AHC won’t go to high and feels bouncy (1 Viewer)

[AtoyotA]

i will measure hub to fender here in a bit. i have nothing special in the back - just a 3/4in plywood sleeping platform that weighs ~80lbs and there are no 3rd row seats.

to reiterate, i can now get it into H so that's no longer an issue. the issue is the truck doesnt think it's level (when it is).

 

[IndroCruise]

i will measure hub to fender here in a bit. i have nothing special in the back - just a 3/4in plywood sleeping platform that weighs ~80lbs and there are no 3rd row seats.

to reiterate, i can now get it into H so that's no longer an issue. the issue is the truck doesnt think it's level (when it is).

With all of that clarified, it is time to consider the Height Control Sensors and the Height Control Sensor Circuits, noting that the effects may be intermittent, based on earlier posts.

On a 23 years old, 202,000 miles vehicle Sensor circuit problems would come as no surprise and this would be the next place to look when there is inconsistent height behaviour.

On 100 series vehicles with AHC, it is not uncommon for the Height Control Sensors and/or circuits -- two in the Front, one in the Rear -- to be problematic especially on older vehicles, and for this to happen WITHOUT recording a Height Control Sensor circuit Diagnostic Trouble Code (DTC).

The proviso is to be sure that the wrong physical heights, or inadequate Front 'cross-levelling' of the vehicle has not built-in some height errors which prevent a healthy AHC system from operating correctly -- hence the emphasis on tape-measured physical heights -- because the AHC system has no idea what physical heights have been set on the vehicle, nor whether it is 'cross-level' or not.

The Height Control Sensors are simple, long-life, robust potentiometers -- or voltage dividers -- involving an applied voltage and a variable resistance arranged, in the case of these Sensors, to be proportional to height. No modern electronics in here!! However, these Sensors are 'wear items', they do not last forever, and on any 100 series vehicle with the original Sensors, these items are now 16 years to 24 years old, and have worked continuously for every second that the vehicle has travelled. Their reliability is now low.

The DTC settings for these Sensor circuits are limited. If there is a DTC, there definitely is a problem. If there is not a DTC, it may easily mean that the voltage signal sent by the Sensor circuit is within the allowable range (so no DTC), but for a worn or degraded Sensor, it easily can be the wrong voltage for the actual height of the vehicle. In these very common cases, it really is only the non-conforming or inconsistent height behaviour of the vehicle which provides the diagnosis. The causes are wear and tear on the internal carbon track, weakening pressure by the tiny brushes contacting the carbon track, poor or corroded contacts within the Sensor, moisture ingress and 'tracking'.

These effects are well known and frequently discussed on this forum and elsewhere.

Problematic Sensors are common. Suspect Sensors can and should be removed and tested as set out in the attachment.

An alternative test based on resistances also has been suggested by @PADDO at Post #2 in this thread:

High as a kite, hard as a rock! (Yes AHC again...)

Hi guys! I have a 2001 VX100 1HD-FT diesel with 300,000 miles on it. I've owned it for two years and love the comforts over the HJ61 it replaced. But the AHC is acting up! I don't want to go conventional, I want to keep the AHC. AFAIK everything except height sensors are original, and I know...

forum.ih8mud.com

Theoretically Sensors can be cleaned up and re-fitted. For long-aged Sensors, this is a mug's game and replacement of ancient Sensors is the far better idea for reliability and no inconvenient surprises or unhappiness.

In addition to issues with the Sensors themselves, there are increasing incidences of problematic Sensor circuits, recently written up very well here -- and if Sensors test as reliable, circuit investigation is next in the batting order -- Post #404 in this thread and references give the idea:

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

Reading this thread again for a bit - there's a lot going on I see. Looks like you are on the right track, usedname. Regarding your worry about the voltage at the areas at the ends of the travel of the sensors: There is no need to worry. What happens to the voltage (or resistance) at the ends...

forum.ih8mud.com

MY06LX EWD needed badly - https://forum.ih8mud.com/threads/my06lx-ewd-needed-badly.1067265/#post-11750050

 

[AtoyotA]

ok so the measurements are:

Front L: 19.5"
Front R: 19.75"
Rear L: 21.25"
Rear R: 21.5"

seems like too much of a rake. i really thought i was gonna need to raise the rear, not lower it. how do you even lower it besides adding weight?

 

[AtoyotA]

...and by raise the rear i mean stiffer springs.

 

[IndroCruise]

...and by raise the rear i mean stiffer springs.

