The ABCs of AHC - How to Measure, Flush, and Adjust all in one place (1 Viewer)

[IndroCruise]

Thanks for that info. I do plan on swapping the sensors at some point soon.

I was able to get it to go into L by putting some more weight in the truck and driving it a bit, and it’s visibly in L now.

Might just need few movements to settle properly -- maybe with the heaviest person you can find sitting on the open tailgate!

Suggest also check

-- springs correctly seated,

-- condition of bushes in Upper and Lower Rear Control Arms and in the Panhard Rod.

 

[fussychicken]

Just want to issue a bit of a PSA here: Just be a bit careful if you start to encounter too much resistance when adjusting your torsion bars.

About a year ago I adjusted my T-bars just fine with the vehicle on the ground when going through the process from this thread. But more recently I did a big front end rebuild and fully disassembled the T-bar bolt and screwed something up when putting the driver's side back together. I might have messed up the threads when using an impact when taking it apart, but I still am not 100% sure. You can see in the picture below what happened. Apparently I'm not alone, you can see more here

Anyway, I'm not trying to scare monger, but I've learned from other threads it is a little bit safer/easier to adjust the T-bars if they are slightly unloaded. You can do this by putting AHC on high or even better, jacking up the front end. While I know this kind of goes against the adjustment advice in this thread, just something to think about.

Although not proven at all, I also think blowing off the dust from the threads and/or using PB blaster to help lube the threads a little would probably help add a little safeguard for AHC adjustments if you didn't feel like raising up the vehicle.

 

[ClassyJalopy]

Just want to issue a bit of a PSA here: Just be a bit careful if you start to encounter too much resistance when adjusting your torsion bars.

About a year ago I adjusted my T-bars just fine with the vehicle on the ground when going through the process from this thread. But more recently I did a big front end rebuild and fully disassembled the T-bar bolt and screwed something up when putting the driver's side back together. I might have messed up the threads when using an impact when taking it apart, but I still am not 100% sure. You can see in the picture below what happened. Apparently I'm not alone, you can see more here

Anyway, I'm not trying to scare monger, but I've learned from other threads it is a little bit safer/easier to adjust the T-bars if they are slightly unloaded. You can do this by putting AHC on high or even better, jacking up the front end. While I know this kind of goes against the adjustment advice in this thread, just something to think about.

Although not proven at all, I also think blowing off the dust from the threads and/or using PB blaster to help lube the threads a little would probably help add a little safeguard for AHC adjustments if you didn't feel like raising up the vehicle.

View attachment 3735491

This looks like it was cross threaded and then run forcefully through with an impact gun. That is a very large hardened bolt that is difficult to mess up. Also, always have the front wheels up in the air when adjusting the TBs

 

[fussychicken]

This looks like it was cross threaded and then run forcefully through with an impact gun. That is a very large hardened bolt that is difficult to mess up.

I know it looks that way, but that was not the case. I used an impact to loosen it, and when I put it back together, I started the threads by hand, used a drill to take up the slack (roughly half the threads), and as soon as the slack was gone, I stopped using the drill and used hand tools. With the hand tools it was going in fine until the very end and then started to get really tight. When I started to take it back out it was just as tight and that is when I knew I was in trouble. Most of this damage all came from me then trying to get the bolt back out.

The only thing I can think of is that when it was originally under tension, and I used the impact to loosen it, it might have slightly warped the threads. And when I was putting it back together, it mostly screwed in fine because the threads at the top of the bolt were mostly ok. And then when I started to engage the bottom of the bolt that is when the slightly warped threads were then again put under pressure. And as I tried to get it to the final position of my paint marks, when the bolt is under the most pressure, something finally gave way.

In either case, I'm of course now paranoid and won't use an impact even to take it apart.

And also, sorry for the thread drift. For those in this thread making AHC adjustments, the point of the PSA was just to say that I think a little PB blaster on the threads and a little caution will help.

Also, always have the front wheels up in the air when adjusting the TBs

Yeah, I guess part of the PSA is sharing this knowledge as well. While I didn't read all 25 pages of this thread, I don't recall seeing this piece of info. I'm guessing that many of us in this thread have done adjustments with the vehicle on the ground.

 

[Pulgasari]

Also, always have the front wheels up in the air when adjusting the TBs

Yeah, I guess part of the PSA is sharing this knowledge as well. While I didn't read all 25 pages of this thread, I don't recall seeing this piece of info. I'm guessing that many of us in this thread have done adjustments with the vehicle on the ground.

