I don't believe that the transmission determines what the PCM references....I always thought it was oil temp for everything since any scanner will give you bogus readings for coolant temp.
My coolant temp sensor is at the top of the water pump adjacent to the hose going to the heater core - single wire. If I ground that wire out the gauge will peg. This is a simple resistance scaled sensor.
The transmission temp "should" be much cooler than actual coolant temp, on my 97 the transmission temp generally is ambient + 100 degrees, while coolant temp is around 220F or so.
Thanks! That is very helpful. I suppose that if the coolant is getting too hot the trans temp would go up, but probably when it started to affect trans temp that much you would be SOL.
I can access the oil temp from the PCM I suppose, which is probably a better secondary measure than the trans temp, given they are both essentially encasing/flowing through the block. I REALLY want to figure this out! It makes me nervous.
As a backup/"giving up" plan, I assume I could just install an aftermarket sensor into the location of the current sensor and wire a gauge into the cab? I found a few on Amazon but it wasn't clear if they had the same thread pattern.
I have tried grounding and testing the sensor every way I can think of, and it seems to be dead-in-the-water. No voltage to it, no reaction when grounding, nothing.
With regards to the manual/automatic difference I mentioned, I really didn't know, but I read the following info on a thread elsewhere that I found on the web:
This is listed for 03
Engine Coolant Temperature Sensor (ECT) - With Manual Transmission Only
The Engine Coolant Sensor is used as the primary input to the Electronic Control System to enable adaptive cooling. This provides a means of providing adequate cooling in severe engine temperature conditions. When ECT is greater than 107°C (225°F) , the fueling rate of the engine is modified to provide cooling protection and prevent engine damage due to overheating.
Engine Coolant Temperature Sensor - With Manual Transmission Only (front of engine)
The Engine Coolant Temperature (ECT) sensor is a thermistor device in which resistance changes with temperature. The electrical resistance of a thermistor decreases as the temperature increases, and increases as the temperature decreases. The varying resistance affects the voltage drop across the sensor terminals and provides electrical signals to the PCM corresponding to temperature.
If the PCM receives a high engine temperature signal from the ECT, it will adjust fueling rates to protect the engine from damage due to overheating.
Engine Oil Temperature Sensor
The Engine Oil Temperature sensor is a thermistor mounted to the oil reservoir whose resistance decreases as engine oil temperature increases. The Engine Oil Temperature signal is used by the PCM to calculate fuel quantity, injection timing, glow plug operation and exhaust back-pressure.
At low ambient air temperatures, and oil temperature below 50°C (122°F) , low idle is increased to a maximum of 1300 rpm to increase engine warm-up. Fuel quantity and timing is controlled throughout the total operating range to provide adequate torque and power.
An Engine Oil Temperature signal detected out of range, high or low, by the PCM will cause the PCM to assume an engine oil temperature of 20°C (68°F) for starting purposes and 100°C (212°F) for operating purposes. The Malfunction Indicator Lamp in the instrument cluster will be illuminated as long as the fault condition exists.