I have a 1980 300D, the other day I was riding w/my lady and the A/C just stopped blowing cold. No power to the compressor. Tried jumping the low pressure switch to see if the Comp. would turn but no power. Removed the on/off switch inside the car(on the control panel) and no power to the switch. Fuses are good in the fuse house. Is there another inline fuse or a relay that could be causing this problem?


OK tried to recheck the fuse and all is well. As soon as I turned on the deforst the compressor did kick in. But not in the A/C mode? Still confused?

[This message has been edited by Backyardmechanic (edited 06-17-2000).]

Recheck the fuses and the fuse holder. Eve if it looks OK, replace it. Clean the holder as well. See if this helps. If not, you've got a bigger control problem.

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Jeff Lawrence
1987 300e
1989 300e
1987 BMW 325

OK rechecked fuses. Took test light and there is power across the fuse. Then I turned on the deforst and the compressor kicked right in. Is there some type of A/C replay under the dash or elsewhere on the car?

If you are saying that it's working now, then you have found the problem.. Very common for fuses to be corroded and/or defective. If it's not working properly then you will have to look further. There is a control relay for the AC, but I don't know it's location for your car. Maybe Steve or Donnie can give you that..

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Jeff Lawrence
1987 300e
1989 300e
1987 BMW 325

I would say that the ETR (evaporator temp regulator) could be the problem. Also, I don't even screw with old fuses. So many have wasted my time that I now replace them!

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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope :)
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

You might have a vacuum problem. There is a compressor vacuum switch that keeps the compressor from coming on till the engine is running. It is bypassed by the defrost position. That is the basis of my suggestion.

A wiring diagram is imperative here. Got a good one at work if you still need it Monday.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

OK Stevebfl, It's Father's Day so first let me say Happy Day to all you Fathers! If you have the time could you send me a copy of the schematics, I think the vac relay switch could very well be the problem.B4I forget Thanks for your help and suggestions, To all of you! Backyardmechanic, school teacher out for the summer. Will be waiting.

Backyardmechanic,

I just want to point something out that may, or may not have anything to do with what you're fighting.

The ignition switch on these cars can be flaky sometimes. The one on my daughters '81 300D had to be turned back to the left slightly after starting in order for the climate control system to get a feed. I just turned a little to the left and everything would work fine. I since replaced the ignition switch, and no more problem.

Since you appear to be possibly fighting an intermittent problem, try this and see if it has any effect. It could be that sometimes the ignition switch is making contact, and other times not. This would definitely set up a variable in your troubleshooting that could drive you as crazy as it did me.

Good luck,

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Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

My scanner is temporarily out of order and I have been using my radio technicians one recently. On Monday I can send the drawing but I have just reviewed the diagram (the good one is not in the climate control book). It is in the ETM. Totally unique wiring diagrams in 78-79. They are huge 19"x15", which makes them hard to scan. They will be the same as an 80 model.

From the diagram I can tell that you must have either a bad ign switch as Larry points out, a bad fuse five or connections, or a bad compressor vacuum switch. These are the only components before the compressor on/off switch. All of this is before the ETR switch or the compressor low pressure switch in order.

There are two major vac switches one knocks out blower, the "blower master vac switch" has a green vac line going to it. The other is the compressor vac switch and it has a yellow vac line going to it. They are both behind the glove box. The defrost by passes both vac switches. They can be by-passed electrically for testing but at least the blower switch shouldn't be left by-passed as it will make the system run on low blow even when the system is switched off.



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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician