “N” height setting (using jacks or some form of lift or by using the “Active Test” procedure), then adjusting the Height Control Sensors as close as possible to zero with the engine OFF. The vehicle is then set to return to this height at “N” height setting, provided the system is healthy and not overloaded. The ECU determines the heights at “LO” and “HI” height settings -- these cannot be adjusted independently of the “N” height setting.
 

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  • AHC - Adjust Heights and Height Control Sensor per FSM.pdf
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  • AHC - Cross level per FSM LoRes.pdf
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Messed with everything a little this morning. Moving the height sensor sliders had the opposite effect that I expected so it was a learning experience. Where I've landed and where I think it's going to stay for now:

FR Height Control Sensor: -0.2 inch
FL Height Control Sensor: -0.1 inch
RR Height Control Sensor: -0.2 inch
Front Pressure Sensor: 6.4 MPa
Rear Pressure Sensor: 6.4 MPa
Accumulator Press Sensor: 10.5 MPa

Thanks for the help @fussychicken and @IndroCruise

Edit: one last question, do I sit in the car when taking the measurements for pressures? It made a .3-.4 MPa difference.

Edit edit: looks like I shouldn't be in the car when measuring.

Last edit I swear:

With me out of the vehicle (after making some more adjustments:

FR Height Control Sensor: -0.2 inch
FL Height Control Sensor: -0.1 inch
RR Height Control Sensor: -0.2 inch
Front Pressure Sensor: 6.7 MPa
Rear Pressure Sensor: 6.5 MPa
Accumulator Press Sensor: 10.5 MPa
 
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Messed with everything a little this morning. Moving the height sensor sliders had the opposite effect that I expected so it was a learning experience. Where I've landed and where I think it's going to stay for now:

FR Height Control Sensor: -0.2 inch
FL Height Control Sensor: -0.1 inch
RR Height Control Sensor: -0.2 inch
Front Pressure Sensor: 6.4 MPa
Rear Pressure Sensor: 6.4 MPa
Accumulator Press Sensor: 10.5 MPa

Thanks for the help @fussychicken and @IndroCruise

Edit: one last question, do I sit in the car when taking the measurements for pressures? It made a .3-.4 MPa difference.
The FSM-specified AHC pressures are for the vehicle with no load, no passengers, no driver, no accessories, no fittings, fuel tanks full. The FSM is done this way because it allows the manufacturer to standardise the specifications.

However, all our vehicles are different and nowadays many are no longer in stock ex-factory condition.

The practical approach is to load the vehicle the way it is to be used most of the time (or for the most important times).

Then with the vehicle in that condition:

... adjust (or if necessary index) Front torsion bars to bring Front AHC pressures into the FSM-specified range, and,

... make decisions about rear spacers and coil springs which bring the Rear AHC pressures into the FSM-specified range.

The main point is always to measure AHC pressures with the vehicle in the SAME condition each time.

When AHC pressures are kept within the FSM--specified ranges, damping and ride comfort will be optimised for the particular vehicle and the way it is loaded and used -- provided that the 'globes' and the rest of the AHC system are in healthy condition.

If there are to be wide variations in load conditions, pressure-adjustable airbags fitted inside the Rear coil springs can be helpful.
 
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Hoping you guys might help shed some light on my situation..................

2006 100 series with AHC (UK)

Since I did some offroading with heavy articulation a couple months ago, my ride quality has gone down hill (jarring on bumps etc.).....

I am getting about 10 graduations of fluid.... however im not in spec so that means nothing??

1) Did an AHC fluid flush (it was black!) and refilled with Toyota AHC fluid
2) Adjusted torsion bars, brought the front into spec :) slight improvement for a while
3) Ride started to feel a bit worse after a week or so, cycling between sport and comfort isnt like it used to be, sport is like a rock now
4) Decided to "pencil test" the globes hoping I had a glaring failure.... nope. All had 5cm of pencil depth in them..... apparently that doesnt mean anything either!
5) Front is 6.6 and rear is 7.7............

Do I chuck new rear springs in? Or go for new globes? I believe both may be original to the car.....

Appreciate your thoughts



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@john100
Definitely get new rear springs. Get 48231-6A780 (Brown paint spot), if you are not carrying a lot of permanent extra weight. They are slightly stronger than the standard purple spot. Toyota 48231-6A780 (482316A780) SPRING, COIL, REAR - https://www.amayama.com/en/part/toyota/482316a780

If the globes are original, I would say they are probably worn out. The pencil test will only tell if the membrane is completely ruptured or not. You need more force than a pencil can take to test the remaining nitrogen pressure.
 

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