Measure AHC neutral pressure?

[RogueM3]

Looking for some advice. Just got my TechStream hooked up and had the below results:

Front: 7.8
Rear: 7.1

So my initial thought would be to crank the front to try and bring that down in the 7.0 range. However, from the feedback above, seems like that is likely to increase my rear pressure. Given I need a .5 pressure drop in the rear to reach max spec of 6.7 as-is before cranking the front, my assumption from above is that when I do go to crank the front, I will now be outside the realm of correction offered by a packer in the rear.

Am I wrong to assume rear pressure will automatically go up when I crank the front given my above readings? If I do in fact need more than a .5 drop in the rear, is my best option just replacing with new rear AHC springs?

Thanks!

 

[ChuckB]

I'm going through this same process now as well.

Earlier this year I had adjusted the front TBs as described in the below thread. Then before I could concentrate on the rear my laptop with TechStream on it was stolen.

I have noticed recently that the rear ride seems to be very harsh over bumps and anything other than smooth roads. I'm assuming this is because of the front only adjustment I was able to complete.

I'm in the process ordering new OEM AHC rear coils/isolators and a tablet to run TechStream. The only mods I have are SleeSliders.

I'll report back in the below thread once I start the adjustment process. If you haven't read through it yet, the below thread is extremely helpful.

Definitive list of AHC maintenance items

 

[uHu]

Front: 7.8
Rear: 7.1
Am I wrong to assume rear pressure will automatically go up when I crank the front given my above readings? If I do in fact need more than a .5 drop in the rear, is my best option just replacing with new rear AHC springs?
Thanks!

There is no reason for the rear pressure to go up by lowering the front pressure.
Before you do that tho', check the height. 1 inch too high at the rear would give about the pressure you have.

Get the heights and the front press right first, then decide about the coils.

 

[JPS]

There is no reason for the rear pressure to go up by lowering the front pressure.
Before you do that tho', check the height. 1 inch too high at the rear would give about the pressure you have.

Get the heights and the front press right first, then decide about the coils.

Is there a list of the appropriate heights with diff tire size? Are we talking about hub to fender heights? Forgive my ignorance.

 

[uHu]

For the AHC suspension system to work well, the height should always be the same, regardless of tire size.
Heigth is measured from the floor, as a difference between the axle centre (hub centre) and, for the front, the centre of the front bolt of the lower A-arm, and, for the rear, the front lower control arm bolt centre.

 

[JPS]

For the AHC suspension system to work well, the height should always be the same, regardless of tire size.
Heigth is measured from the floor, as a difference between the axle centre (hub centre) and, for the front, the centre of the front bolt of the lower A-arm, and, for the rear, the front lower control arm bolt centre.View attachment 1615233

WOW! Did that ever clear things up. Thank You!

 

[uHu]

Hub centre to fender lip will do the same tho', for ease of measuring. A tad more difficult to get the 1mm accuracy, but good enough.

 

[MuseChaser]

After replacing the AHC fluid two days ago (new-to-us '05 LX470), I got hold of an OBD reader and TechStream software and just read the pressures. Yikes. 10.7 front and 8.2 rear. Vehicle has 135K miles on it, and I doubt anyone's ever done anything with the suspension based on the condition of the fluid I replaced. The up/down stuff works fine, no error codes, and within time frames specified.

This is all very new to me - AHC, Techstream, torsion bars.. never messed with any of that stuff. Mostly, I can swap fluids, do brake work, and have gone a little deeper on old Volvos and Volkswagens (intake manifolds, fuel rails/seals, suspensions), but I don't want to mess anything up. SHOULD I just go ahead and go a few turns clockwise on each of the front torsion bars? Need to lift the car to do so? Any hidden lock nut or anything, or is it just put a 30mm socket on the bold head and go? If/when I get the front in spec, what about the rear?

Going by the diagram @uHu posted a few posts up, my A-B measurement is about 26mm (just over 1"... is it even possible to be that far off?!?) and my C-D is about 63mm (2 1/2"). At least, thankfully, the rear is fairly close. Worried about the front.. going to check again.. just a sec..

OK.. back.. maybe I measured the wrong thing. The pic looked like the "B" measurement was taken at the bolt mounting the bottom of the shock, and that's what I did the first time. Is the "lower suspension arm front bolt center" more towards the center of the vehicle, i.e., the inboard side of the control arm, sort of hidden behind a metal shield? If THAT'S it, then I'm ok and on spec of 3 1/4".

