A/C troubleshooting... - PeachParts Mercedes-Benz Forum

Amuso · #1 06-12-2002, 12:23 PM

I have my Klima Relay out in the process of cleaning up my cars electrical system and getting rid of all gremlins.

I opened it for fun, since I am always curious and into electronics... I found the relay inside totally fried. Parts of the circuit board too.

I am wondering what can cause this? Bad compressor shorting the circuit? Ideas? For the record the A/C has never worked since I got the car but has charge in it - had it measured.

jsmith · #2 06-12-2002, 12:28 PM

in general, a bad compressor can blow out a fuse in the relay circuit. some systems have both a fuse and a circuit breaker. it shouldn't have reached the circuit board unless the fuse was jumped or a higher amperage one was substituted.

Mike Richards · #3 06-12-2002, 02:09 PM

I'm curious. If the system is not functioning(fried Klima), how was it determined that there's sifficient charge?

jcyuhn · #4 06-12-2002, 03:17 PM

Hmmm... I've spent my share of time staring at the electrical schematic for the 124 air conditioning system, and I can't recall a fuse between the Klima and the compressor. I'd guess a shorted clutch coil on the compressor. It can be measured with an inexpensive DVM - I think it should have something like 4 Ohms resistance. Anything less, and it's fried.

BTW, it's impossible to determine whether the a/c system is fully charged without running the compressor. Certainly you can see if there's pressure in the system, but without actually running, it's impossible to distinguish between overfilled and barely filled.

stevebfl · #5 06-12-2002, 04:39 PM

Of course there is a fuse. I forget the car here but a 300E gets its power to the Klima relay from fuse #7.

The fuse is an 8amp and the clutch coil shouldn't take more than 3 amps. Most burned spots in circuits that are fused come from poor contact and high resistance. A shorted coil that take 5-8 amps will overload the whole circuit and the fuse is there to protect the wiring. Control units have to protect themselves and usually do this with some form of current limiting (electronically). I haven't had too much trouble with the Klima relay being taken out by clutch coils but I have had two mAS units that it happened on. The MAS is used for the compressor relay and there is a different compressor on those cars. Honda alo has great trouble with there coils on their Nippondenso compressors, so it seams to be a common problem overall.

Amuso · #6 06-12-2002, 04:55 PM

It must be monday or something

The A/C was chargetested when it was working I meant.

What is actually burned off inside the Klima relay is the circuitpath from 15 (ignition coil) to the relay that output 87. The relay itself has stuff burned off from one of the pins too.

Probably had some bad connection making it go on/off/on/off flickering and burned it out. This car has had a lot of amatuer hacking to the electricals and half the stuff just goes dead over bumps and comes back.

Have the fusebox out now, and ready to install my new custom one

jsmith · #7 06-12-2002, 04:56 PM

hmmm, no fuse huh? whatever happened to good old MB over-engineering? i was just speaking in general terms as i had just assisted someone with debugging a non-working compressor circuit in a toyota. it's not in the manuals but through a diligent search of usenet, i found that there was a fuse and a circuit breaker in the relay circuit. sure enough the fuse was blown and the breaker tripped...