Air Suspension Discussion

If you want to go this route (and I strongly recommend it), I would recommend finding the parts off a '91-96 Buick Roadmaster (sedan or wagon), or similar year Cadillac Fleetwood (what I have). These RWD cars have the "Auto Level Control" installed on GM's long-lived 4-link live axle rear suspension:

http://parts.nalleygmc.com/images/parts/gm/fullsize/960731MB07-003.JPG

For the auto-level system, there is a ride height sensor and controller (#6) mounted directly over the top of the differential pumpkin. The sensor has a small adjustable link (#15) connecting the axle to a swinging arm coming out the side of the sensor's enclosure (#6). Inside the enclosure is a circuit board that is permanently encased in potting.

The controller has several functions:

35-40 second delay after the ignition key is turned on to when the compressor is allowed to run (so that it doesn't run until the engine is running)
4 second compressor purge after initial startup delay to ensure that the air shocks always have a minimum of 8-14 psi (this can be modified)
Opens the exhaust valve for 1.5 seconds when the compressor is started, to relieve pressure on the compressor and lower its start-up demand
8-15 second delay between sensing an out-of-spec ride height and commanding up (compressor) or down (exhaust) to correct ride height to spec (+/-1")
Compressor and exhaust (solenoid valve) circuits are limited to 6 minutes of continuous on time. After the 6 minute maximum is exceeded, the system disables these functions until reset by cycling the ignition key
Short circuit protection: after detecting an overcurrent condition, the controller will retry the function/circuit 32 times with a 10-20 second timeout between tries. After 32 retries, that particular circuit will be disabled until an ignition key cycle resets the system
System remains active for 6 minutes after the ignition key is switched off (so that suspension can still adjust if weight is added or removed shortly after the vehicle is stopped and shut off)


The sensor harness has fused battery power, ground, and "chime" (ignition key on) wires coming in, plus adjust up (compressor) and adjust down (exhaust) leads heading back out. The compressor harness has three fused 12V inputs (exhaust solenoid, relay control circuit, and relay load [compressor motor]), ground, and the adjust up/adjust down leads coming from the controller. The compressor harness is all powered through a single fuse, so you can easily disable the whole system by simply pulling its fuse. A 1/16" diameter plastic air line leads back to the shocks from the compressor.

You can see in the parts diagram how the compressor assembly (compressor, compressor relay, exhaust solenoid, air filter and dryer) simply mount to a frame rail on a bracket, and similarly, the sensor/controller simply mounts to the chassis on a bracket. After unbolting these two brackets from a donor car, it'd be pretty simple to mount them on another vehicle. The sensor link mounts through a small hole in one of the control arms, but you could weld a small tab or bolt a small bracket on just about any rear axle to make this work on another vehicle.



My Cadillac has this exact system. It's really very simple but works very well. A previous owner installed tow package rear springs on my car, so the rear suspension always rides firm, but never harsh. I've thrown 800lbs of softener salt into my trunk with no visible sag; the gas station attendant asked me if I was sure my car was going to be all right, but didn't say anything after the compressor ran for a while and I drove away straight and level. I do a fair amount of towing with my car (up to 3000lbs), and the auto level control works great for that. The one drawback is that sometimes it's difficult to get a feel for tongue weight while loading stuff onto the trailer, because the car compensates. I really should pull the fuse for the level control while I'm loading up the trailer (then replace it before driving away), but I never do.






On air shocks, however, I don't have much good to say. My first car ('74 Camaro) had wrinkles in its body sheetmetal from some previous owner running air shocks to "jack up" the car's rear end. Even in my current Cadillac, I hear some unusual creaks in the back when towing a trailer with a lot of tongue weight and the trunk full. This is one of the problems with air shocks: they force the chassis to carry a lot of load through the shock mounts, which are almost never designed for the amount of load an air shock can put on them.

Another problem with air shocks is that their damping is almost never the top priority in their design and construction. Because of this, most owners of the above-mentioned Buick Roadmasters and Cadillac Fleetwoods ditch the factory air shocks and run Airlift 1000 air bags that install inside the rear coil springs. This works much better for load carrying anyway, and frees up the option to use shocks with much better damping (Bilsteins are the norm on these cars).

I came across this on the inter nets and was impressed by the description of operation of the ride height control.

Good troubleshooting info. Thanks for posting it.