Just some thoughts Jay, not really anything definite by maybe this can shed a lil light on to some of the wiring diagrams.

Just looking those wiring diagrams again...
Engine coolant temperature has two pins, it's a 5volt reference wire, engine coolant temperature sensor is a thermistor type. Temperature goes up, so does resistance, thus the amount of voltage leaving the sensor is less. 5volt reference wires often only put out 4.7-4.8volts on the wire. Verify the 5 volts with key on, verify a different voltage (less) when plugged in to sensor (will require back probing while plugged in).

I'll add more tomorrow, I'm going to bed.
Doug

Doug, Thanks for the call this morning! This post is to just sum the fix up.

Doug identified a connector that holds the ground switch wires for the fans in the engine bay (A-26 I beleive). It was located under and to the right of the air filter. Upon locating this connection, I realized it was dangerously close to the edge of my cold air blockoff plate (Sheet metal) and so I removed it. Lo and behold, the plate had cut into both of the control wires and grounded them both. This was causing the fans to switch on any time the ignition was on. I trimmed the blockoff plate and rewrapped the wires and all is good now.

Thanks also to Jason M who suggested that I unplug the ECU completely to isolate the problem.

Glad you were able to fix this in a short amount of time. It's really a great feeling when you're able to fix a problem yourself or with a few pointers.

For those who want a little more detail this is what Jay and I were dealing with. As it said, both fans stayed on all the time when the key was turned in to the on position. There were some things we both knew about this system but they actually added to the confusion. We knew that the a/c system is designed to turn on the condensor fan if the pressure gets too high. The cooling fan will cool off the condensor thus dropping pressure in the system. So... was there a problem with the sensor circuit that made the computer think the pressure in the a/c lines is too high thus making the fans come on? One problem I saw was going to be in the wiring diagram. If I read it right, the a/c system circuit energizes the low speed and high speeds of the condensor fan but it really didn't look like it was attached to the radiator fan circuit too, just the condensor fan.

The next side of the system is the one most people know about. Engine coolant temperatures also determine when the radiator fan comes on. The problem was that the engine coolant temperatures were within spec, both according to the DSMLink as well as the gauges on the dash. The engine coolant temperature sensor circuit is described in my last post on here. So according to the wiring diagram and the sensor readings, the temperature sensor signal was probably NOT the cause for the fans to come on.

Well, after pondering some more things, Jay disconnected the ECU in hopes that the ECU was commanding both fans to come on. But after unplugging it the fans still came on. So the problem was in the ignition side of the circuit. Looking at the diagram again we found that there was a connector between the fan relays and the ECU. It was connector A-32, it's black in color, 6 sided and 12 pin-outs. The wires in this connector are for: Radiator fan relay (low speed), radiator fan relay (high speed), Condensor fan relay (high speed), ground wire for the radiator fan motor, dual pressure switch for the a/c system, voltage regulator in the generator, power steering pressure switch, and the a/c compressor relay.

Knowing what circuits were being affected, finding a connector that shared all these circuits and a little bit of elbow grease Jay was able to find the wiring problem and fix the problem. So there it is for those who wanted tons of details.
Doug

I have a 91 talon n/t just put in a motor, and as soon as I turn the key the fans both come on high, and dont shut off! Whats causing this, a faulty temp sensor? How do I fix it?

If you follow the path that I used to find my problem these would be the steps:

1. Notice it but do no think much of it

2. Start to do a mod to the car and identify that it is actually a problem

3. Stress about it ;-)

4. Start unplugging sensors that could control the fans (one at a time), after you disconnect each one, turn the key to On and see if that cures the problem. If you find a suspect sensor, check the shop manual to find testing procedures. Here are some sensors to check, not sure about location on a 1G:
a. Any Coolant temparature sensor (I have read that there are two on the 1G, one at the bottom of the radiator, not sure about the other)
b. Preassure check sensor for the AC system (if there is one on a 1g)

5. Check your fan relays to make sure they are not stuck in the on position. I swapped each of the relays out with a known good one to see if they were causing the problem.

6. Disconnect the battery to reset the ECU. Reconnect after a few minutes and see if the problem persists.

7. Unplug the ECU harnesses and then turn the key on, if the fans are still running then it is not likely any problems prior to or with the ECU (this would include sensors and the wires that connect to them). If the fans are no longer running then either your problem is something that is telling the ECU to turn on the Fans, or the ECU itself could have a problem.

8. Identify which side of the ECU you are working with per number 6, and then work your way from there.

9. It helped me to find an expert that could trace wiring diagrams and point me to connection points for the sensors/fan relays. If you do not know an expert, get our a Shop manual or backup manual CD and start looking at the wiring diagrams for cooling.

10. What I finally found was a couple of pinched wires at one of connectors that were grounding out and turning the fans on.

11. Replace any sensors, fix any broken or pinched wires, and post your results.

12. If you contacted an expert, buy the expert dinner for his/her help.

I hope that this is somehow helpful. Basically just test one piece of the system at the time so you can eliminate each point as the cause. Good luck to you.