Does anyone happen to know the range of resistance for the A/C Thermistor?
I may have to bypass mine with a fixed value but wasn't sure if what value was safe...can I just short across the leads to bypass?
Correct about the clutch cycling! My compressor would run for about 10 seconds and then turn off for about 18 then repeat.
Right now, I have the AC controller out of the car. I'm running my AC by jumpering switched +12VDC to the controller connector pin 2 (papajam diagram). The compressor runs whenever the car is on. I can get away with AC always on in the Florida heat!
The controller uses a comparator circuit to drive a relay that enables the compressor clutch. The two inputs to the comparator are the thermistor and the AC temperature control. With the controller in the circuit, I'm able to cycle the compressor by adjusting the temperature, so I'm pretty sure that that signal is correct.
The thermistor is the unknown. I'd like to bypass the thermistor if mine is bad. I'd be able to put the controller back in the car and use it to turn the compressor off and on. I've heard that the thermistor is hard to get to...gulp.
Thought I'd try to answer my own question here....
I hooked up a meter to read the thermistor resistance while I drove to/from work.
The thermisitor is not connected to the AC controller, I simply have the leads from my meter connected across the two thermistor wires. The A/C is on as I have jumpered 12Vdc to the compressor clutch.
Prior to starting the car, the thermistor read 10k ohms. As I drove to work the value increased to almost 40K ohms. With the car at idle the value decreased to 22k ohms. As I started to drive again, value again increased into the mid 30K ohms range.
I'm going to try this again at lunch as it is very hot today and the readings may be different.
Lowest value observed 8.5K (yesterday after work)
Highest value observed 40K (this moring on the way to work)
My guess is that the readings are in the 'normal' range and the thermistor is good.