I have a question;
The 'cross-level' seems to be a strange thing for me. Does the AHC, and the sensors, not ensure the front of the vehicle is level? Would the system no simply increase pressure to compensate for less pre-load on the TB's, thus making the measure part redundant?
I have similar front pressures L+R, and the same measured height, yet my TB are tensioned differently.
@Rebus Knebus already has answered the question succinctly.
Here is a long note written for someone else long ago and which may be of use to others following this thread and who are new to AHC ….
The Toyota AHC/TEMS system was designed around 30 years ago in the early 1990’s -- and appeared on LX470 (and LC100 where fitted) from 1998. It is a simple two-channel system – Front and Rear. It is not a four-channel system. It cannot and does not raise or lower each wheel independently. This is why on Techstream or other scanner only Front and Rear AHC pressures can be seen, along with the separate measurement of the Height Control Accumulator pressure.
The Control Valve Assembly contains only two
Levelling Valves – Front and Rear – and so the system is capable only of raising, lowering or self-levelling the Front and Rear, not each wheel. These valves are
‘normally closed’ and are opened by the Suspension Electronic Control Unit (ECU) only when the height of the vehicle is being changed by the driver at the centre console switch, or, the when the ECU causes the AHC system to self-level the vehicle at Front and Rear (not side-to-side) in response to signals from the Height Control Sensors. It is these Levelling Valves which hold back the AHC pressure and prevent the vehicle from sinking when the engine and/or the AHC system is OFF.
When the vehicle is stationary, or, in motion with the Front wheels straight ahead, the two
Gate Valves located in the Control Valve Assembly – one for Front, and one for Rear – are in their
‘normally open’ position. This means that AHC Front Left and AHC Front Right are hydraulically connected and must be at the same pressure in these situations (unless there is a fault or a blockage). Ditto Rear Left and Rear Right.
It is only when the vehicle turns at speed (I don’t remember what speed) that the Gate Valves are closed by the ECU, making the side on the outside of the turn more stiff than the side on the inside of the turn – and in this way providing more roll resistance.
Because of the interconnection, the Front Left and Front Right pressures can never be measured individually or adjusted separately using readings on Techstream or other scanner, nor even with pressure gauges. Again, ditto Rear Left and Rear Right.
There is no
hydraulic connection between the Front and Rear AHC hydraulic circuits. The ECU determines what happens in the Front and Rear parts of the AHC system.
The Front and Rear AHC pressures can be measured only when the vehicle is stationary and when the FSM procedure is followed: temperature sensor disconnected, height change N > LO > N, wait ~30 seconds to ensure that vehicle has settled and Height Control Accumulator has been recharged by the AHC Pump and the AHC Pump has stopped, then record pressure readings.
There is only one Pressure Sensor in the AHC system – mounted next to the AHC Pump. The ECU records each of the three pressures in a sequence – Front, Rear, and Height Control Accumulator – and displays these on Techstream or other scanner. The measurements are not instantaneous in real time as happens with, say, engine oil pressure. The alternative is to read actual AHC pressures at any instant by installing pressure gauges in place of the bleeder screws.
The AHC/TEMS system has no sensory connection with the torsion bars (or rear springs). The Suspension Electronic Control Unit (ECU) has no way of knowing how these are set – or whether the vehicle is leaning side-to-side – or whether the vehicle is sloping incorrectly between Front and Rear. Only the Front and Rear AHC pressures caused by the torsion bar and spring settings can be detected by the ECU, along with the Height Control Sensor signal at each wheel.
The individual Height Control Sensors (two Front, one Rear) each provide a voltage signal to the ECU. The signal is proportional to the height at the relevant wheel position. Techstream (or other scanner) translates this signal into a reading in inches or millimetres because in theory this can be related to the
FSM-specified vehicle height at Front and Rear (as distinct from the IH8MUD hub-to-fender reliable approximation which we all use ).
‘Cross-levelling’ at the Front is the fundamental first step in measuring or adjusting the AHC system.
The checks or adjustment of Front ‘cross-level’ must be done on a level surface (like the concrete floor in a garage or workshop, not in the street), without unequal added weight on either side of the vehicle, with fuel tank(s) full, with no persons or temporary loads on board, with front wheels straight ahead. The FSM specifies ‘engine OFF’ – maybe this does not matter so much because of the Left-Right hydraulic connection – but 'cross levelling' should be done as specified by FSM to avoid confusion and for personal safety and to avoid any unwanted response from the AHC system during torsion bar adjustment.
The purposes of ‘cross-levelling’ are two-fold:
- to avoid – or rectify – any side-to-side ‘lean’ of the vehicle, noting that the AHC system cannot make side-to-side adjustments by itself,
- to equalise the share of vehicle load carried by each torsion bar – noting that for a spring, force is proportional to the depressed distance (deflection) of the spring, so if the heights on both sides of the vehicle are approximately equal, then the loads on the torsion bars will be approximately equal (unless there is a mechanical fault in the non-AHC part of the suspension or running gear, or the body or the chassis is distorted).
Some further notes:
- Unequally loaded torsion bars will result in different vehicle behaviour in Right and Left turns,
- Different to a conventional suspension, on a AHC-equipped vehicle the torsion bar adjusters are not used -- and cannot be used -- to set the ‘ride height’ of the vehicle,
- After Front ' cross-levelling' has been done correctly, Front and Rear ‘ride heights’ and vehicle Front to Rear rake are set using the Height Control Sensor adjusters,
- If ‘cross-levelling’ is not correct at the Front of the vehicle, then it cannot be correct at the Rear of the vehicle (unless there is an issue somewhere in the mechanical parts of the suspension or running gear or in the chassis or the body),
- If ‘cross-levelling’ is not correct at the Front of the vehicle, then the errors here will make it impossible to correctly set or adjust the Height Control Sensors, which in turn will lead to further problems and confusion,
- If the ECU receives widely different signals from the Height Control Sensors, the ECU may place the system in 'fail safe function' in which AHC function and the variable damping function by TEMS are prohibited,
- The number of threads visible at the Right and Left torsion bar adjuster bolts may be different – and they often are – but this is not a reliable indicator of torsion bar loading,
- Equalisation of Front heights as measured with a tape-measure (not Techstream or other scanner) is the required method to equalise torsion bar loadings.
If not seen previously, the attachments may assist understandings of how the Active Height Control (AHC) system and the Toyota Electronic Modulated Suspension (TEMS) are meant to work when all parts are healthy and correctly adjusted. The function of individual components are described with diagrams. The hydraulic circuits are described in words and diagrams in various different situations. Strongly recommended -- definitely worth a re-read any time when AHC and/or TEMS questions arise.
Hope this helps! Congratulations if you got this far – apologies offered if TL:NR ….
