The A/C compressor on my 91 300E (Model 124-030) isn't running. Have checked just about everything, including that the refrigerant charge. The suspected culprit is the A/C Compressor Control Unit (N6), but I can't locate it. My parts locator shows it behind the battery near the ABS Control Unit, but my car doesn't look like the picture on the Mercedes CD. Can someone please give me a clue where to look?

Thanks,

Walt Lake

If it's what I'm thinking of, it is a black square relay looking thing with a bunch of white writing on it, and it should say "Klima" on it pretty prominently. ("Klima" is German for climate, as in climate control system). On my '87 300E, there is a black plastic divider behind the battery that you can just pull out (it is not held in by anything), and the Klima, overvoltage protection, and another big relay are right behind it.

when u locate the KLIMA pull it out .and with a pen knife cut the silicone seal around the bottom ..then pull apart . more often it has a loose solder connection on one of the leads..if so re heat it and re install ..you just saved yourself about 125 $ ..look at the connections with a magnifying glass. your problem sounds like it is IT.. Let us know if we can be of any further help. Pete

The '91 has a MAS not a Klima relay.

Have you tried jumping the low pressure switch terminal connections on the receiver/drier?

Lift off the 2 connections and bridge them together with a small wire. If your compressor kicks in, you are probably low on refrigerant, or have a bad switch (unlikely)

Good Luck

__________________
-Paul-
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi

After looking in vain for a compressor control unit, I discovered that my car has a MAS. The Mercedes CD never mentions this fact in the checkout procedures. After finding the beast behind the battery, I found that I have voltage to the MAS from the pressure switch, but no power out to the compressor clutch. Jumpered the compressor clutch and it works. So it seems that I'm stuck with a new, rebuilt or used MAS. To add to my problems, the blower motor stopped running. Luckily this was covered in another thread, and I found the little black fuse box with the strip of metal between contacts. Now the blower runs again, but the annoying thing is that the Mercedes CD manual doesn't mention this fuse. Their maintenance manual is a real pain.

Thanks to everyone,

Walt Lake




W

When my OVP relay was failing i had no A/C function.

I have an older car

Pelle

300E 4M -89

Walter,

How did you shorted the clutch?

Reply to benzidude - to jump the compressor, remove the 3 conductor connector at the compressor and jumper from the battery plus terminal to the clutch connector (the blue/green wire). It's a little tricky getting the connector off because there is a little knob on the plastic piece in the front of the connector that fits into a hole in the connector. You have to push the plastic piece away from the connector with a screwdriver while lifting up on the connector. Hope this helps.

Different subject for anyone who can help. I installed a new (not rebuilt) MAS control unit yesterday, and the A/C worked beautifully. Unfortunately today the compressor stopped working again, and I have the exact same symptoms as with the old MAS; i.e., power to the MAS from the pressure switch, but no power to the compressor clutch. In addition, fuse #7 blew, and I can't connect this with a failure of the new MAS. I hate to just throw in another MAS without some assurance that I don't have an underlying problem. Any suggestions?

Thanks - Walt Lake

Another bit of information to add to my last message about destroying a new MAS control unit. The check engine light came on about the same time that the compressor stopped. The trouble code is 17, which is for a oxygen sensor lead either shorted to ground or B+. The oxygen sensor is new, and the check engine light has been silent since it was installed. It seems like too much of a coincidence to ignore, but I fail to see any connection between an A/C failure and the oxygen sensor. Any thoughts would be appreciated.

Thanks,

Walt Lake

A slipping belt will cause the compressor to kick out.

However, with a blown fuse and a dead MAS, I'd look for some shorted wiring.

Mice love to chew the insulation off of wires, so look for mouse damage in particular.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Fred - thanks for the reply. I also think that I have a short somewhere but have been unable to locate one. A problem is that the Mercedes maintenance CD doesn't have a wiring diagram that matches my 91 300E. The CD says that I have a A/C control relay while I actually have a MAS control unit, so I'm checking for shorts in the blind. No sign of varmints or belt slippage.

I'm going to try to isolate the problems (A/C compressor and fault code 17 for the oxygen sensor) for a while longer, but I probably need professional help. Does anyone know a technician in the San Diego area that can be recommended?

Walt Lake

This isn't direct help, but good info to acquaint yourself with, from this site's DIY section:

http://www.peachparts.com/diy_stevebfl_0.htm

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

Golden Rule - Thanks for the lead. I copied the article you recommended, and one statement hit home. It said to check the amperage draw of the compressor clutch, or I would be using the MAS control unit like a very expensive fuse. I had previously checked the clutch with a jumper, and it ran beautifully. Today it barely engages, and sucks up a lot of current. I popped a 20 amp fuse in line with an ammeter. I don't like the result, but at least I know what has to be done, i.e., new compressor clutch. Thanks again for your help.

Do you have any idea why the code 17 trouble code keeps popping up, even after installing a new oxygen sensor. The code says that the signal lead is shorting to ground or B+, but I can't track the problem down. The check engine light doesn't come on until the engine is hot, and usually just before the end of a drive. Very strange. Any suggestion would be appreciated.

Walt Lake

Try unplugging the heater on the O2 sensor (the two wire plug). If it is defective, it could short to the electrode in the O2 sensor and cause your fault.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!