300E A/C - please help diagnose - PeachParts Mercedes-Benz Forum

300EE320 · #1 08-27-2002, 11:30 AM

Hi,

Yesterday the A/C in the 1990 300E abruptly quit. My wife used it on the way to the store and it worked fine. On the way home - nothing. The blower is fine, but the air that comes out of the vents is suddenly not cold.

It doesn't seem like it needs a charge because it was working very well. We had it charged about 2 years ago (with R12) when it slowly lost its effectiveness. The filter drier and high pressure switch were replaced at the time as well.

I checked the fuse (#7) for the compressor and it was OK. I changed it anyway for the heck of it. I also looked at the sight glass on the fliter drier. It looks clear. Not opaque and no bubbles. I also checked the electrical connections near the filter drier. All seemed okay.

Unfortunately, my manual does not cover the A/C system. I do have a generic automotive A/C book and I understand the theory of how it works. Can anyone give me some pointers on troubleshooting this or point out some of the common things that could cause it?

Thanks very much.

pmizell · #2 08-27-2002, 11:40 AM

How tight is your serpentine belt? If it's loose, and it slips when you start the car, the compressor will shut off. As was the case with my 300E, I didn't know it was slipping because it never squealed, but my tensioner was bad and causing the same problems.

If you have a pulse counter you can also check diagnostic pin 7 on the X-11/2 for fault codes for the AC system.

Also, try and disconnect/reconnect the neg terminal of your battery -- it worked for me.

Good luck

300EE320 · #3 08-27-2002, 01:26 PM

Thanks Paul,

I gave it a quick parking lot inspection and the belt tension looks okay. I replaced the belt a few thousand miles ago. The compressor pulley is turning, but the clutch is not engaged.

Can someone walk me through the procedure for jumping the high side pressure switch and / or the thermostatic switch? Where are they located?

Of course I'll check for voltage at the compressor first to rule out the clutch coil. I'll also try your trick of disconnecting / reconnecting the battery. I don't have a pulse counter. Is that the same is the DIY "code reader tool" that has been described in old posts? Please elaborate.

Thanks again!

joe p · #4 08-27-2002, 01:40 PM

No clutch and a full charge of feon? Check the Klima relay behind the battery.

Joe

#5 08-27-2002, 03:35 PM

Joe, Paul's car has, I believe, a MAS relay instead of a KLIMA. More functions, bigger bucks. BTDT . . .

Richard Easley
Waco, Texas

joe p · #6 08-27-2002, 04:35 PM

90 can go either way. :-(

Joe

#7 08-27-2002, 04:39 PM

Yeah, but if his luck is like mine, it'll be the wrong (MAS) way . . .

Richard Easley
Waco, Texas

300EE320 · #8 08-27-2002, 04:50 PM

I'll know tonight when I check it out... I will report back.

Thanks guys!

300EE320 · #9 08-28-2002, 12:06 PM

Yes, in fact my luck is the same as Richard's. It has the MAS box, not the Klima relay. I read up on the Klima Relay. Does the MAS box have the same type of solder failures?

I checked the belt tension - no problem, and the belt is fairly new.

I disconnected / reconnected the battery. No change. While I saw from other posts that a tachometer malfunction is often also related with a Klima relay, I don't know if it is controlled by the MAS in my car. If it helps, my tach is operating properly, as is everything else on the car.

Next steps? Troubleshoot the MAS? Jump across the high pressure switch?

Thanks in advance!

joe p · #10 08-28-2002, 02:44 PM

I assume you've checked the static charge of the a/c right?

MAS control the a/c as well as a few other things. Make sure you replace it with the same part number, there is a MB microfiche mistake on the 92 TE MAS relay part number, yours might fall in with that.

I've never opened a MAS to look for desoldering but, it might be worth a shot but, only of you determine thats the problem. Try jumping it out for the compressor clutch.

Joe

300EE320 · #11 08-29-2002, 11:52 AM

I don't have the tools to check the static charge. Two years ago when the system did lose Freon it just slowly lost its effectiveness. This time it just stopped working immediately. It's possible that something ruptured and lost the charge, but I just don't think it's likely.

I would like to jump out the MAS to check the compressor coil. Can you tell me which pins on the plug to jump?

Would someone be willing to send a diagram if the CD includes it? My manual does not cover A/C for some reason...

Thanks again.