I need to adjust my TBs and also would have done so with the wheels on the ground had I not seen this comment

 

[Bisho]

I'll disagree a bit with @ClassyJalopy for an AHC equipped truck. The TBs don't support the weight of the truck with AHC like they do on the traditional suspension, they are just helpers. Its very easy to adjust the TBs on an AHC truck with the wheels on the ground, because the hydraulic system is doing most of the work (although I recommend the suspension be in "high"). Its near impossible to adjust the traditional suspension without lifting the truck first, although many have tried and mucked up the bolt as a result.

 

[parkitparker]

+1 here for disconnecting temp sensor. I did my globes and things looked great. I checked again a few weeks later and was showing 2.9 in the rear pressures. I couldn’t figure it out until I remember to unplug the temp sensor. Brought me back closer to spec.

 

[blacktimber]

I just followed the steps above on my 2003 LX. Heights were good, so no adjustment needed. Checked pressure in TechStream:
Front pressure @ 8.2,
Rear pressure @ 7.3.

Cranked the torsion bars 7 turns and ended up with:
Front pressure @ 6.5
Rear pressure @ 6.9
so pretty close to spec in the front, still high in the rear. The ride is drastically improved. I thought it was good (a bit mushy, but comfy) before.

I did order some new springs, the LC w/ AHC springs. They are slightly stiffer than the LX springs (or so I've read). I do plan to tow a boat occasionally, and like to load up the family for road trips. I want to make sure the rear is supported well. I will post after install in case others are curious about the difference.
I currently get about 7/8 graduations of AHC fluid. This is on a truck with 260,000 miles and original spheres!

Update:
So after installing the LC AHC springs my pressures are at 6.8 front and 6.5 rear. I backed off the torsion bar one rotation from my previous reading to get the front closer to spec of 6.9.
The pressure with the new springs are not a huge difference, but driving the car, especially over speed bumps, is much improved. The rear doesn't sink as low after going over a speed bump as before, and the rear generally feels more planted on the road. One note, when adding replacing rear springs, remember to disconnect the height sensor bracket, or you may be in for some of this.

With a little reading, $90 in AHC fluid, and $250 for new rear springs, I have a comfortable suspension that rides almost like new. One day I will need new globes, but now that I know what AHC is supposed to feel like, I am certainly going to keep it.

Hello, just purchased a 2003 LX470 and want to address the AHC pressures. I use techstream v12.20.024 and hook up to the LX computer with no issue BUT there is no AHC data listed. I was wondering what version of techsteam and cable you used successfully. Much appreciated.

 

[JBerger]

@blacktimber Are you saying there is no AHC data or you have no AHC Live option in Techstream?

 

[MTBNCYN]

I just followed the steps above on my 2003 LX. Heights were good, so no adjustment needed. Checked pressure in TechStream:
Front pressure @ 8.2,
Rear pressure @ 7.3.

Cranked the torsion bars 7 turns and ended up with:
Front pressure @ 6.5
Rear pressure @ 6.9
so pretty close to spec in the front, still high in the rear. The ride is drastically improved. I thought it was good (a bit mushy, but comfy) before.

I did order some new springs, the LC w/ AHC springs. They are slightly stiffer than the LX springs (or so I've read). I do plan to tow a boat occasionally, and like to load up the family for road trips. I want to make sure the rear is supported well. I will post after install in case others are curious about the difference.
I currently get about 7/8 graduations of AHC fluid. This is on a truck with 260,000 miles and original spheres!

Update:
So after installing the LC AHC springs my pressures are at 6.8 front and 6.5 rear. I backed off the torsion bar one rotation from my previous reading to get the front closer to spec of 6.9.
The pressure with the new springs are not a huge difference, but driving the car, especially over speed bumps, is much improved. The rear doesn't sink as low after going over a speed bump as before, and the rear generally feels more planted on the road. One note, when adding replacing rear springs, remember to disconnect the height sensor bracket, or you may be in for some of this.

With a little reading, $90 in AHC fluid, and $250 for new rear springs, I have a comfortable suspension that rides almost like new. One day I will need new globes, but now that I know what AHC is supposed to feel like, I am certainly going to keep it.

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 is 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

I have a relatively new to me (2000 LX 180K) that I just got AHC system working after replacing ECU. I spent far too many hours adjusting TB to level. I never could get them equal. An issue seemed to be my low side would raise as I tightened and at some point got easier and that side dropped. Is this a torsion bar issue? I need to increase TB tension to get my pressures lower but as I increase tension my side to side differences gets extreme. Where should I go from here? Replace TBs? Low side or both? First image is low side where most of thread is used.

 

[MTBNCYN]

I ended up jacking up the vehicle and setting the screws the same. It was almost level! I leveled then was able to add tension to get it in spec. Thanks for all the posts. I had to read a lot to get the entire picture. I have gone from non working AHC to fully functional.