I have 1998 EB Expedition 4X4 with front and back auto leveling. MY compressor motor went out and I replaced it along with the relay. When I hooked overything back up the compressor came on and picked the truck up to its max point. Then the Check Susp Light came on. I let the truck sit overnight and the next day the rear end was on the stops and the front was OK. I started up the vehicle and the compressor would not come on. I pulled the harness off the relay and tried jumping A & B to verify that the compressor worked, and it was fine. All day long I screwed around with this thing. Just as I was getting ready to go buy a new truck, I read a post that said something about turning your automatic 4X4 switch into 4 wheel drive or 4 Low. When I did this the front came up first and then the back came up. Nut when I go back into AWD the compressor doesn't come on. Any ideas AWD?

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Air Suspension can be a beggar if you don't understand how the system works. I've never worked on an Expy in particular, but they're not much different than the Lincoln systems. Your Expy has Air Springs in the rear and Air Shocks up front. It sounds like you have a leak in the rear.

Keep in mind that the main failure item on any air suspension system is the air springs\air shocks. A leak in one can cause the whole vehicle to settle in certain situations. Your truck is entering the age when the rubber air bladders can start to crack and leak where they fold in on themselves. A small leak can be "made up" by the compressor for some time without you even knowing it, but the compressor isn't designed to maintain the vehicle height and it's life span depends on keeping the system air tight. A leak can quickly trash the compressor.

As for the 4x4 switch, there could be an input to the AS system to keep it from adjusting at a certain setting or to raise\lower the height in certain situations. This would be explained in the shop manual if you have access to one.
To get you started, check out this Tech article:
http://www.lincolnsonline.com/tech/00150_a.html
Also see this one for pics and relevant diag:
http://www.lincolnsonline.com/tech/00061.html

Turn off the AS and raise the rear of the vehicle, letting the rear end hang. Look closely at the bottom of the air springs, where the bladder folds under. If you see cracks or checking, you've found your problem.
Take a look here for parts: http://www.arnottindustries.com/