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Air suspension does not work (compressor will not start)

Tiny
IVO.IVANOV
  • MEMBER
  • 2007 MERCEDES BENZ GL450
  • 4.5L
  • V8
  • 4WD
  • AUTOMATIC
  • 170,000 MILES
Fuses are okay, relay is okay, but the car doesn't rise.
I took out the compressor (thinking it was bad). When I apply 12V to the terminals of the compressor, it works. I put it back on the car, still it won't work.
So fuses okay, relay okay, compressor okay. What else could be wrong here?

I-
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Saturday, August 21st, 2021 AT 6:55 AM

5 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,192 POSTS
When you checked the fuses, did you confirm there was power to them? The fuse could be good but there could be a problem before the fuse. Also, have you checked for power to the compressor?

The first thing I would recommend is this. Because of the year and model, I would recommend having the can-bus scanned for diagnostic trouble codes. CAN stands for controller area network. Basically, the computers/modules are tied together via a few wires. This will retrieve codes regardless of the module it's stored in. Here is a video showing how it's done:

https://youtu.be/u-4syLc-ifQ

Next, I attached the wiring schematic below of the system. Confirm there is power to fuse 108 in the under hood fuse box. If it has power and the fuse is good, go to the compressor and confirm there is power to the pink wire with a black tracer.

Here is a link you may find helpful:

https://www.2carpros.com/articles/how-to-check-wiring

Also, here is a link that shows how to check the fuse for power.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Let me know what you find.

Joe

See pics below. Note: The schematic was two pages long. I had to cut each in half to make it readable for you. I did overlap them so you can follow from one to the next.
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Saturday, August 21st, 2021 AT 8:15 PM
Tiny
IVO.IVANOV
  • MEMBER
  • 3 POSTS
Yes, fuse 108 is okay (with power), and there is power on the pink/black wire going to the compressor.
Here is the confusing part for me: the compressor has 2 wires going to it (pink/black and blue/black). Then there are 2 more wires going to the solenoid valve (which is part of the compressor unit). The 2 wires going to that valve are gray/white and brown (the brown here is clearly marked as ground). I do get 12V reading between the pink/black (compressor) and the brown (valve). But I get only 0.3V between the two wires going to the compressor (pink/black and blue/black). Shouldn't I get 12V here as well?
Yet, when I remove the compressor from the car and apply 12V to the 2 terminals on the compressor (pink/black and blue/black), it runs.
The point here is that it seems like the compressor works, but somehow it is not powered.

I-
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Monday, August 23rd, 2021 AT 3:50 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,192 POSTS
Hi.

Yes, it should be getting 12v, but the black/blue wire is a signal wire. That is why the voltage is off.

At this point, we know the compressor is good if it runs, but we aren't getting the signal to actuate it. At this point, we really need to scan the can bus. The airmatic control unit receives input signals from several sensors/components via the CAN data bus. That should provide a specific code that will point us in the right direction. Otherwise, it becomes a guessing game.

Let me know.

Joe
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Monday, August 23rd, 2021 AT 6:22 PM
Tiny
IVO.IVANOV
  • MEMBER
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First, let me say thank you for your comments here in this thread.

Well, I am confused here: the 2 wires going to the compressor are the pink/black and blue/black. On one hand you are saying that blue/black is a signal wire, and that's why the voltage between them is low; but on the other hand, when I apply 12V directly to the 2 terminals of the compressor (where these 2 wires go), it starts running.
So it seems to me that in order for the compressor to run, it requires to get 12V on these 2 wires, doesn't it? Is it the case that the voltage must be low when it's off, and it goes up to 12V when it is supposed to kick in?

I haven't had a chance to do the scan yet, but let me share this though: a few days ago, before even testing anything and before taking anything apart, I was convinced that the compressor was bad. So I just went ahead and ordered a new one online. While waiting for the new one to arrive, I started working on the car, taking the old one out to prepare it for the new one. But when I took the compressor out and tested it by applying 12V on its terminals, it worked. So my conclusion was that the compressor is okay, but something else must be wrong. As I stated before, the relay and the fuses are okay, too. So I started looking online for answers and started this thread here.
Here is the surprise - the new compressor just came in, and I put it on the car. And it works. So it looks like it was a bad compressor after all. But I am still curious as to why the old one works when off the car, but it won't work when is on the car.

I-
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Wednesday, August 25th, 2021 AT 7:26 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

The voltage to the compressor motor is gray/white, ground is brown. The two wires you are powering are the signal wires (auto-leveling). The compressor should also run with the gray/white wire + and the brown wire grounded. If that doesn't run it, something must have come apart internally and causing a short. I attached a copy of the above pic and circled the motor power and ground.

I hope that helps. Regardless, I'm glad to hear it's working. Please feel free to come back at any time in the future if you have questions.

Take care,

Joe

See pic below.
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Wednesday, August 25th, 2021 AT 6:20 PM

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