Does anyone have the wiring diagram for an lt1 a/c system? I am just finishing up installing a hotrod air a/c system in my car. I have a pressure switch in the system and would like to wire it up to use the lt1 computer to read the pressure and also bump the idle when the compressor kicks on.
Any info would be appreciated.
Kory

 

http://www.lt1swap.com/pictures/100_1777.jpg

http://www.lt1swap.com/1994_f_body_w...ss_schemat.htm

 

Thats exactly what I wanted.
Thanks,
Kory

 

From what I can tell - 2 sensors control whether the PCM allows the A/C to turn on or not.

The the evaporator temperature sensor is a probe that is stuck into the evaporator. Its purpose is to tell the pcm when it reads at or below 36°F when the driver has the a/c switched on. When the pcm sees 36°F or below, it will shut off the a/c compressor and the pcm will throw a DTC 71 code that will NOT cause the service engine soon light to come on. After looking over a LT1 tune in tunercat, that is all I'm seeing the evaporator temperature sensor is used for in the pcm.
More information: http://www.mifbody.com/DTCs/DTC%2071.jpg
http://www.mifbody.com/DTCs/DTC%2071_2.jpg

I would recommend not using the evaporator sensor and bypassing it because
1) Its purpose mentioned above meaning its not really needed unless you think you are not smart enough to turn off your A/C during the middle of winter
2) extra cost to acquire the sensor if you don't have it
3) time wasted trying to figure out how to install it (and who knows IF it can be installed)
4) Its an extra thing that could fail and cause you to not have a/c until you fix it

Two possible theories of bypassing the evaporator temperature sensor:
1) Reprogram PCM. When the sensor is open or not connected, the lowest the PCM will read is 19°F. A stock PCM will disable the A/C at 36°F. Simply change the tune by changing "A/C Disable Evap Temp Threshold" in the "ECM constants table" from 36°F to something below 19°F, like 0°F. Also change "A/C Enable Temp Threshold", this is the value that tells the a/c to re-able after shutdown, to something like 1°F.
2) Hardwire in a resistor across the two leads that would go to the sensor. The temp sensor is a thermistor meaning its a resistor that changes with temperature. Read the resistance of the evap temp sensor at say 80°F with a ohm meter then wire in a resistor with that fixed value. The pcm will always read the same value because the resistance will always be the same. I should have access to a 95 T/A next weekend so could try to measure the resistance while I'm looking at other things on the car.

Note: The PCM information may vary for any LT1 pcm/ecm that isn't a OBDI pcm (94-95).

The other sensor, the pressure sensor, obviously is needed if you want the pcm to control A/C.

 

Back from the dead! Any idea what resistance was needed to get past the evaporator temperature sensor?

 

I never got around to finding a LT1 car and measuring the resistance of that evaporator temp sensor (or resistances?)

I think it may be a little more tricky than just measuring the resistance. There are 3 wires for it so it might be looking for a voltage. Seeing that it gets its voltage from the 5V feed off the pcm instead of the 12V battery feed, I'm guessing its looking for something under 5V. Maybe 4V would work?

Got a couple options. You don't know exactly what the pcm is looking for but its under 5 volts.

To get an adjustable voltage output of 4V or somewhere around there, a three-terminal 12V potentiometer could work instead of 2 resistors. Hook up 12 volts, ground, and just adjust the output until you get the voltage that the PCM likes and allows the a/c system to work. They don't cost too much at radio shack: www.radioshack.com/product/index.jsp?productId=2102789

Or try 2 resistors: http://www.bcae1.com/resistrs.htm
Look at the attached image. It uses a 4700 and 2200. If you would hook up a wire between the 2 resistors, you probably get the voltage that the pcm is looking for but its kind of a guessing game.