Why won't compressor engage?

[RickR1983]

I have a 1983 300SD Turbo Diesel. It still uses R-12 and I don't have a problem getting it. I charged the system up this summer and it was working great. Then I started having a problem with the system blowing hot air. I'd then mess with the buttons back and forth and get it to start cooling. Sometimes I'd get hot air out of some vents and cold out of others. Then just hot. Then just cold. It was kind of intermitent. Then I started to get HOT all of the time. I just replaced the mono valve and while I don't seem to be getting super heated air anymore I am not getting any cold at ALL. I replaced the control head last year because I was told it was bad and that helped until the intermitent problems started. The compressor will not engage at all at this point. If the R-12 has leaked out will that keep the compressor from even engaging? I can't hear the cluth engaging at all either, no clicks of any kind.

Any help appreciated. I just want to get this fixed. Could it be the control head again? Belts are tight and turn freely.

[RickR1983]

So I just took another test drive. Every single button on the climate control now yields the same thing. I am no longer getting supper heated air. I am simply getting outside air. On the heater wheel.......no hot..............on the cold wheel.........no cool. If I flip to full defrost I simply get vent temperature air up on the front windshield. This seems eerily like my last bad control head. No compressor engagement, so no clutch engagement. The reason I am leaning control head is that I am not getting anything from the heat side either.

[Yak]

I'd recommend doing a little research on the A/C system to assist in troubleshooting.

Yes, if the pressure is too low, then a switch - probably on the R/D - will keep the compressor relay from closing, which will keep the clutch from engaging on the compressor.

But your symptoms don't sound exclusively like low freon. The system is a combination of vacuum, electrical, sensors, and "heat exchangers" in the condenser, evaporator and heater core.

The control unit is a reasonable supposition, but may not restore perfect operation. You may be able to repair or reflow solder joints on the unit.

And if you're leaking R-12, you may want to have a leak-check done, even if it's easy for you to get.