A/C thermostat testing

[john maples]

I am installing a/c in 1976 non a/c car using updated components. After mounting the vent unit (Porsche original) under the dash, thought it would be a good idea to test the functions. The variable speed fan works (new controller and motor) fine but I do not get 12V thru the thermostat.

I have 12v on one spade terminal but do not receive the voltage thru the thermostat when turned "on". Shouldn't I get 12v at ambient temps to start the compressor?

Thanks

[86 911 Targa]

Originally Posted by john maples

I am installing a/c in 1976 non a/c car using updated components. After mounting the vent unit (Porsche original) under the dash, thought it would be a good idea to test the functions. The variable speed fan works (new controller and motor) fine but I do not get 12V thru the thermostat.

I have 12v on one spade terminal but do not receive the voltage thru the thermostat when turned "on". Shouldn't I get 12v at ambient temps to start the compressor?

Thanks

Unless you are in an extremely cold environment, the T-stat should pass 12V.

Try to connect the two thermostat wires & see if the compressor activates.

If it does you have a defective switch.

If it does not, check your wiring connections.

pm me as needed.

Best,

Gerry

[kuehl]

You can usually get a "typical" 911/930 thermostat pass through current in ambient temps above 55F. So, if you have the aluminum sensor (capillary tube) from the thermostat inserted inside the evaporator and its 55F+ in the room or area you are testing, when the thermostat knob is turned fully CW (clockwise) current should pass through it. You can test this with current to one of the spades and a VOM set to DC or its resistance (ohms) or continuity feature.

If the ambient 55F does not turn it on then remove the probe from the evaporator and warm up the last 3 inches of it with your hand. If that fails and you are sure you have good contact at the 2 male spades on the back of the thermostat with your meter, you might have a failed thermostat. Most failures occur when the aluminum sensor is damaged (bent, twisted, broke tube) and the refrigerant gas that is inside the sensor tube escapes. When the refrigerant gas expands or contracts it reacts on a metal below inside the thermostat housing. A thermostat that lost its gases requires a proper funeral. "Occasionally" the point contacts inside the switch might be corroded or fused. You could fiddle with it but then you know how that can turn out.

If you are using a 'used' thermostat that came with the knee pad vent panel chances are it might be a dead thermostat. Often the aluminum tube will break from storage, shipping, handling installation, etc. of the knee pad vent panel.

The circuit for the thermostat is simple: when the evaporator fan switch is turned on power travels to the evap motor (variable speed controller in your case) and as well to the thermostat. If you have power coming to the male spade on thermostat and not coming out the other, when the stat is turned fully CW, you might make the proper arrangements .... bag pipes and Becks are normal celebration.

Trouble Shooting the Porsche Air Conditioning System

[john maples]

I had installed a new thermo switch but did not test it until the knee pad vent panel was in place. Perhaps I kinked the capillary tube during the install or the new switch was defective.

A replacement has been ordered.

John

[kuehl]

Oh. I see.

Well. Now you know how to test a new thermostat before you install it.

And, be aware that bending the tube where it exits the switch box
too much can lead one to buy a third switch.