I have searched over several forums and other web pages on the air suspension problem but still am not able to solve my problem, hope someone will chime in and help me out.

I have a 1997 Expedition 5.4 V8 with 330,000km on it now as I got it new back in 97. Over the years the compressor has failed 3 times and were replaced.

Nearing the end of the winter season, the rear end dropped and compressor wouldn't start when put in drive. Then 1 time I started the truck and the compressor started and pumped the truck back to regular height. Over time the rear dropped again and the compressor never kicked in. The front suspension remained inflated.

I bought a new compressor from Arnott Industries last week and replaced it after checking that the fuses are ok. But the new compressor still didn't engage and the rear didn't get pumped up. So I checked the old compressor and the new compressor and they both work when power is applied to it directly. (now I gotta sell the new compressor since I cannot return it).

I then checked the compressor relay and by unplugging the wiring harness from the relay and one thick wire is always HOT (12v) and from reading the forums, the other thick wire sends power to the compressor. I then "jumped" the 2 thick wires from the harness and the compressor started but the rear didn't get inflated even when the truck has doors closed and in gear. The other 2 thinner wires at the plug is normal like the other posts had mentioned.

Next I had the harness plugged back in the relay and unplugged the 4 round pins connector between the compressor and the harness coming from the relay. I had the truck started and in gear with doors closed, I then checked the currents on the round pins. Now the 2 thick wires are live as expected (truck in gear and control modules sends signal to trip the relay to send power to power up the compressor), so this led me to believe the harness and relay are ok.

I think the other possibilities are either control module failure (inside dash) or rear ride height level sensor failure. From checking the relay, I would think the control module is sending signal so I don't believe the control module is bad. The front is inflated so its just the rear not inflated and this led me to believe the rear ride height sensor is broken.

I read somewhere that if any of these sensors are bad/faulty, the whole air suspension system will not work, but then I have no way of finding if the front sensors are good or bad.

I am not planning to keep the truck long due to its age but thought I'd give it 1 last kick and see if it will last me through the summer, so with that said, I am not planning on dumping more money to it.

If this is an easy and cheap fix, I'd do it and use it for my move in June, if not I am selling it to the junkyard for $500. Just seems like a waste where I JUST put 2 new cats and 2 new O2 sensor in, new windshield, new drive shaft and firestone Winterforce tires with loads of tread left. Not to mention I spent the time and money in fixing the blend door, custom exhaust and had the exhaust manifold studs replaced all within the last year.

Sorry for the long read as I don't know what else to do at this point.


EDIT*** just found another thread about a solenoid under the driver's side door on the frame and when he applied power to it, it realeases the air to the rear and the rear is lifted. Maybe thats why when I jumped the wires, the compressor engaged but rear is not pumped up. But then if its the solenoid, the compressor should still work but not pump the rear. I have to check the solenoid with a voltmeter. Anyone can tell me exactly where it is located and what it looks like?

 

That test is a good indication that the MOSFET (read $$$) relay has failed.
The compressor may not have run long enough to lift the rear - you did indicate the rear was ALL the way down. Or the compressor may have got too warm and the thermal breaker shut it off. Or there may be a leak in one of the lines or rear bags. Its possible the bag may leak more or less when it is at different heights.
I'm gonna assume when you wrote you "checked the currents on the round pins" you meant you checked for voltage on the pins in the "round" connector near the compressor. With the round connector disconnected one thick wire (gray) should have ~12 volts coming from the relay and the other thick wire (black) would not have any voltage with the connector unplugged as it provides the ground for the compressor motor.
The two thinner wires are used by the Air Suspension Module (ASM) to open and close the vent solenoid with in the Air Compressor Assembly. The ASM provides 12 volts on the pink wire to turn the vent solenoid on and 0 volts to turn the vent solenoid off, while the thin black wire provides the ground for solenoid control.

If you were trying to run the compressor with the vent solenoid wires disconnected (or the ASM not in control of the vent solenoid), I believe the compressor would deadhead against the vent solenoid and probably heat up and hopefully trip the thermal breaker.

Hope this helps you git 'er going............

 

To me it sounds like an issue with the rear height sensor. If the control module does not get a 'raise' signal from the height sensor, then the module does not open the solenoid, nor turn on the compressor.