So now suggest follow the sequence set out in Post #9 in this thread and repeated below -- this now includes comments and also answers to your questions:

1. Check that vehicle vehicle is on level ground (not in the street), wheels straight ahead, fuel full, no poorly distributed loads,

If this is OK, nothing to do here. If not, suggest find a more suitable location and re-check tape measurements and Techstream readings.

2. Check condition of Height Control Sensors – all should be close to zero +/- 5 millimetres or +/- 0.2 inches (AHC system cannot work correctly with faulty or incorrectly adjusted Height Control Sensors – absence of a DTC does not provide assurance of healthy Sensors),

If there is no reason to suspect Height Control Sensor problems -- such as erratic or inconsistent height behaviour at different times -- then assume Height Control Sensors and circuits are good, at least for now, and assume that there is no immediate need to investigate Height Control Sensors as described at Post #17 and Post #22 in this thread.

Techstream Height Control Sensor readings at Post #3 in this thread look reasonable -- a little low on the Front Left at negative 0.4 inches, likely because the Front Left hub-to-fender at 19.5 inches by tape-measure is physically a little lower than Front Right at 19.75 inches. This is reasonable. Some would say further tweaking is not worthwhile so leave it alone. Others would say tweak a little using the attached ‘cross levelling’ procedure and torsion bar adjusters with engine OFF to better equalise hub-to-fender measurements.

This also is likely to bring the Techstream Height Control Sensor readings closer to each other and closer to zero.

With Front Left a little low, the diagonally opposite Rear Right hub-to-fender tape-measurement is a little higher than Rear Left. This is as expected on a perfect chassis and perfectly attached body -- may be a matter of luck on an older vehicle! Tweaking the Front ‘cross level’ also is likely to adjust the Rear hub-to fender tape-measurements favourably.

3. Check/adjust/equalise Front ‘cross-level’ with torsion bar adjusters -- measured by tape-measure, engine and AHC “OFF”, then,

Your call. As mentioned above, tweak these a little if you wish to equalise Front hub-to-fender tape-measurements more precisely.

4. Check ‘ride heights’ at N height, measuring hub-to-fender by tape-measure – engine “OFF”, ignition “ON”, adjust if necessary using adjustment procedure and Height Sensor Control adjusters (not torsion bar adjusters), then,

Use IH8MUD recommendations by long time AHC guru @PADDO for a stock vehicle -- Front 19.75 inches, Rear 20.50 inches at N height setting. The well-known and widely respected @2001LC suggests that Front is better left at 19.50 inches at N height setting. Whatever, the idea is to make the Front hub-to-fender tape measurements as equal as practicable. Note that these ideas are not FSM numbers but are the widely used and reliable approximations of the much more fiddly-to-measure FSM height specifications.

In answer to your questions, note also that upgraded springs (or torsion bars) with a higher spring rate (such as KING KTRS-79 coil springs) do not increase the ‘ride height’ of the vehicle. ‘Ride height’ is controlled by the Height Control Sensors and the Suspension ECU. When upgraded Rear coil springs are used, the ‘ride height’ is not increased and remains the same as set at N height. Instead, the Rear AHC pressure is reduced, meaning that a higher share of the vehicle weight is carried by the springs, and a lesser share is carried by the AHC ‘shock absorbers’ -- which actually are weight-bearing hydraulic struts on an AHC-equipped vehicle. The advantage of the upgraded Rear coil springs is that they allow increased load carrying capacity while also keeping the Rear AHC pressures within the FSM-specified range and providing better 'ride comfort' in loaded conditions or if carrying permanent additional fittings. The ride will feel slightly firmer, but not unacceptable, when the vehicle is empty or if permanent fittings are removed. (Personally, I am very happy with the ride comfort and better road-holding with the KING KTRS-79 Rear coil springs especially when loaded for long distance travel -- no regrets).

To reduce Rear ‘ride height’ (and also reduce the Rear AHC pressure which is too high in the screenshot at Post #3, so therefore less damping and rougher ride), move the Rear Height Control Sensor adjuster down the slide. Suggest experiment with this, say one-eighth of an inch at a time. This will make a large difference in Rear height, maybe three-quarters of an inch. Strongly recommend that any such adjustments are made with the engine (and therefore AHC) OFF and with vehicle stands also in position under the chassis rails to prevent any possibility of the vehicle body inadvertently coming into contact with the human body if a mistake is made. After adjustment, start the engine and measure the effect. Fine tuning may be necessary after a drive around the block.

