I have a 84 300DT with 140K. I just put on a rebuilt compressor, new expansion valve, new drier, new orings, and flushed the system prior to refill with R12.

Question: Why does my compressor run all the time when in AC mode? Isn't it supposed to cycle on and off? It gets nice and cold but I do not care to burn it up. Do all of the compressors make some noise? At idle, it sounds a little clunky but has not changed in about 500 miles since I repaired the system.

The temperature control seems to work just fine and it is consistant. I appreciate your advise. This is the first 300DT that I have owned.

Thanks,
MDE

Do you know which brand and style compressor you have ?
Before you did those repairs to the system.... did you own the car while the AC did not work ? What was the reason or condition that caused you to do those replacements ? In other words, is there a possibility that the prior compressor ran all the time, thus went kaput, and that you did not know you needed to be looking for the reason that happened.... ? As compared to this being a new problem which has only existed since the fixup was performed .... ?

You did not enter your location in your profile and you did not indicate ambient temperature in your post. We need more information.

Good luck,

Thanks, tonite I will look at the old compressor to get the exact model. The replacement is a duplicate of the original. The system was almost empty when I bought the car and I do not know what the original problem entailed. I decided to go with new parts as I did not know the car. I live in Maryland and this year we have had very few days above 80F. June has averaged 75F average high for the month with much rain and high humidity.

You won't burn up the compressor as long as there is refrigerant and oil in the system.

The temperature is regulated by the monovalve regulating hot water flow through the heater core. The a/c system basically refrigerates most all the time.

If it ain't broke don't fix it.

Good luck,

Thanks for the assistance. The compressor is a R4 and I have oil and R12. The system works great and gets ice cold so I will let well enough alone at this time. Thanks again!

Perhaps the diesel and the gas systems operate different, but I recently had the same problem with my SL. Turns out the problem was in the climate control unit which mounts behind my glove box. Had replaced the OEM one with another of a different part # which the vendor said would work. The replacement part had one less pin on the right side than the OEM one. Couldn't figure out what the difference was until I put the OEM part back in after replacing the voltage regulator.

So, my guess is your problem could be in the CCU box, but it's just a guess.

__________________
Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000