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ML320 AC Problem Compressor not Engaging

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132K views 64 replies 17 participants last post by  0x00  
#7 ·
As of MY 2002, the AAM is no longer a factor in turning on the compressor. That duty has now been assigned to the AAC Push Button Control Module. Your problem, assuming that you have a sufficient amount of refrigerant, is either the Pressure Sensor or the AAC Module or the compressor itself if everything is working properly.

Remove the connector from Pressure Sensor, key on. With a digital volt meter, neg. probe to brown wire and pos. probe to pink wire = 4.75 - 5.25 v.

Brn-neg Pink-voltage Grey-pressure signal Org-temp signal

Instead of jumping wires you should buy a new pressure sensor if you are getting the 5 volt reference, since they are not that expensive.

If all proves good, then you will have to take the AAC module apart and check for damage.
 

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#11 ·
np90fxrp said:
43sqd Just looked at the photo again (with my glasses on) and saw the burnt connection on the AAC so I guess that answers my question about what kind of damage i might be looking for. Is something like that repairable by soldering or is a new module the answer? Also still would like to know where it is located. Thanks
Click to expand...
The AAC is the A/C and Heater Controls. Conduct the tests first to check for voltage.
 

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#27 ·
When you checked the voltage (which is correct) at the Refrig. Pressure Sensor, did you use the PINK wire as GROUND?

There is either one of two problems:

1. there is something wrong somewhere in the A/C system that is preventing the actuation of the compressor.

2. or there is an internal fault with the AAM which delivers 12v to the compressor via pin #28
 

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#29 ·
frustration_X_2 said:
I will pull that pressure sensor again and make sure I've connected my leads correctly. Why do you have to pull the pressure to see if the connector is on properly?

Can I 'jump' the pressure sensor plug to see if the compressor clutch engages or am I going to blow something if I do? You cannot jump the sensor. Don't think about it.


The AAC on a '98 ML is under the fuses, correct? So I've got a bunch of plugs to pull apart if it's NOT the sensor. The AAM is under the fuse/relay module.
Click to expand...
Remove both covers to the fuse/relay box. Then remove the three additional screws and pull the fuse/relay module out of the fuse box. Then turn it over and lay it on top of a folded towel.

Among the many connectors, locate connector X22, wire B4. One side of B4 will be a PINK wire and the other side will be BROWN/VIOLET.

Start the engine and then introduce 12v to either the PINK or BRN/VIOLET wires and see if the clutch engages. Only allow the compressor to run for 5 seconds and no longer.

The reason why your compressor does not engage is that you have a leak somewhere and there is no refrigerant in the system.
 

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#34 ·
gadha, this is not the problem. You are probably low on refrigerant.

You should always state what your problem is instead of self diagnosing it. Just tell a short story.
 
#37 ·
Static pressure can only tell you that there is some refrigerant in the system, but cannot tell you if there is enough or the required amount.

Troubleshooting with Gauges FAQ

There is no Merc. A/C code B1231.

The first thing to check is fuse #5 in the right side kick panel fuse box. If it is not blown.................

Remove the radio with required keys, then remove the Center Section of the I.P.

Disconnect the four electrical connectors to the AAC Control Module and detach the AAC Module from the Center Section.

Then remove the seven tiny screws with a T7x40 screwdriver and remove back cover.

Check to see if the circuit is burnt.
 

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#41 ·
Do not jump any wires. You can damage the AAC Control Module.

1. With the connector removed, the cooling fans come and stay on at all time while the engine is running (whether or not the AC is requested). That is the way the A/C system is protecting itself. There is nothing wrong other than the sensor was disconnected.

2. With key out, key in, key in ignition position or engine running (at the connector): voltage between Brown and Pink is ~5V. That is the correct test with the correct wires and the correct results..

3. Interestingly enough, voltage between Brown and Grey is ~5V; voltage between Brown and Orange is ~5V. Forget about the other wires. They will not function if there is not enough pressure in the system.

You are assuming that you have a sufficient amount of refrigerant to operate the compressor and without definitely knowing that everything will be conjecture from this point on.

The only way to find that out is to have the system evacuated and refilled with the precise amount of refrigerant.

Did you take apart the AAC Control Module to see if there is a burnt circuit?
 
#43 ·
Gadha, you are wasting your time and money with this thought in your head about the pressure sensor. They don't go bad.

But if you feel that you must change it, the answer to your question is NO, you will not lose any refrigerant but it must be unscrewed quickly and the same goes for installing the new one.
 
#49 ·
No, it is not a faulty AAC. It's a proper scanner that is needed.

You should first have the leak fixed, hopefully it will be.
Then find a find a shop that has a Mercedes SDS scanner, or a table model by SnapOn, MAC, etc., and read and erase codes stored in the AAC Module.
 

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#52 ·
gadha707 said:
I bought iCarsoft i980 and scanned the system.

it is a b12/2 sensor trouble code. The error does not erase. or perhaps it ges erased and re-appears instantly.

when i read actual data, the pressure reading is 8 bar but the temperature is "invalid".

faulty temp/pressure sensor definitely? or it could still be something else.




Chirs,

you you said made sense.
Click to expand...
Be a little helpful.

What was the code number?

What temperature is invalid?
 
#56 ·
gadha, I have been trying to help you by supplying all the information that you need to perform these tests.

You have not repaired the leaky condenser nor have you checked the AAC for a burnt circuit. The only thing you need to do that is to buy a tiny screwdriver, of which I supplied you with the proper dimensions.

You have already asked about if you would lose any refrigerant by removing the Refrig. Pressure Sensor and I answered that but I do not see where you changed it.

And now, because you do not want to fix the leak or buy a screwdriver or change a sensor, which you were considering, you want me to assume and guess as to what the problem is.
 
#58 ·
gadha707 said:
Just a quick tip/correction on previous advise, it is not correct that you wont loose any gas while replacing the sensor. I lost all of it to the atmosphere as soon as I removed the sensor! plus a fair bit of oil too. So, do it the right way, Evacuate, Replace, Re-gas - as I originally thought.
Click to expand...
Then you had a faulty/stuck open Schrader valve in the aluminum line. But you probably didn't know that.
 
#60 ·
If the AAC Module has a burnt circuit the A/C will no longer operate at all.

When the problem occurs, go to the firewall and feel the two aluminum pipes; one should be ice cold and the other hot. Are they?
 
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