NOTE: Posts by @2001LC usually recommend minimum rake (Front lower than Rear) of 3/4 inch.

5. Check AHC pressures by Techstream at N height after N > LO > N movement (allow 30 seconds after green AHC dashboard light stops blinking after arriving at N), then record Techstream results for Front, Rear, Height Control Accumulator, then,

When Front and Rear ‘ride heights’ are where you want them to be, use Techstream to measure AHC pressures -- Front, Rear and Height Control Accumulator.

6. If AHC pressures are out of the FSM-specified ranges, then (i) use Front torsion bar adjusters to vary the share of Front vehicle weight carried by torsion bars and the share of Front vehicle weight carried by Front AHC system, thereby adjusting Front AHC pressures, and, (ii) consider spacers or replacement/upgrade of Rear coil springs to vary the share of Rear vehicle weight carried by springs and the share of Rear vehicle weight carried by Rear AHC system, thereby adjusting Rear AHC pressures. (Note: Front AHC and Rear AHC are not hydraulically connected but changing the Front and Rear weight distributions may cause some small interactions between Front and Rear AHC pressures and some ‘fine tuning’ may be required), then,

After adjusting load share between springs / torsion bars and AHC system, especially at Rear, to achieve best damping characteristics, smooth travel over bumps and good ride comfort -- fine tune as necessary.

7. When heights and AHC Front AHC and Rear AHC pressures are correct, check overall ‘globe’ condition by observing the difference in AHC Fluid level at AHC Tank between HI and LO heights. Per FSM, difference of around 14 graduations means ‘globes’ are near new; when the difference has declined to around 7 graduations, replace all four (4) ‘globes’. Results of this test at incorrect AHC pressures are not comparable with FSM numbers.

This test is the last test in the AHC adjustment/test sequence, conducted when heights and pressures are correct. Current difference reported is ~9 graduations. If done at FSM-specified AHC pressures, this would indicate that 'globes' are in fair condition, although there will be some loss of damping and ride comfort.

Note that AHC Front and Rear pressures, correct or otherwise, tell nothing about the overall condition of the 'globes'.

 

[AtoyotA]

So now suggest follow the sequence set out in Post #9 in this thread and repeated below -- this now includes comments and also answers to your questions:

1. Check that vehicle vehicle is on level ground (not in the street), wheels straight ahead, fuel full, no poorly distributed loads,

If this is OK, nothing to do here. If not, suggest find a more suitable location and re-check tape measurements and Techstream readings.

2. Check condition of Height Control Sensors – all should be close to zero +/- 5 millimetres or +/- 0.2 inches (AHC system cannot work correctly with faulty or incorrectly adjusted Height Control Sensors – absence of a DTC does not provide assurance of healthy Sensors),

If there is no reason to suspect Height Control Sensor problems -- such as erratic or inconsistent height behaviour at different times -- then assume Height Control Sensors and circuits are good, at least for now, and assume that there is no immediate need to investigate Height Control Sensors as described at Post #17 and Post #22 in this thread.

Techstream Height Control Sensor readings at Post #3 in this thread look reasonable -- a little low on the Front Left at negative 0.4 inches, likely because the Front Left hub-to-fender at 19.5 inches by tape-measure is physically a little lower than Front Right at 19.75 inches. This is reasonable. Some would say further tweaking is not worthwhile so leave it alone. Others would say tweak a little using the attached ‘cross levelling’ procedure and torsion bar adjusters with engine OFF to better equalise hub-to-fender measurements.

This also is likely to bring the Techstream Height Control Sensor readings closer to each other and closer to zero.

With Front Left a little low, the diagonally opposite Rear Right hub-to-fender tape-measurement is a little higher than Rear Right. This is as expected on a perfect chassis and perfectly attached body -- may be a matter of luck on an older vehicle. Tweaking the Front ‘cross level’ also is likely to adjust the Rear hub-to fender tape-measurements favourably.

3. Check/adjust/equalise Front ‘cross-level’ with torsion bar adjusters -- measured by tape-measure, engine and AHC “OFF”, then,

Your call. As mentioned above, tweak these a little if you wish to equalise Front hub-to-fender tape-measurements more precisely.