Any and all advice is WARMLY and GRATEFULLY received. @PADDO just because.. and thank you.

 

[johsti]

SHOULD I just go ahead and go a few turns clockwise on each of the front torsion bars? Need to lift the car to do so? Any hidden lock nut or anything, or is it just put a 30mm socket on the bold head and go? If/when I get the front in spec, what about the rear?

No need to lift the car, nor is there a hidden lock nut. When you look underneath you will see the bolt on each side. You will lower your front pressure by about 0.2MPa for each full turn so you may be turning the bolts awhile to get to a 6.7/6.8 front pressure. The most I've done is about 5 or 6 turns and I'm not sure how many turns you can go before having to re-index the torsion bars. Tightening the torsion bars can also raise the rear pressure so you may want to consider new rear springs and/or 30 mm spacers, or both. How did you check the pressure? Did you start in low and raise it to neutral?

 

[MuseChaser]

No need to lift the car, nor is there a hidden lock nut. When you look underneath you will see the bolt on each side. You will lower your front pressure by about 0.2MPa for each full turn so you may be turning the bolts awhile to get to a 6.7/6.8 front pressure. The most I've done is about 5 or 6 turns and I'm not sure how many turns you can go before having to re-index the torsion bars. Tightening the torsion bars can also raise the rear pressure so you may want to consider new rear springs and/or 30 mm spacers, or both. How did you check the pressure? Did you start in low and raise it to neutral?

First, thank you for the help! As far as starting in low then raising to neutral.. no. This was my first time w/ OBDII/Techstream/TIS, and I couldn't figure out how to get it display pressures for a while... it found my car, then I clicked on AHC, then "data display," or something like taht I think. Bunch of stuff came up, but the front and rear pressures displayed as 0. Clicked on "Active Test," then accumulator solenoid... activating that turned it off, and I couldn't turn it back on w/ the computer OR with the button in the car. Sooo.. clicked out of that window, unplugged the car, turned off the car, waited a few minutes, started the car, ran through the various height settings via buttons in the car console successfully, then plugged the OBDII sensor back in, hit "retry," and the values displayed. Are you suggesting that the values were off because I didn't start in low, then raise to neutral before reading? Man, that would be great.. gonna go try that. Sigh... sometimes a little knowledge in the wrong hands (mine) is worse than no knowledge.. i'll report back shortly, and thanks again.

 

[PADDO]

I’ll wager you’ve somehow caught a reading going to H. You only need to raise from L to N and I’ll bet your front pressure will be around 7.5MPa (its about 3.3MPa uplift from N to H).
Measure each corner from hub centers to fender bottoms, this will show your front and rear heights and any front side to side lean which you’ll need to dial out by individual adjustments on your torsion bars. If, say, you’ve got a 1/2in lean or more you’d lower (ccw turns) the higher side by 2 turns and raise (cw) turns the low side 2 turns. You split the difference. Rinse and repeat until you’re getting <1/4in difference in front height measurements. Once the front is cross level you then do any height adjustments with the sensors if necessary and the last step is to correct your front neutral pressure by equal cw (tightening) turns (usually the case) on both bolts.The rear has no adjuster for the coils so to get the rear pressure in the design range of 5.6-6.7MPa (around 6 is desirable) you’ll need to fit new coils and probably coil spacers. Don’t get hung up on reports of the rear pressure raising when the front pressure is lowered. Spurious results and it’s simply not borne out when you fit mechanical pressure gauges to the system to read actual pressures instead of the numbers Techstream throws out.

 

[MuseChaser]

Thank you both. I just (finally) got the readings after going from low to neutral and letting it stabilize. Here they are..

Front Right Height sensor 0.4"
Front Left Height sensor -0.3"
Right Rear Height sensor 0.0"
Front Pressure 8.2
Rear Pressure 6.9
Accumulator pressure 10.3

I was sitting in the driver's seat at the time, the rest of the vehicle was empty.. not sure if that would impact the FR and FL sensor readings.

edit.. just read some stuff from the FSM you may have posted for us all in another thread.. it says to take all reading w/out any passengers in the car, which I take it to mean w/out a driver, too. Could me being in the car be causing what are only slightly high readings now?

