Here are my findings in regards to the AHC riding harshly over small bumps: At least 3 of my 4 AHC dampeners failed and one was marginal.
Cause: excessive heat buildup due to fluid not changed in 91k miles and large height accumulator seizing up and vehicle operated in hot southern climate.
When the large accumulator works it serves to raise and lower the car when parking, as a side benefit circulates the oil out of the shocks and mixes with fresher fluid stored in the large height accumulator. My LX lived in the heat of the Texas, Louisiana and Georgia which I suspect contributed to the overheating. As far as i can tell the initial starting position for step one is not correct. This in effect keeps the small hole that the fluid passes thru closed too much at all the 16 settings. This is most likely an electrical issue since the stepper motor goes thru an initialization when the engine is started. I observed this several times with one old dampener plugged into the LX and dangling loose to observe the steps. The total movement from step 1 to 16 is approximately 1.5 mm. So a step is about 1/10 of a mm! How did I find this? Simple, replaced each part until the ride started improving. My ride was so bad that I would not keep the vehicle. If I had know they could fail, I would have replaced with a standard suspension- it is still hard to believe but I have the 4 old dampeners and took one apart - very expensive paperweight. They look indestructible and even with my dirty fluid had only a small sign of over heating. The most difficult design component to understand is that the total dampening is a combination of the AHC accumulators(gas globes) and the very small opening that the 16 step stepper motor opens and closes in the electronic dampeners. I had weak globes to start with(9.5 graduations) and out of spec dampers. When I replaced the globes the resistance increased enough to cause a harsh ride - feel all the small road bumps. I have read other posts where folks had the exact same experience with no resolution identified. Now that I have replace all 4 dampeners the ride is much smoother and I expect the ride to become ever smoother as the globes gas out and the graduations are no longer sitting at 14. I replaced in order - AHC fluid reservoir, front AHC shocks, 4 gas globes, rear upper/lower control arm, panhard bar, rear AHC shocks, rear dampeners, front dampeners. Each time I replaced a part, I drove over the same very rough road and noticed improvement in ride quality. I replaced the dampeners one at a time and noticed an increase in that floating feeling returning with each one. Now my LX rides fine, I will purge the gas out every 10k miles, watch the color of the fluid and replace at least every 30k. When it fails again goodbye. Shame on Lexus for not changing the AHC oil.
. I believe in keep it simple stupid approach, just hard to find it in today's vehicles. Looks like your going to be a great resource for others with AHC.
