I have a 1985 300SD with 192,000 miles and have been fighting this A/C problem for the past three summers. Some days the air conditioning works just fine but on other occassions it won't turn on (fan or compressor) until I shut off the engine and restart. Sometimes the A/C clutch with turn on and off rapidly and the tachometer will "flutter" and/or "under-report" the true engine speed. Sometimes the A/C compressor and tachometer are completely dead -- revving the engine may or may not bring them back to life.

Any advice you can offer would be most appreciated.

Bill

Bill,

Just two quick thoughts I'm not an expert on your particular system but I'm pretty sure you have a circuit that compares the compressor speed with the tach signal and the compressor slows down the circuit disengages it so you don't fry the belt. Check for proper belt tension and condition. Also I think but am not sure, that by 85 MB had added the grimlin infested KLIMA relay to the system. If so it resides inboard from the fuse box just aft of the compartment bulkhead. Pull it out and check for burned contacts where it plugs in.

Bob D.

Bill,

Did you get your problem fixed?

My 91 350 SDL exhibits the same tach and compressor issues ... would like to know what cause was on yours.

Thanks!

__________________
George Stephenson
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg

Check the fuse. Dont go after how they look. Read my previous post . Looks on fuse ( ceramic ones) are fooling. They look ok but have mico faults in them at times leading to abrupt on and off behaviour.

Also inadequate amt of freon in the AC system causes the clutch to engage and disengage more repeatedly.( I call this a threshhold amt). My Benz did that once , I added 1/2 can and bingo works good now.

But do check following thread from last week.
http://www.peachparts.com/shopforum/showthread.php?threadid=39083&highlight=micro+fuse

MVK

__________________
One penny saved is three penny earned.

1985 300D Turbo 179,000miles

stephenson,

Yes!!!!! I got it fixed. On the '85 300SD, the EGR circuit uses information from the engine speed sensor to adjust the EGR valve position. The circuit also controls the tachometer and AC compressor clutch. I re-posted this question (search: lost tach and A/C) and feedback suggested to check the wire connecting the EGR ciruit to the + lead on the battery and the fuse on the overload protection relay. Both were fine. "M.B. Doc" (thanks M.B.Doc) said that the engine speed sensor should output 5v at idle speed. I measured mine and it was about 25mv. Replaced the engine speed sensor and I now have AC and Tach. Just in time too, the temperature hit 95 the day I got it fixed.

On my car the engine speed sensor is located on the bell housing, on the drivers side of the engine. The sensor cable feeds through the first firewall to a connector. Pretty easy to replace once your determine the correct way to dislocate your arm to get to the bolt that holds the sensor to the bell housing. Cost from MB dealer was about $55.00.

Hope you get yours fixed. Cool air is good air.

Bill

My AC doesn't work so I'm having it checked out on Monday. Meanwhile I'm trying to learn as much as I can about the system so I don't get ripped off.. I've never been to a mechanic here yet so I don't know who the best one is.

My question is.. do I have the circuit on my car that compares the compressor speed with tach info etc.? It's an '82 300D Turbo.

Thanks.

__________________
Nello Tare

1982 300D (266,001 mi.) looking and running great

Previously owned:
1976 450 SLC
1983 300D
1976 300D
also
1982 Jaguar XJ6 (loved, but gone)

My 350 SDL's intermittent tach and a/c problem got worse ... I checked the speed sensor pickup at the bell housing - it had a few metallic flakes on it, but looked good otherwise. Check every cable connector and could find nothing but great connections.

Finally, looked at Klima (part number 0035452605) ... one of the terminals was a little dark ... started and ran engine ... still not tach or a/c .... smacked the Klima with a wrench and got all functions back normal. Touched Klima again and lost 'em all.

Guess I will have to replace the relay ... initial part search turns up ludicrous price of $200!! Any other options?

__________________
George Stephenson
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg

I have a 1991 350SDL. The speed sensor is on the bell housing? I am also having a problem with my compressor cutting out.