I have had 2 friends complain about harsh ride and never checked there tire air pressure. Both had pressures in the 40's set by there dealer. Problem solved by checking and adjusting.
One was going to spend several $k for a new suspension
I threw it out there just in case you were one of the few who do not check.

 

No way mine is tire pressure, I can see on the axle housings where the bump stop has been rubbing.


Im not sure I even have KDSS, how do I tell?

 

mann777, do you know if this part is expensive to fix?

 

I had a gx that had a crazy bouncy harsh ride. Problem turned out to be height control sensors. Minor bumps had this thing bouncing like crazy. 1 or more of the sensors were out of range causing it. Hope this helps or eliminates a potential problem.

 

I had my sensors replaced already. I know what that's like. But this is different. The height is ok. It just bottoms out over bumps and is stiff.

 

Chris, s mentioned the part is cheap, but the manhour involved would be long.

 

Interesting, I have noticed the same thing on my GX. Also, it seems that most people complaining about a harsh ride noticed similar symptoms: when empty (no cargo, and low gas) it bottoms out - tested going over speed bumps - front passes no problem, back bottoms out on the rubber stops (hard); when loaded (some cargo, full of gas) it does not bottom out, or very lightly - again tested going over speed bumps (local Shop Rite :-) ). Measured riding height - same when loaded and empty - so the leveling sensors are working right. Took it to the dealer, explained in detail, they listened (rrright...), kept the car for a day, said that they ran computer diagnostics and everything is fine. I have repeated the tests again after I got the car from the dealer with the same results. Now, if the results are repeatable, and the car behaves as described, why can't the dealer figure out what's wrong, or is this a design defect ?

 

I am having the exact same problem. If there are no springs on the rear suspension, then the only thing that supports the axle housing is the air bags. So the only reason it could be bottoming out is lack of pressure in the air bags. The question is why. So we fall back to the sensors. So maybe there is a worn spot on the sensor when the car is at normal height and no rear load, and the sensor tells the air bag to deflate a little, thus causing less pressure in the bags and therefore bottoming out over bumps. Now when you load the car, the sensor gets moved to another spot on the electronics, and this spot is not worn, so the adjustment is made to accommodate the load and the pressure is increased stopping the bottoming out.

Now the problem with this theory is that Chris has already replaced his sensors, unless they are bad too.

I've had some luck with putting the suspension in high mode as I back out of the garage, when it goes back to normal it seems to be somewhat better about not bottoming out.

 

Well, problem with your theory is that, when you release the air pressure from the air bag, it will lower the vehicle. Remember you have over 2000lbs pressing on the rear axles. If you have the vehicle loaded, you need to increase the pressure in order to raise the vehicle to preset level since it lowered due to higher load. In theory, the air bags should act the same with less pressure/light load and higher pressure/heavier load. When going over bumps lighter load will exert less force than heavier load (f=ma), so in theory you need less force to decelerate it.

One thing I'm trying to understand though. I've read some other suspension posts and some of them refer to fluid pressure. This would mean that GX does not have pneumatic only system, but some sort of hybrid hydraulic/pneumatic? How does that work? Maybe there lies a problem. Once air can not handle the force the fluid takes over (like at the end of the compression/deceleration cycle - the bump) - that would mean that the fluid does not have enough pressure, hence when you have more air in the bags it handles bumps better (it will decelerate quicker)? I just don't know, but I still think that the dealer should have been able to handle this better, and address the problem AND FIX IT.

 

The hydraulics are in the shocks. Thus the ride settings switch allows for the firmness or loosness of the shocks.

 

Therotically there is no relationship of the Airbag theory in GX with the rear suspension Compression. The design is totally different. They have pneumatic for some and Hydraulic for some. It is the Compressor which triggers to function via ECT. Could you look at the Exhaust Height Control Solenoid Valve, Sometimes due to moisture intrusion, these sensor go bad, effecting the height Control Sensors. All you need to replaced the solenoid valve. These were a problematic ones on the 2003/2004 model GX. It is also called as Compressor Height Control Assy and the Part # 48910-60020. This once replaced will only be reset thru a Diagnostic Kit which is available at the dealer. As lot of interface is done to the ECM to understand the part number installed and clear the previous codes installed. I am bit surprised if the dealer say he does not know, try to change you dealer from your area to another one. Only one important thing the dealer should remember that there is Code DTC C1735 is not removed and it should be retained in the Suspension Control ECU. These store Height Control Values.

 

do you guys notice that the duration of the motion to full height via the height switch in the cab takes much longer? It is much faster on the down stroke. how many seconds should the upstroke take?

 

The techs hang out in the maintenance forum

 

Have you guys noticed a difference with weather change. It warmed up to 40 one night here in chicago and it seemed to go away. This proving my theory with fluid dynamics. I think once the hydraulic fluid thins out it flows easier through some orifice and does its job. So I definitely think it is the control assembly like Mann777 said. Now if I can only get the dealer to take care of this. I am going back tomorrow. We will see what happens. Anyone else have an update on this?

 

I have not had my drive shaft replaced yet. The clunking seems to have gone away for now. I think it is the cold weather.
As far as the bottoming out. I'm in Georgia, the warmer temps don't seem to effect the rear suspension.
I am out of warranty, so if you get the dealer to repair yours, please get some details on what they did so I can attempt to do the same thing.
Thanks