Physical measurements, center of hub to fender bottoms, are very close.. 19 5/8" drivers, 19 3/4" passenger. Not sure how to do height adustments on the sensors.. is that for bringing the Techstream data so taht the FR and FL readings are the same? Where/what is the physical adjustment/procedure? About how many turns should I expect to make on the torsion bars afterwards to get the 8.2 down in range?

I'll keep researching in the meantime. Thank you both SO much!

 

[PADDO]

Your +0.4 and -0.3 sensor readings offset each other and you get a net processed error signal that determines where the vehicle actually settles at N. To be honest, particularly at this stage of your AHC knowledge journey, I’d recommend not touching your height sensors today, you can tweak them in to get less of a spread later but as your actual front height is good and you don’t have a lean worth worrying about I’d just turn both torsion bars 6 cw (tightening) turns and go for a drive then recheck pressures and adjust if necessary. Tuning the system can be somewhat of an iterative process, particularly as you’re learning how it responds to variables.

The FSM states to do all your readings and adjustments on flat level ground, full fuel tank, no extra weight and no one in the vehicle. Certainly being on level ground is important but the differences you get between a full tank, 1/2 tank etc aren’t that great and it all comes out in the wash. If you can achieve front pressures in the 6.7-6.9Mpa range, rear around 6MPa at your chosen N heights and at your normal driving configuration (stock heights are ~ 19.5in F and 20.5in R hub center to fender bottom) and you have <0.3in spread on your front sensors then you’re in pretty good shape.

 

[MuseChaser]

Great, Paddo... thanks so much! I'll address the torsion bars soon and report back. Can't tell you how much I appreciate this!

 

[lcdude]

For the AHC suspension system to work well, the height should always be the same, regardless of tire size.
Heigth is measured from the floor, as a difference between the axle centre (hub centre) and, for the front, the centre of the front bolt of the lower A-arm, and, for the rear, the front lower control arm bolt centre.View attachment 1615233

I am puzzled with my hight readings. The rear is within the norm, but the front is severely off (A-B=14.5-13.5 = 1 inch). How can I be over 2 inches off? If I adjust TS, my front may be higher then rear. Any suggestions? I am positive I am measuring B from the right bolt (see pic below).

 

[PADDO]

I am puzzled with my hight readings. The rear is within the norm, but the front is severely off (A-B=14.5-13.5 = 1 inch). How can I be over 2 inches off? If I adjust TS, my front may be higher then rear. Any suggestions? I am positive I am measuring B from the right bolt (see pic below).

View attachment 1878723

Wrong bolt, use the center of the A arm bolt and not the shock mount bolt. You can crawl around and try and get accurate and reproducable readings down to less than 1mm - how this is reliably achieved with a rule off a grease cap center and a bolt head center, I don’t know. But I’ve tried. For all intents and purposes it easier and just as relevant to take readings from the grease cap center to the fender bottom where 500mm ~ stock front height and 520mm for the rear. After all, you adjust TB tension to optimize pressure at your actual height, be it stock or sensor lifted.

 

[Beto Cruiser]

Speaking from experience.

I was going to purchase 2 new rear lx470 springs my rear pressures was 7.4, but then I started reading about ahc sensor lifts. I saw people talking about using king springs and thought I'd give them a try.

I moved the 3 height sensors to the top of the slider and achieved a 3/4" lift and installed the king springs at the same time. Adjusted the front torsion bars 5 turns to lower the front pressure in the front, with the sensor adjustment and new king springs my rear pressure was now 5.4.

I then shortened the sensor link arms 1/2 each and made a little metal bracket to adjust the rear sensor arm higher. This resulted in a 1.5" lift. Turned the front bars another 4 turns

My front pressure is now 6.8 and rear went from 5.4 to 6.2. The truck now rides just as good as before while having lower rear pressure and a very modest 1.5" lift.

Revival. Thanks for the info. One question, when you say you shortened the sensor link arms 1/2 each, do you mean you cut them like Hoser (AHC Sensor Adjustment for Lift) and made a bracket for rear like DirtDawg
(2-2.5" AHC Lift Using King Coils & Shock Spacers)? And if you did cut them, did you cut 1/2 of each end of the rod?