4. Check ‘ride heights’ at N height, measuring hub-to-fender by tape-measure – engine “OFF”, ignition “ON”, adjust if necessary using adjustment procedure and Height Sensor Control adjusters (not torsion bar adjusters), then,

Use IH8MUD recommendations by long time AHC guru @PADDO for a stock vehicle -- Front 19.75 inches, Rear 20.50 inches at N height setting. The well-known and widely respected @2001LC suggests that Front is better left at 19.50 inches at N height setting. Whatever, the idea is to make the Front hub-to-fender tape measurements as equal as practicable. Note that these ideas are not FSM numbers but are the widely used and reliable approximations of the much more fiddly-to-measure FSM height specifications.

In answer to your questions, note also that upgraded springs (or torsion bars) with a higher spring rate (such as KING KTRS-79 coil springs) do not increase the ‘ride height’ of the vehicle. ‘Ride height’ is controlled by the Height Control Sensors and the Suspension ECU. When upgraded Rear coil springs are used, the ‘ride height’ is not increased and remains the same as set at N height. Instead, the Rear AHC pressure is reduced, meaning that a higher share of the vehicle weight is carried by the springs, and a lesser share is carried by the AHC ‘shock absorbers’ -- which actually are weight-bearing hydraulic struts on an AHC-equipped vehicle. The advantage of the upgraded Rear coil springs is that they allow increased load carrying capacity while also keeping the Rear AHC pressures within the FSM-specified range and providing better 'ride comfort' in loaded conditions or if carrying permanent additional fittings. The ride will feel slightly firmer, but not unacceptable, when the vehicle is empty or if permanent fittings are removed. (Personally, I am very happy with the ride comfort and better road-holding with the KING KTRS-79 Rear coil springs especially when loaded for long distance travel -- no regrets).

To reduce Rear ‘ride height’ (and also reduce the Rear AHC pressure which is too high in the screenshot at Post #3, so therefore less damping and rougher ride), move the Rear Height Control Sensor adjuster down the slide. Suggest experiment with this, say one-eighth of an inch at a time. This will make a large difference in Rear height, maybe three-quarters of an inch. Strongly recommend that any such adjustments are made with the engine (and therefore AHC) OFF and with vehicle stands also in position under the chassis rails to prevent any possibility of the vehicle body inadvertently coming into contact with the human body if a mistake is made. After adjustment, start the engine and measure the effect. Fine tuning may be necessary after a drive around the block.

NOTE: Posts by @2001LC usually recommend minimum rake (Front lower than Rear) of 3/4 inch.

View attachment 3289652

View attachment 3289653

5. Check AHC pressures by Techstream at N height after N > LO > N movement (allow 30 seconds after green AHC dashboard light stops blinking after arriving at N), then record Techstream results for Front, Rear, Height Control Accumulator, then,

When Front and Rear ‘ride heights’ are where you want them to be, use Techstream to measure AHC pressures -- Front, Rear and Height Control Accumulator.

6. If AHC pressures are out of the FSM-specified ranges, then (i) use Front torsion bar adjusters to vary the share of Front vehicle weight carried by torsion bars and the share of Front vehicle weight carried by Front AHC system, thereby adjusting Front AHC pressures, and, (ii) consider spacers or replacement/upgrade of Rear coil springs to vary the share of Rear vehicle weight carried by springs and the share of Rear vehicle weight carried by Rear AHC system, thereby adjusting Rear AHC pressures. (Note: Front AHC and Rear AHC are not hydraulically connected but changing the Front and Rear weight distributions may cause some small interactions between Front and Rear AHC pressures and some ‘fine tuning’ may be required), then,

After adjusting load share between springs / torsion bars and AHC system, especially at Rear, to achieve best damping characteristics, smooth travel over bumps and good ride comfort -- fine tune as necessary.

7. When heights and AHC Front AHC and Rear AHC pressures are correct, check overall ‘globe’ condition by observing the difference in AHC Fluid level at AHC Tank between HI and LO heights. Per FSM, difference of around 14 graduations means ‘globes’ are near new; when the difference has declined to around 7 graduations, replace all four (4) ‘globes’. Results of this test at incorrect AHC pressures are not comparable with FSM numbers.

This test is the last test in the AHC adjustment/test sequence, conducted when heights and pressures are correct. Current difference reported is ~9 graduations. If done at FSM-specified AHC pressures, this would indicate that 'globes' are in fair condition, although there will be some loss of damping and ride comfort.

Note that AHC Front and Rear pressures, correct or otherwise, tell nothing about the overall condition of the 'globes'.

incredible post! thank you so much for taking the time to reply! invaluable info. i will definitely adjust the rear sensor heights and do the cross-leveling - i'm guessing my measurements explain why my Front Right tire is wearing so much more than the Front Left!