Electrical problems (I think)

Install a grounding temp sensor in the block or lower intake to run the fans. Get that load off those electronics before you do more damage!

So you tapped into the signal return for the ECT or something? Is that what you're saying?

Sounds like it. If its in the start ciruit and you damaged it, then most likely you damaged the computer. Have you got a buddy who will loan you his computer for an hour so you can swap it out and either determine your problem or rule out the computer?

The signal wire from the ECT provides temp readings via measured resistance to the ECM. If you completed a circuit through this wire such as with the fans, you more than likely have damaged a number of capacitors in the ECM. On a newer vehicle, I would say read the datastream via a scan tool. '90 Mustangs have no datastream and a breakout box will only give you an ohms reading from the ECT. A scan tool on a newer vehicle would tell you the reading the computer is using, not just the reading the ECT is sending.

I understand what you did. You thought the ECT was an "open/closed" switch similar to a ported vacuum switch. The switch opens or closes when it reaches a certain temp, rpm or speed depending on the switch.

The ECT provides resistance thru a ground to the ECM. The higher the temp, the higher the resistance. Some vehicles have an ECT that works just the opposite. Either way, another circuit terminating on the signal wire will send a spike(either power or ground depending on the type of circuit being terminated) to the ECM.

EEC-IV and EEC-V systems are some of the toughest out there, especially for the time period from which they were produced.

The high idle is a result of a combination of two things. First, whatever memory is left in the ECM is running in open loop/limp-in mode. This causes the ECM to use a predetermined set of readings to operate by, not the actual live readings. Next, the ECM is, for lack of a better term, confused. It may be "seeing" a reading of -40F from the ECT no matter what the ECT is actually sending. Negative forty is default for most ECT sensors. Combine this with normal readings from the other sensors and you get an ECM that is torn in two directions. It will then try to do the only thing it knows, trust the ECT and thus try to build heat in the engine.

Bottom line, as an auto technician, I would tell you to start by trying another computer as stated above. I would also replace the ECT BEFORE trying anything. The ECT more than likely recieved no damage, but if it did it possibly could send another spike thus ruining the test ECM. Get a good ECT such as a Motorcraft. Cheaper parts lines tend to show weaker readings and not be as durable.

Hope this helps. Keep us informed.

arent there 2 coolant temp sensors...one for the gauge and one for the comp?

Yes. There are two coolant temp sensors. One next to the thermostat housing and one on a heater hose. Both use almost identical methods to provide information. The sensor for the gauge also provides measured resistance to ground. The one for the gauge is farther away from the front of the vehicle, I was assuming it to be less likely used in this case than the sensor in the very front. Had Brammage attempted the same with the gauge signal wire, the most that could have possibly happened was the coil in the gauge being destoyed. No problems other than the gauge being non-functional would have resulted from this.