A/C problem with my SLC... - PeachParts Mercedes-Benz Forum

revray · #1 11-20-2002, 01:00 AM

Aloha,

1979 450SLC (R134A) - A/C is on and the compressor doesn't spin. There's blowing going on, but not cold. Further, I don't see any movement in the inspection glass of the receiver/drier. I'm thinking that the dash switch may be faulty (actually I'm praying that the switch is faulty) and would like to test it before moving on to suspecting the compressor itself. (needless to say, I'm praying that that's not the problem)

Can anyone help me with some instructions on how I can test the switch, or any other suggestions that might help me get my cold air back?

Thanks in advance,

#2 11-21-2002, 07:18 AM

Most likely the system is low on refrigerant. There is a low pressure switch that interrupts the clutch circuit to protect the compressor from damage due to low refrigerant.

You need to start by putting manifold gauges on the system to check pressures. This is your starting point with most any A/C problem involving blowing of hot air.

Good luck,

revray · #3 11-21-2002, 04:14 PM

Have 44 pounds. I looking through my books and manuals to see if that's high, low, or just right.

GregS · #4 11-21-2002, 05:32 PM

You said you have 44 pounds, but you also said you compressor isn't spinning - it doesn't work that way. The compressor has to be working (spinning) in order for you to determine high and low side pressures.

One way to check if you are low on refrigerant is to turn on the dash switch and then jump the wires at the low-pressure cut off switch. If the compressor clicks on then you have either low pressure or a bad switch.

Good luck,

Greg

jbaj007 · #5 11-21-2002, 06:44 PM

At Hawaii temps., even a static 44psi tells us you're way undercharged. Time to go leak hunting.

#6 11-22-2002, 06:58 AM

The compressor does not need to be turning to check static pressure. Static pressure will always be a little over 80 pounds if there is ANY liquid refrigerant in the system at all. Since you are reading 44PSI, the system is virtually empty.

The a/c system does not USE refrigerant like an engine uses oil. If it is empty, this means you have a leak. You will have to find the leak, repair it and recharge.

I am not a fan of converting to r134, but since it is an r134 system, this makes it a little cheaper to find the leak. Go find a can of r134 UV dye and charge the system. Once you put in as much refrigerant as it will take without the engine running, you will have enough pressure to actuate the low pressure switch and kick in the compressor. After you have run it a little, use a black light to search for the leak. If you don't readily find the UV tracing anywhere under the hood on lines, receiver, condensor, compressor, etc. then run the system enough to get condensation to drain underneath the car. Use the black light to look at the condensation water to see if there is any UV dye present. If there IS UV dye in the condensation water, then the leak is in the evaporator. If there is NOT, then gain access under the dash and look for UV dye traces around the evaporator connections and the expansion valve.

Good luck,