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89 300E AC Compressor Controller

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Doyle's picture
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My AC has stopped working over the winter. I thought the Freon might be low and started to add some. The compressor is not turning and the freon pressure is at the high end of the scale.

The auxiliary fan does not come on when I turn on the AC. The voltage on pin 7, the wire to the A/C Comp Ctrl unit drops to near zero when I turn the AC on. When I monitor the voltage to the compressor clutch, the voltage with momentarily come up to 12 VDC after I cycle the engine off and on again.

It appears to me that I have a problem with the compressor control and likely a inhibit function. I'd like to monitor the compressor Control terminals, but I can't locate the controller.

Questions:
1. Can anyone give me a clue as to what to do next?
2. Can anyone tell me where to locate the AC compressor control.

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Re: 89 300E AC Compressor Controller

Simple questions, not so simple answers.

First question: Follow the directions in the answer to the second question.

Second question: Off the top of my head there are at least 7 different, seperate and distinct components that will cause the compressor to not run. Mercedes did not design this climate control system for simplicity. Since the explanations and troubleshooting take up more than 40 pages, I must refer refer you to the repair manual.

If you simply want to throw money at it, here are the 7 items:

MAS module

push button unit

clutch coil

low pressure sensor

temperature sensor

compressor speed sensor

engine speed sensor

W124Service Library CDs are still available from Mercedes. They were not available early in 2007 as stocks had run out. As of 1/7/2008 there are over 100 copies on hand and available for sale.

Price is US$64.98 plus US$5.00 shipping plus applicable state sales tax.

To order call 1-800-367-6372. Ask for Mercedes part number P-2700-124-099.

After obtaining the CD, you should download Steve Nervigs Index for the Service CDs. It allows you to use the CDs easily without having to learn what Mercedes meant when they structured their proprietary product. You can download the index at: http://web.mac.com/dakota/iWeb/Mercedes/Indexes.html

Additional resources are available by joining the Mercedes Benz Club of America. MBCA membership application is at: https://www.mbca.org/mbca/index.htm

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87 300SDL
95 E300D

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Re: 89 300E AC Compressor Controller

The main compressor (Klima) relay is in the electronics compartment behind the black plastic cover on the firewall behind the battery. It's there along with the OVP, KE controller, ABS controller and fuel pump relay. Assuming the push button CCU itself hasn't failed, the most likely inhibitor is the low pressure switch on the receiver.

Failure of the aux fan to come on is likely unrelated to compressor issue. Fan circuit can be tested by bringing jumper from battery + to the 1-wire side of the pre-resistor behind the driver side headlight. That should give low fan; jumper to 2-wire side gives high fan speed. If no to both tests, the fan motor is probably bad (altho it may just be gunked-up and stuck--tap it to see if it frees up), or the aux fan relay is bad. If yes to hi fan and no to low fan, the pre-resistor and/or the pre-resistor relay are suspects.

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Re: 89 300E AC Compressor Controller

Two more things to add to my list above:

slipping belt

compressor tightening/locking up (usually caused by lack of oil. Loosen belt and see if compressor (not the pulley) turns with just a bit of resitance)

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Re: 89 300E AC Compressor Controller

I'm obviously not communicating well. I know where the Push Button Control and the Klima relay are.

In troubleshooting the system using the wiring diagram, I've found that Pin 7 of the Push button control is changing when I call for AC. However, there is no corresponding change to compressor clutch. I need to troubleshoot the AC Comp (Compressor?) Control and cannot locate that unit.

Or.... are you saying that the Klima Relay is the AC Comp Control?????

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Re: 89 300E AC Compressor Controller

A couple more items.
1. The compressor will come on if I apply 12 Volts to the compressor clutch.
2. The aux fan will come on if I apply 12 Volts to the fan motor pins.

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Re: 89 300E AC Compressor Controller

Good information. N6, behind the battery, provides direct control of the compressor clutch among other things. Section 86 of the W124 Service Library contains tens of pages of troubleshooting proceedures on all or most of the things that feed N6. They are in my previously provided list as well as on the diagram. Page 306 (Automatic Climate Control) in Section 24 of the Library contains the schematic for compressor control on your car. Since you have a schematic, I have to assume it is the same one. If not, send me a PM with an E-mail address and I will send it to you. It exceeds the maximum allowable file size for this system.

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Re: 89 300E AC Compressor Controller

Doyle;170039 wrote:
Or.... are you saying that the Klima Relay is the AC Comp Control?????

Yes, the Klima relay is the A/C compressor control unit.

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Re: 89 300E AC Compressor Controller

FYI, my reference material is the Mercedes Manual on CD. For those who are doing some or all of their own work, it is indispensable.

W124Service Library CDs are still available from Mercedes. They were not available early in 2007 as stocks had run out. As of 1/7/2008 there are over 100 copies on hand and available for sale.

Price is US$64.98 plus US$5.00 shipping plus applicable state sales tax.

To order call 1-800-367-6372. Ask for Mercedes part number P-2700-124-099.

After obtaining the CD, you should download Steve Nervigs Index for the Service CDs. It allows you to use the CDs easily without having to learn what Mercedes meant when they structured their proprietary product. You can download the index at: http://web.mac.com/dakota/iWeb/Mercedes/Indexes.html

Additional resources are available by joining the Mercedes Benz Club of America. MBCA membership application is at: https://www.mbca.org/mbca/index.htm

__________________

Virginia Beach VA

87 300SDL
95 E300D

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