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Pete, Mercedes Mechanic
Category: Mercedes
Satisfied Customers: 29094
Experience:  Over 15 years experience as a Mercedes dealer technician
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We have a 2009 ML350. The AC is not working. it has a fault

Customer Question

We have a 2009 ML350. The AC is not working. it has a fault code 9006 (Compressor short circuit) The wiring update at the compressor was already performed at the dealer last summer. We replaced the ACC control unit, everything was fine for 2 days, then the AC started blowing warm again. We rechecked the refrigerant levels; they are ok. We scanned the ACC controller and the same short circuit fault is back, but now the SDS is showing 120 bar of refrigerant pressure and 25volts of power at terminal 30 of the ACC control unit. The actual voltage at the terminal is 12-13 volts. We swapped the new ACC controller with the original unit and the psi and voltage readings are normal again (but of course the AC is not working). We installed a second new ACC controller, programmed it, and it is showing the same high psi and high voltage. My question is (finally) are these ACC controllers prone to being defects out of the box? or am I missing something here?
JA: Have you checked all the fuses? Do you have a wiring diagram?
Customer: Yes we have the diagram. We checked the fuses and the grounds. We also jumped 12v thru the wire at the back of the ACC connector that leads directly to the compressor and the compressor activates and blows cold.
JA: Are you fixing your ML350 yourself? What have you tried so far?
Customer: Yes I am fixing it. My issue is, I am not sure which way to go here, everything points to a faulty second new control unit, but what are the chances of that?
JA: Anything else you want the mechanic to know before I connect you?
Customer: Thats it for now.
Submitted: 1 year ago.
Category: Mercedes
Expert:  Pete replied 1 year ago.

Hi my name is ***** ***** I will be happy to assist with this today.In some cases I may need more info to assist better.I apologize for any delay.

Did you have the ac controllers programmed at all?

Let me know,thanks Pete

Customer: replied 1 year ago.
with the latest version of the SDS.
Expert:  Pete replied 1 year ago.

Do you have access to service bulletins as there is a bulletin out on this model for this code set here?Would you like me to post the info on this?

Let me know,thanks Pete

Expert:  Pete replied 1 year ago.

I will post the bulletin below on this:

No cooling output, fault code 9006 in air conditioning control unit

Topic number LI83.30-P-049644
Version 3
Design group 83.30 Air conditioning
Date 06-05-2013
Validity All veh. of model series 164/251
Reason for change Remedy and parts availability adapted

No cooling output, fault code (FC) 9006 stored in air conditioning (AAC) control unit.

- Poor system ground at ground point W29/2

- Faulty software in AAC control unit

- Poor ground at refrigerant compressor

1. Check ground point W29/2 (A-pillar, see WIS documents in attachment for details) and repair if necessary.

-> Optimize contact, tightening torque: 9 Nm.

2. Check crimping of ground line of AAC control unit at ground point W29/2 and repair if necessary.

In vehicles produced prior to 04/2010:

3. Flash AAC control unit with flashware (FW)(###) ###-####using Star Diagnosis.

Then perform the following test on the vehicle:

4. Perform the Star Diagnosis tests stored after fault code 9006, including the test of the refrigerant compressor.

5. Check ground line between AAC control unit and ground point W29/2 for open circuit and repair if necessary.

6. Check positive line between AAC control unit and refrigerant compressor for open circuit and repair if necessary.

In vehicles up to code 808 (YoM 07/1) :

7. From the connection of the regulation valve on the refrigerant compressor one cable with a cable lug is bolted onto the housing of the refrigerant compressor as a ground line; the other cable goes via the connector to the engine wiring harness. An additional ground line with a cross section of 0.75 must be properly routed from the bolt-on point of the ground cable on the housing of the refrigerant compressor to ground point W16/4 in the front right of the engine compartment below the engine hood hinge (see photos in attachment).

In vehicles as of code 808 (YoM 07/1) :

8. Convert electrical line on refrigerant compressor from 1-pin to 2-pin. (Part number of 2-pin wiring harness A1644406641) When connecting the 2-pin wiring harness, ensure correct polarity otherwise the regulation valve will be irreparably damaged. The regulation valve is thus grounded via the operating unit.

Replacing the refrigerant compressor and the A/C operating unit does not remedy the problem in this case.

Even if the dealer installed a wiring harness this needs to be gone through to see if any issues are found.If this is done and found o.k. let me know.

If more help is needed, use the reply tab to continue our conversation. If no further assistance is needed, kindly rate my service,your 5 star rating is appreciated.You can rate at any time and we can continue to work on your question as this will not close out your question.Keep in mind in some cases it can be difficult to fully diagnose or help repair your vehicle without seeing it and I work on delivering the best possible answer from what I see to work with.

Thanks Pete