Engine control computer

12 volts is applied to the idle air control solenoid on the same circuit that supplies the injectors and other parts. Idle speed is controlled by the Engine Computer bringing the other side closer to ground.

Ford had a pile of trouble with the coolant temperature sensors in the early '90s. There's only one part inside it so they're normally extremely reliable, except for these. The resistance can bounce around confusing the computer into thinking it needs to raise engine speed in response to cold air coming in. That's a good suspect to start with.

I learned this after a former student had this problem with his Taurus. I never taught my kids to measure sensors except to verify that they found the cause of the problem and to learn how the circuits work, but in this case a resistance reading showed the value to be bouncing all over the place. That is extremely unexpected since temperature sensors have only one part inside. That is a temperature-dependent resistor, known as a "thermister". I have to guess there was a mechanical break in the resistor or one of the wires attached to it. After I mentioned that to a friend who worked at a Ford dealership, he just shrugged his shoulders and said they ran into that very often. For that reason, suspect that sensor first before looking for other causes of your idle speed problem.

If you have a temperature gauge on the dash, you should have two coolant temperature sensors. The gauge uses a sensor with a single wire. You want to look for the Engine Computer's sensor which uses two wires.

If the CTS doesn't solve the problem, you'll need a scanner to view live data to see what the computer is seeing when it reacts by raising idle speed. You'll also be able to see if the computer is actually requesting that higher idle. If it is not, suspect a vacuum leak. That will cause a high idle speed without a corresponding increase in power.

Come from a old school, first thing we would check is the wiring, make sure there is not a frayed or wire that is grounding out. That would send a false signal to the sensor.

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Sometimes it takes time for the new computer to learn the transmission, here is a wiring diagram, you can see the wires come right off of the computer, have you done a pin to pin check?

Sorry here you go, please let me know.

I came from an old school, so do not know if this will work, pre 80s cars you could reprogram the computer by just unplugging the positive terminal for a second then hook it back up. A lot of people would do this prior to bringing the car in for emissions check, then somewhere in the 80s they stopped it. Then one would have to drive the car about 40 miles until the computer would read everything and reset. With another ecm you put in, it might work in reading the engine and transmission. As I said I come from a school that had flat head engines and no emissions, so I don't know if it would work on your car. Ken has more experience in that than I do.

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