I work on my neighbors 01 expedition quite often. He's an older guy living on a measly pension and can't afford shop bills. Anyway his rear suspension was on the frame last week and after a day of troubleshooting I found the notorious air line union snapped in two. I replaced it and thought all was good. Now the rear leaks down intermittently and pumps back up when it feels like it. It never goes all the way down and seems to come back up after he drives it a bit. I know there's a leak, I'll look for it with soapy water soon but here are some questions...

- These trucks have ride height sensors, so no matter how soon it leaks down, it should air back up right?
- Are the ride height sensors like a throttle position sensor? Can they wear out in the position they are normally in? (that would explain only going down so far before airing back up)
- Could the compressor relay be on the outs, working only when it wants to?

I know there is a leak, but if all the components of the system are working correctly, the compressor should keep the suspension up even with a slow leak, right?

 

You didn't specify if the vehicle has 4-wheel or just rear air suspension. So I'll assume it's rear only for now.

Note: When the vehicle is turned off the AS drops the vehicle height 25mm.

The air suspension (AS) holds vehicle height when the rear hatch or any door is open. The AS module stores the rear height the moment any open door is detected. The AS then maintains this height regardless of the addition or removal of a load. The AS should return to it's programmed height when all of the doors (and rear hatch) are closed or the vehicle exceeds 10 MPH.

Note: If the door ajar light on the dash is lit even when all doors, the rear hatch, and the rear glass are closed, the AS will not work properly.

 

Sorry, yes it's 2WD rear air suspension only. He bought the thing new in 2001, and he says what it's doing is abnormal. No door ajar lights are on, and 25mm is only about an inch. It's sitting a lot lower than that when it goes down. With these symptoms where would you look next, besides leaks?

 

Once the system kneels down it's 25mm it should not drop any lower. When it drops lower than that a leak is present somewhere. Finding it is the headache. Besides looking for leaks in the hoses or joints, I would check the air flow solenoids mounted in the top of each of the rear airbags. Since the system worked good prior to the union breaking, are you sure your union repair isn't leaking?

 

Yeah, the union was the first thing I checked with some soapy water and it
s holding up fine. He does have a "check suspension" light on the dash. Is there a way to pull diagnostic codes with the air suspension system?

Back to my main question though...even if he has a leak, the system should kick the compressor on to keep it raised right? The only evidence of a leak would be the compessor running more often than normal if I'm thinking this thing through correctly.

 

Only with a $3,000 diagnostic tool.

I don't believe the compressor will keep running to raise the vehicle when the engine is off. If it did the battery wouldn't last very long.

 

Well, I've been sick and short on time. I still haven't leak tested the system yet, but here's a symptom. The truck leaks down, not all the way to the frame, but a good three inches. It will not air back up on it's own until I bypass the relay to start the compressor. Even after I "hot-wire" the relay it won't air up. Then I plug the relay back into the harness and wait 30 seconds. The thing airs back up like nothing was ever wrong. Which raises some questions...

1. How does the system leak down the 25mm? Is it a relief valve? If so, controlled by what? How does it know to stop?

2. Does the system have protection for the compressor? By "hot-wiring" it does it give it enough back-pressure to take over from there?

3. Could the ride height sensor be bad? Maybe never telling the system it went down the 25mm after the truck is shut off and it just keeps going down?

4. Is manually starting the relay enough to convince a worn out relay to work a few more times?

5. If the truck had a really bad air leak, I mean enough to go down three inches over night, why wouldn't it go ALL the way down to the frame?

This is really the only place I've found that has any information on this. So sorry for all the questions, but not being able to sort this thing out in my head is like nails on a chalkboard. I'm having him drive it to the end of the road and back in the morning. I hope getting it over 10mph will give me some more symptoms to go on.

 

There is a vent solenoid included in the compressor unit. The vent solenoid is controlled by the air suspension module, which monitors the rear air suspension height sensor.

The vent solenoid has an internal 160 psi relief valve.

Could be. The height sensor may have come free of its bracket and may not be reading the height correctly.

I don't know. The relay is solid state - uses a MOSFET transistor to switch the compressor on and off. My experience is that when they go dead, they are 'dead'. If you need a new one, bring at least $125 to the dealer

If it was a 'really bad' leak, but it might only leak when the pressure is above 110#s or what ever. If one of the rear bags is leaking, as the bag slowly rolls over itself on the way down, the roll may seal the leak at some point.

 

Thought I'd close this out... I replaced the compressor and switch. All seemed fine for a month or so. Then it would slowly leak down once a month or so. I would go over a jump the compressor to air it up again. I did this maybe two or three times, then it just stopped leaking. I know this isn't the "fix" most of you all are looking for, I just thought I should post the outcome. It's been a few years now and all is well still.

 

I put replaced the air sus on my 99 just recently... needed needed compressor, front struts, relay and air lines... less expensive to replace.