My 1994 E320 A/C was working intermittently, cold one day, warm the next.

Took it to a Shop: Tech replaced A/C Relay and stated that it still did not work. It has not worked (even intermittently) since I got it back from the shop. The A/C Clutch comes on for 2 seconds, then cuts off each time I turn on A/C.

I bought a A/C Low Pressure gage to do a little investigating myself. The low side pressure is 80 PSI! I measured the pressure with engine off, since the clutch does not turn but for a moment.

My guess is that the Tech tried adding 134A to the system, but added too much?

Questions:
1. Is 80 PSI way too much (with engine off - low side)
2. What is proper pressure?
3. Can I remove some 134a myself? How? I hate to take it back to those "Techs".

Thanks, John

Pressure is normal.

You have several possible problems.

One is that the speed sensor isn't indicating the compressor is turning, so the climate control turns it off. This is usually the result of a bad belt or belt tensioner. If the belt is missing most of the rubber ribs, it wont' pull the compressor, so when you first start the car, it runs until the Klima decides it isn't turning fast enough. The relay will shut the compressor off, and it won't start it again until you turn the key off and restart.

The other possibilities are worse --

Speed sensor gone (rare unless this is a recently rebuild compressor and the rebuilder didn't do it right)

Wrong compressor for the car if replaced, so the Klima doesn't get the correct signal.

Wires bad, speed signal won't get to Klima.

Climate control shot, Klima shuts compressor off.

I'd bet on the belt or tensioner, myself. I think my 300D is doing the same thing, just need to get out and change the belt.

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!

Thanks Peter, I checked the belt it's fine.

After I that , I started checking wires.

That was it, a wire was broken going to the compressor!!

Fixed and cool............. ahhhhhhh......

John

Hi, could you please specify which wire was broken, and cause this behavior (starts and cuts after seconds)...I have the same problem in my car (300D 1987)...I am trying to replace the compressor speed sensor,
please help,
thanks

Starts and then stops after a few seconds isn't a wiring problem unless the wires from the speed sensor are bad. You will need to verify at the Klima socket that all the inputs are correct.

You will have to vent the AC to change the speed sensor.

Usuall cause of cutout after a few seconds is a bad belt or tensioner.

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!

Thanks "psfred"
I checked the inputs at the klima, the compressor speed sensor input is between 0-0.1 v (AC) at idle, it increases to 0.4 or 0.5 at 2000 rpm...the CD says it must be at least 0.3 v at idle (750 rpm)...at rare times the compressor stays on at idle but cuts again if the car driven...
I am not sure if the previous owner had the compressor rebuilt, this is a possibility since the system was converted to r134 professionally (there is a tag and a leak detection process etc..). They might have rebuilt the compressor...
Now, should I "vent" the AC and take off the sensor, what signs of failed sensor should I look for, should I just clean it and change the o ring and reinstall for a test, or the magnetic inside the sensor is old and weak therefor I should install a new one (~ $85)...can I do that while the compressor in the car, it is tight space but I think it is doable??
After that, should I vacuum the system, if it is fixed with the sensor replacement, before charging, or this is process preserve the system cleanliness and no need for vacuum...
Lastly, how about the oil, does vacuum take off oil..
I really appreciate your advice,
thanks

Two possibilities here, maybe more:

Wrong compressor installed -- the diesel isn't the same one as the gasser, so the speed sensor signal is wrong. I've heard of this before, major PITA.

Speed sensor producing the wrong signal due to being too far from the crankshaft of compressor -- this won't be fixed by replacing the sensor, the crankshaft isn't isntalled properly (bad rebuild, untested).

You need to check the part number of the compressor and veriy that it is correct, and it if is, the rebuilder needs to send you a tested known good one in exchange for the bad sensor equiped one.

You could also have a bad connection at the sensor harness -- pull it apart and check for corrosion or dirt.

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!