A/C Compressor question

[thorsen]

I'm almost convinced that the compressor on my 1984 300SD runs whenever one of the three buttons on the left side (Defrost, down and up arrow, and down arrow) of the climate control unit are pressed. It's a little hard to tell when driving so I'm going to wire a light in series so I can see when the compressor clutch is engaged.

If it is on all the time as I suspect, is this correct? If not, are there any hints on where to start looking? The A/C system seems like a black hole to me.

Thanks!

[engatwork]

When any one of the first three buttons from the left are mashed the compressor should be running. Sounds like your system is fine.

[Diesel Giant]

Yes correct the comp. is supposed to on with those 3 buttons. The A/C is not a black hole.

Read the pictorial and get the confidence to do it yourself.

http://dieselgiant.com/repairyourac.htm

[thorsen]

Just to clarify - it seems like it runs ALL the time, ie- even if the temp dial is in the middle of the gauge.

Does the compressor to cycle to keep the temp at a certain level, or does the compressor run continously and the heater core is used to moderate the temp?

Thanks for the link DG, I'll read up.

I agree, the AC should be running. If you don't want it to run, use the "economy" mode (4th button).

[Diesel Giant]

Your temp dial may not be working. Does the comp. run even if on the "econ" button?

[thorsen]

The compressor runs if one of the three left buttons are pressed, it does not run if the Econ or off is pressed.

The temp dial seems to work because if I turn it warmer, the air coming from the vents gets warmer.

I'm just concerned that the compressor is running 100% of the time when one of the three left buttins are pressed. I would have thought that the car gets to the desired temp and the compressor cycles off until it gets too warm.

[BobK]

Compressor runs all the time unless the temp gets too low in the A/C system and thus low system presure cutout. Or the coolant temp gets too high and it shuts down on high head presure. yeah, not cheapest system to run.

[Brian Carlton]

Bob is correct.

The compressor runs until the evap temp sensor signals "too cold" and it then shuts down. If the system is marginal, or the ambient temperature is very warm, the evap temp may never reach the point of "too cold". This is quite common on R-134 systems. I'd bet a proper R-12 system would cycle normally.

If the cabin finally gets down to the desired temperature, the compressor must cycle, otherwise the evap will freeze.

Why do you believe the compressor does not cycle? It's not always readily apparent on a 617.

Originally Posted by thorsen

I would have thought that the car gets to the desired temp and the compressor cycles off until it gets too warm.

I believe the compressor is run just to dry the air, even when you need heat.

[thorsen]

Originally Posted by Brian Carlton

This is quite common on R-134 systems. I'd bet a proper R-12 system would cycle normally.

Why do you believe the compressor does not cycle? It's not always readily apparent on a 617.

That makes sense. Mine had been converted to R-134 years before I purchased the car.

I guessed the compressor was on 100% of the time because I drove the car for 90 miles on the highway and got the inside nice and cool. I then had my wife slowly turn the temp dial to hot as I watched the compressor and the clutch never kicked off.

[Brian Carlton]

Originally Posted by thorsen

That makes sense. Mine had been converted to R-134 years before I purchased the car.

I guessed the compressor was on 100% of the time because I drove the car for 90 miles on the highway and got the inside nice and cool. I then had my wife slowly turn the temp dial to hot as I watched the compressor and the clutch never kicked off.

The system had very poor efficiency at idle. The condenser is inadequate and the airflow across the condenser is inadequate. So, you can bet that the compressor will never get the evaporator down to a level where it shuts down the compressor.

Now, on the highway, it's a different story. Massive airflow across the condenser raises the efficiency of the system and will allow the evap to reach the cutoff temperature.

You can check this if you lift the hood and suspend your wife from the M/B hood ornament while travelling 60 mph.

[Larry Delor]

Originally Posted by Diesel Giant

The A/C is not a black hole.

Heh.....My wallet begs to differ.

[dmorrison]

http://mb.braingears.com/123_DISK2/program/Climate/83-600a.pdf

Here is the Service Manual for the Automatic Climate Control for the W123. You car is basically the same, some very minor differences. But at your level of knowledge, read the manual and learn about your system.
It does sound like your system is running normally.

Dave

[Matt L]

Originally Posted by BobK

Compressor runs all the time unless the temp gets too low in the A/C system and thus low system presure cutout. Or the coolant temp gets too high and it shuts down on high head presure. yeah, not cheapest system to run.

His car has a low-pressure switch in the high-side only. It will prevent the compressor from starting, but it's not a cycling switch and won't stop the compressor based on the pressure in the low-side line.