I initially put this in the Definitive list of AHC maintenance items thread, but at almost 800 posts long and growing, it's going to get buried and the thread itself is very unwieldy.
In an attempt to make as focused a summary as possible, I present "The ABCs of AHC." It's intended as a comprehensive, linear checklist for anyone who needs to get their AHC back into factory tune. Information gathered from as many sources as I could find on this forum; I take no credit for developing any of this, just bringing it all together as much as possible. Special shout outs to @PADDO, @uHu, @IndroCruise, @PabloCruise and @2001LC for many of the details, specs, and procedures I've included in here. Feedback more than welcome, will make edits as appropriate based on what I hear.
A - Assess: Figure out where you’re starting from
B - Bleed: Flush out the old AHC fluid
C – Crank/Check: Adjust the side-to-side height and neutral pressures front/rear
Good luck! Hope folks find this a helpful all-in-one reference for tackling their AHC. Maybe it will prevent a few of you from ripping out a great system. Save the AHCs!
In an attempt to make as focused a summary as possible, I present "The ABCs of AHC." It's intended as a comprehensive, linear checklist for anyone who needs to get their AHC back into factory tune. Information gathered from as many sources as I could find on this forum; I take no credit for developing any of this, just bringing it all together as much as possible. Special shout outs to @PADDO, @uHu, @IndroCruise, @PabloCruise and @2001LC for many of the details, specs, and procedures I've included in here. Feedback more than welcome, will make edits as appropriate based on what I hear.
A - Assess: Figure out where you’re starting from
- Wheels on level ground, no ramps, jacks, or stands needed for parts A, B, or C. DO NOT PUT YOURSELF PHYSICALLY UNDER THE VEHICLE FOR ANY OF THIS.
- Get your rig loaded/unloaded to what is “typical” for your use.
- FSM calls for no driver or passengers, full tank of fuel.
- Measure ride height at all 4 corners from center of wheel cap vertically to fender lip on a level surface
- Front OEM: 19.75” (50.2cm)
- Rear OEM: 20.5” (52.1cm)
- Drive a bit, then park in the same level place. Take 3-5 measurements and get a mean or median.
- Measure difference in AHC tank level between H and L suspension heights
- >7 graduations acceptable, 14 graduations new from factory
- Need to replace globes/spheres if <7 after parts A, B, and C are complete.
- Backlight AHC fluid tank with a light to help see fluid level
- Again, 3-5 measurements and take a mean or median value
- Measure your front/rear pressures, accumulator pressure, and sensor heights using Techstreamand a Mini-VCI cable, a manual inline gauge, or an advanced OBDII reader (iCarsoft, TYT II, OBDLink, VGate, etc.)
- Front spec: 6.9MPa +/- 0.5
- Rear spec: 5.6-6.7MPa
- Accumulator spec: 10-10.6MPa
- Start car, connect Techstream, make sure you choose your exact model, not the default Land Cruiser with no AHC installed
- Open AHC channel and select "data list" to monitor values
- Cycle from N to L to N again. Take a screenshot of your values. There are Front R and Front L height sensors but only one rear sensor for pressure and height. May need to disconnect the AHC temp sensor to get accurate rear readings per FSM.
- Again, 3-5 measurements and take a mean or median value for each data point.
B - Bleed: Flush out the old AHC fluid
- Procure three 1L bottles of OEM AHC fluid (08886-81221)
- Put AHC in Low, turn engine off
- Suck all old fluid out of AHC tank (~1L if level was between Max/Min at N height)
- Pour in new fluid to top of tank (~2.5L)
- Bleed accumulator until fluid stops (long cylinder on left side frame rail) (~300mL, 10mm wrench)
- Bleeder torque spec: 62 in-lbf (5.5 ft-lbf, 7.5 N-m)
- Bleed one front and one rear damper (globe unit) until both front and rear are on the bump stops and fluid stops flowing (~300-400mL each)
- Bleeder torque spec: 73 in-lbf (6.0 ft-lbf, 8.1 N-m)
- Careful, as the car will lower significantly during this process. Don't put yourself under the vehicle!
- Start car, put AHC in Neutral until pump stops, turn off again
- Bleed the remaining front and rear dampers on the other side of the car for just a second until fresh fluid and no air comes out (~50mL each)
- Start car, cycle to AHC to L then back to N
- Top off AHC fluid in the reservoir. Should be between Max/Min at N height.
- Recheck graduations in the tank as described in step A2. May see some improvement, may not.
C – Crank/Check: Adjust the side-to-side height and neutral pressures front/rear
- Remeasure your ride heights as in Step A1. Measure twice, cut once.
- Fix any left-to-right ride height imbalance, i.e. lean. This is commonly called “cross-leveling” throughout this thread. This is done entirely by adjusting the front torsion bars (TBs), not height sensors.
- Tighten (CW) a TB nut to raise a low side, loosen (CCW) to lower a high side. 30mm wrench/socket
- Mark your TB nuts (permanent or paint marker) so that you don’t lose track of where a full turn is
- Place AHC in H to lessen pressure on TBs and make turning easier. Put back to N for measurements.
- ~1/8” correction in left-to-right height imbalance per full turn of a TB nut
- Best to turn TBs equal amounts in opposite directions until level
- Drive a bit, repeat, measure until satisfied with left and right ride heights being equal
- Adjust the three ride height sensors as needed to achieve as close to 0mm on all values at N in Techstream.
- Engine off, ignition on. Monitor changes in real time on Techstream.
- Raise: Shorten heim joint (the adjustable threaded rod) or move the top of the joint up in its slider
- Lower: Lengthen heim joint or move the top of the joint down in its slider
- Rear sensor only has a slider
- <10mm (0.3-0.4in) between the two most discrepant sensors is acceptable
- Recheck ride height as in step A2. If all sensors at/near 0mm, ride height should be very close to OEM spec, otherwise sensor(s) may be malfunctioning.
- Adjust front N height pressures by cranking the TBs
- Place AHC in H to lessen pressure on TBs and make turning easier
- Lower pressures: tighten (CW) both TBs the same amount
- Raise pressures: loosen (CCW) both TBs the same amount
- ~0.2MPa change in pressure per full turn of a TB nut
- Cycle H to L to N. Recheck pressures as in step A5.
- Check rear N height pressure
- OEM specs in step A5
- May need to disconnect the AHC temp sensor to get accurate rear readings per FSM (Decreases pressures ~0.8-1). Try it both ways.
- Rear pressures can be changed with new springs (OEM or King), spacers, airbags, or rear sensor height adjustments (but this last one also change the height of your rear and the car’s rake front to back).
- Check graduations in the tank as described in step A2. Again, may see some improvement, may not.
Good luck! Hope folks find this a helpful all-in-one reference for tackling their AHC. Maybe it will prevent a few of you from ripping out a great system. Save the AHCs!
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