The Lexus LX470 Automatic Height Control System (AHC) incorporates oil filled ‘shock absorbers’ on each corner. The AHC system adds (or releases) oil from the shock to adjust the height. When you drive over a bump, the oil must go somewhere if the shock is to compress and ‘absorb’ the bump. The place it goes are accumulators—one for each shock.
The Lexus and Toyota accumulators are factory pre-charged with nitrogen to several hundred PSI. The gas is on the top side of an internal diaphragm separating the gas from the AHC oil. The gas slowly leaks through the diaphragm just like air leaks out of a tire, only much slower.
Eventually (after 4 to 6 years, or 50K to 100K miles) enough gas has leaked that the accumulator becomes mostly oil filled-just like the shock. Then, when driving over a bump, the oil has no place to go and the shock cannot compress and the vehicle gets tossed up and rides real bouncy.
There are two methods of testing the accumulators. Lexus advises that when you change the height setting from ‘low’ to ‘high’, the oil level in the AHC reservoir should lower by more than 7 graduations on the reservoir (new accumulators will change level by more than 12 graduations.) Since higher pressure is needed to raise the vehicle, some of the oil goes into the shocks to raise the level, the rest into the accumulators, compressing the remaining gas.
The other method is to set the vehicle at normal ride height and then bounce real hard on each corner to see how much the shock will compress. With flat accumulators, there will not be much ‘give.’ New suspension accumulators will allow several inches of movement when you bounce hard on the corner. Another way of accomplishing this is to drive each wheel over a speed bump at about 5 mph and note the ride quality. Good accumulators will have a nearly smooth, well damped movement. Flat accumulators will produce a sharp and bouncy movement and bouncy ride.
Why can’t I just disconnect or deactivate the AHC system?
The AHC system is integrated with the Stability Control System to reduce the body roll and sway. Deactivating the system will compromise the vehicle’s roll-over resistance. Also, the oil pressure in the shocks supports a significant part of the vehicle weight and the vehicle height will drop to the minimum height stops and ride extremely harsh.
What damage may result if I continue to drive with flat accumulators?
Driving with flat accumulators will cause very high transient pressures in the shock absorbers when you go over bumps. This is, of course, very hard on all the suspension components, including the shock absorber mounting bushings.
The system has a safety valve which releases this dangerously high pressure back to the AHC oil reservoir. However, this is not designed for continuous activation. When the high pressure oil is released from the shocks, the vehicle will ride lower than it was. The AHC system will then add oil back to restore the height, requiring the electric oil pump to run.
The AHC computer system has an automatic shut-off to prevent damaging the pump by frequent or continuous running since that would indicate a leak or other damage. Driving on bumpy roads will cause the pump shut-off to activate and the AHC system to switch to default mode. Now when you drive over another bump, oil is released by the overpressure valve, but no oil is added back—and the vehicle level drops lower and lower on each bump.
It will reach a point that when the wheel is in the air after hitting a bump, the spring pushing the wheel back down will cause negative pressure in the oil supply line from the pump. This will suck air into the hydraulic pump past the motor shaft seal. The Lexus factory test procedure will show this condition as a ‘faulty pump’ requiring replacement (at a cost of more than $2500).
The accumulators are mounted between the front and rear wheels on the outside of the frame rails, under the running boards.
The accumulators simply screw on and off the control valves mounted on the frame rail. They can be removed easily with either a ‘chain wrench’ or a 36mm open end wrench (spanner) that is no more than 9mm (3/8”) thick.