Lexus LX 470 suspension system

The Bouncy, hard ride syndrome

In their efforts to create the best-performing full-size SUV in the industry, Lexus engineers developed sophisticated suspension feature on the LX-470 for added convenience and performance on and off-road. With its rigid chassis, independent front suspension, Adaptive Variable Suspension (AVS) and Adjustable Height Control suspension (AHC), the LX 470 delivers far superior ride characteristics among large SUVs

Adjustable Height Control suspension (AHC) allows the driver to adjust the vehicle ride height to three levels - high, normal and low. The high mode provides improved drivability over rough terrain, while the low mode affords improved ease of in and out access. The height control switch and indicator light work to help the driver know at what height the vehicle is as well as to indicate the optimal height control depending on driving conditions. If set in the high mode, the AHC will automatically lower the LX to the normal mode when the vehicle exceeds 19 mph. When parked in low mode, the suspension will automatically raise to normal height when pulling away from a stop. The Adaptive Variable Suspension (AVS), a semi-active shock absorber system, can react as fast as 2.5 milliseconds (about every 5 inches of road travel at 60 MPH), continuously changing shock valving individually at each wheel. The computer system selects from a range of 16 different settings -- one being softest and 16 being firmest - in four range groups depending on feedback from various road sensors. The AVS system senses road bumps and continuously adjusts shocks to a stiffer or softer value as needed. The driver also is provided an adjustment switch for personal preference.

The Component Causing the Problem: A critical component of this system are the hydro-pneumatic accumulators mounted to each of the 4 valves controlling the shock absorber stiffness. The role of the accumulator is to absorb (take in) the oil forced out of the completely filled shock absorber when you drive over a bump, AND to supply high pressure oil to force the wheel down when you drive over a hole. The hydraulic pump role is supplying the overall amount of oil needed to the accumulators to achieve the correct vehicle height.

What Happens?

However, there is a thorn in the rose bush. All hydro-pneumatic accumulators (simply accumulators hereafter) slowly loose the gas precharge. The vehicle then rides extremely bouncy and hard. This is because there is no space inside the accumulator for anymore oil--all the gas is gone and the accumulator is flat. The LX-470 accumulators are bladder type accumulators, similar in construction to those of Citroen, Mercedes Benz, Rolls-Royce, etc. They are initially pressurized (precharged) on the gas side of the bladder with dry nitrogen to several hundred PSI. Unfortunately, the LX-470 accumulators are very costly due to the elaborate and non-standard threads and sealing groove they use to attach to the valve assemblies.

When installed on the vehicle, the oil-side pressure is whatever is needed to support the vehicle at the ride-height required. Lexus (and most other car manufacturers) use conventional springs to support approximately 60 percent of the empty vehicle weight, and hydraulic pressure the balance.

As one switches from Lo to Hi height setting, the hydraulic pump has to supply additional oil to extend the shock absorbers, and because the springs are also extended, their contribution to load support drops, requiring higher hydraulic pressure in the shocks. Obviously, adding payload causes the hydraulic pressures to rise even more. Since the accumulators are directly connected to the shocks, the pressure in them rises also, compressing the nitrogen into smaller and smaller space. When fully loaded and at maximum ride height, the pressure in the accumulator is several times higher than when empty and at low setting. When absorbing a bump, the pressure goes even higher- much higher, or much, much higher if the nitrogen pre-charge is low or gone. You feel this much, much higher pressure as a hard, bouncy ride because the shock absorber cannot compress any because there is no space left in the flat accumulator to receive the oil.

The system is sort-of protected from over-pressure damage by pressure relief valves that allow the excess pressure (oil) to flow back to the pump reservoir. You hit a bump, shock pressure sky-rockets, causing oil relief back to the reservoir--but now what? There is less oil in the shock and the ride height is too low-so the pump --which is designed for infrequent and short duation operation only--turns on and supplies the oil needed to restore the correct height--but -here comes another bump etc. etc.etc. After a few minutes of this losing battle, the pump shuts off (computers do have some good safety features) because it obviously is not going to fix the ride height problem. Now you soon bounce down to the bottom--riding on the bumper stops--an unhappy low-rider.

Unfortunately, this condition can cause air to be sucked into the small hydraulic pump, past the shaft seal which is intended to keep oil IN, not air OUT. Remember the flat accumulators do not have any gas to expand and force oil into the shock as the springs push the wheel down while you are flying through the air, with the wheel off the road surface. The spring extending the shock causes a vacuum in the oil supply system--all the way back to --the pump seal.

Apparently, the Lexus factory diagnostic and repair procedure does not detect and correct this 'air-locked' condition. Many perfectly good, but air filled, LX-470 hydraulic pumps are being replaced with new (precharged, oil filled)pump assemblies--for several thousand dollars.

AHC PUMP AVAILABLE SEPARATELY $325 +/_ !!!!!!!

Thanks to a customer who had the 'pump won't put out pressure' symptoms (usually dealer replaces entire assembly for some $2400 in parts alone) found Lexus/Toyota do sell just the pump itself (and O-rings needed)

Lexus part numbers: pump sub-assembly #48901-60010; o-ring #90301-0612; o-ring # " -7003; AHC Lexus/Toyota suspension fluid #0886-01805.