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

[jaewastooshort]

That’s why I’m wondering whether, instead of chasing the exact factory ride heights, it might be better to start by lowering the rear a bit first (which in theory should reduce the pressures). Those were roughly the ride heights my AHC had before the globe replacement, and everything actually worked pretty well then. Only if that doesn’t improve the comfort, it seems to me that it would make sense to try setting the factory ride heights and continue troubleshooting from there.

Unless I’m misunderstanding how the AHC system works, and raising the vehicle through adjustment could somehow reduce the pressures.

The apparent stiffness of the system is likely due to the high hydraulic pressure, as the AHC system is working too hard to try to control the load. By increasing the preload on the torsion bars, you allow the hydraulic system to do less work to keep the vehicle at the correct height (as indicated by the sensors), as the spring is doing more of the work. Lower hydraulic pressure will allow easier motion of the system, which should give a more compliant ride over bumps.

If you're out of adjustment on the torsion bar preload bolts, you may be able to re-index the torsion bars (another topic that is covered in various threads ad nauseum).

 

[chriscosta416]

What if your accumulator spec is lower? Cycling from N to L and then L to N gives me good pressures in my front and rear pressure sensors but my accumulator pressure is reading 7.5Mpa-g.

I did not unplug the temp sensor for the AHC pump.

 

[IndroCruise]

After raising the vehicle Front and Rear, the last step in the raise sequence (LO to N, or, N to HI) directed by the Suspension Electronic Control Unit (ECU) is to cause the AHC Pump to recharge the Height Control Accumulator (long cylinder located midway along LHS chassis rail).

Suggest make sure that the AHC Pump has stopped after completing the raise -- allow at least 30 seconds after the green AHC light in the dashboard has stopped blinking -- then take readings.

The only function of the Height Control Accumulator is to expedite the raise of the vehicle. The Factory Service Manual (FSM) shows the following table for a new vehicle at stock weight in the unloaded condition. If concerns continue, suggest check raise times:

Note that raise times may be longer if

• the Solenoid Valve at the front of the Height Control Accumulator is not functioning -- and the vehicle relies only on the AHC Pump during a raise, or,
• the vehicle is heavier than stock -- and takes longer to raise, or,
• there is air in the AHC system -- and the AHC Pump spends time compressing air, or,
• there are restrictions in the hydraulic circuit between the AHC Pump and the Height Control Accumulator -- such as due to sludge building up in the strainers inside the AHC Pump sub-assembly or in the separate strainer inside the Return Valve.

Check by feel/listening for

• Solenoid Valve operation at Front of Height Control Accumulator,
• Levelling Valve operation in Control Valve Assembly,
• and/or use FSM test procedure:

C1731 / 31 to C1736 / 36 | Repair Manual | DIAGNOSTICS | ACTIVE HEI... - https://lc100e.github.io/repair-manual/diagnostics/active-height-control-suspension-skyhook-tems/c1731-31-to-c1736-36/

Further information:

Hydraulic circuit during vehicle raise:

Hydraulic circuit during recharge of Height Control Accumulator: