1977 911S A/C trouble shooting help needed
 

I just replaced the a/c compressor and the dryer in the front fender. I've had the system charged by a reputal service. I can't seem to get the clutch to engage by adjusting the t-stat knob in the center console. The fan runs fine on all the speed settings. I can jump the clutch directly with a wire from a hot source and the clutch engages. Any suggestions as to what my next step should be?

Your problem will fall into a couple of catogories.
1) You have already lost your freon and your low pressure switch is preventing the compressor from engaging. (check pressures and or fix leak)
2) Your low pressure switch located on the receiver/dryer is defective (check switch)
3) Interior thermostat switch has lost it's sensing tube pressure. (listen very closely for slight click sound as rehostat is rotated - test with key off- replace thermostat)

It will probably be a loss of freon. If options 2 or 3 was the problem the A/C shop would not have been able to charge the system.

Good Luck
Michael

The fault became evident as the system was being charged. We jumped the clutch in order to charge the system.
What is the best way to check the low pressure switch?
Thanks

Most '77 911's didn't come with a low refrigerant pressure compressor cutout switch and there usually isn't wiring up front at the R/D for a R/D mounted switch. If a conversion to R-134a was done, a high pressure cut off switch is technically required but not a low pressure switch but sometimes combined units are installed. If a low pressure switch had been in place, the shop would have had to jump around it to start the charging process but once the minimum pressure was reached they should have undone the jumper and let the thermostat switch takeover. Either there is a fault in the circuit to the compressor (some of these systems have a circuit breaker and a relay that can fail) or as mentioned above the thermostat switch is bad or disconnected. Cheers, Jim

I'm sure this system has the low pressure switch if thats the switch thats mounted on the R/D, located up front under the left fender. Would it be wise to replace the expansion valve and the t-stat switch just as a precautionary measure? They are not very expensive.

One has to open up and re-evacuate the system to replace the expansion valve (same for low pressure cutout switch). Thermostat can be replaced without opening up the system, but I would first find out what the real problem is before changing out parts.

Is there a way to test the low pressure switch?

The first test is to see if it has continuity (passes current with a test light). If okay, then check if it is receiving current from the thermostat switch or compressor relay. If no continuity then attach a refrigeration harness to fittings on or near the compressor and check the low and high side pressures. If pressure is too low then the lack of refrigerant may be the problem. Jim

I'll give that a try. Thanks for the Help.