I have a Flex-a-lite one and I will assume it is the same or similar. What you want to do is tap into the ground wire that controls the low speed fan relay. This is going from the PCM to the CCRM on a '96-97. When the switch closes due to a rise in temp it will ground out the wire. This will turn on the fan when the key is in the run position.
You failed to give enough details about your car to give any more specific directions.
Looks like the Tan and Orange wire on pin 14 labeled LFC relay is the one. And yes you just run a ground from one terminal and tap into the OEM wire. Do not cut the wire and use only the fan control. If you tap into the wire, then the PCM can still control the fan for when the A/C is on. If you cut the wire and disconnect it from the PCM then it will only work when the temp raises and closes the switch.
I tried to do the wiring but found something weird.
The Tan-Orange is normally ground. With A/C OFF (fan OFF), my test light turned ON when I checked the Tan-Orange line with the positive terminal of the battery. When I turn the A/C ON (fan ON), I can read 13 Volts from the line top the ground but if I try to test it with my test light (Tan-Orange line with Ground), the fan turns OFF. I did not install the wiring yet.
Now that I think about it the pre '96s used reverse logic compared to the post '96s. Just use a 1A fused hot in run wire, if you want the fan to run after the car is shut off use an always hot wire. Be sure to use a fuse.
Yes I have a voltmeter. What i mean is that if the Fan is OFF, the Tan-Orange is ground. A test bulb lights up if I tap one connector to the Tan-orange wire and the other connector to live hot line (positive battery terminal).
If I tap a fused live wire on it and provides a positive voltage on it, the fuse (I think) will blowup. It's like tapping a live hot wire to ground.
What should be happening is the PCM is grounding the coil on the relay. The relay coil receives it's hot wire from within the CCRM. By switching the ground on, the relay is activated.
Try switching the voltmeter to ohms. The check the T/O wire and the positive battery wire when the battery is disconnected. If you get a reading of 50 ohms or so them you are reading across the coil. This means the T/O wire is a ground wire.
Using the Ohmmeter, I get 2 results - 1942 ohms if I tap the Positive (red) terminal of the tester to the T/O wire and 502 ohms if I tap the Negative (Black) terminal of the tester to the T/O wire.
Anyways, using a fused wire and with ignition ON, I tapped the T/O wire to the positive terminal of the battery and IT WORKED! You're right! I adjusted the thermostat temp to my heart's desire and tested the A/C - no problem!
Thank you very much! Its really feels great to have it working!
It appears that Ford change it's logic when the OBDII came along. The '95-'96 was the break point. There is what is called sinking and sourcing logic when referring to DC switching. Sourcing is switching the the power before the load as in your case (load being the relay coil and actually switching the 12V+ wire). Sinking is switching the power after the load (ground of the coil or the 12V- wire).
The T/O wire is 12V+ that is coming from the PCM and feeding the coil on the relay. It is safe to do this and should only take a small amount of current so small wire can be used. The T/O wire is a small wire isn't it?
Be sure to fuse it.
I would set the fan to come on at 170F. This is what my '97 is set at and the temp of the coolant will be the temp of the intake above the rear head. The V8 SHOs are very sensitive a to heat.
FWIW I will swap my low temp t-stat out in the winter and put back in the OEM 190F t-stat.
Or you could just wire the whole works up so the fan runs when the ignition is on. My '89 Camry seems to be hooked up that way. The twin fans are always on, even at 0 deg F, and the heater still gets toasty warm. My wagon seems not to have fan function at this point, sending to shop tomorrow.