Key on engine off I can read signal on a scan tool from the air temp, coolant temp.... and other sensors. As soon as I start the engine both temp sensors drop to the bottom and read -40. Of course at this point it thinks it is freezing out and sits at high idle, runs rough, and dumps the fuel. Shut the engine off but turn the key back on and I can once again read my temp sensors accurately. 5 volt reference coming out of ECM is good, all powers and grounds at ECM appear to be good. Unplug my coolant temp sender and short it out it reads max at 304. Start the engine and it still drops to -40. Sensor unplugged I have 5 volts on my yellow, plug it in it drops to 3.3 or so. Start the engine and it jumps all over from .5 to 2.1 but ECM constantly reads -40. Any ideas? I see in the ECM there is 5 volt reference and it goes through a resistor before exiting to go to the temp sender. Should it maintain 5 volts even with the temp sensor plugged in? What controls 5 volt reference in the ECM?

 

Looking at the diagram, I believe at the R input terminal to the ECM is constant power with which the ECm will use a micro regulator to produce the initial 5v signal and the reference signal. It looks like the coolant temp sensor is a two wire model, one should be input voltage of constant 5v, and the other is signal through the thermal resistor in the temp sensor. If while plugged in and jumpers into the sensor read 5v on one side and a non-constant on the other (jumps around), I would suspect the sensor. If while plugged in and engine running you are seeing more than 5v in, you have an issue with the ECM or a short in the wiring harness feeding into the coolant temp circut.

You should also see the MIL on, and a code 15 when you pull it if the temp sensor is having issues.

 

The ecm has good voltage and ground. With the sensor plugged in the voltage and constant (or supplied ground) are steady with the engine off key on. When you start the engine the 5 volt reference is what is not steady. The reference is what jumps around. I am wondering if anyone knows of anything other than the common grounds and powers to the ecm that can affect the 5 volt reference feed sent to the sensors....

 

Correction. ecm provides ground to tps, coolant temp sensor, and manifold air temp sensor through one common circuit. Ecm provides 5 volt reference that remains at 5 volts to the tps at all times. There is 5 volts used as reference for both temp senders, but the voltage drop from 5 volts is what the ecm uses to calculate the temp. Why would these 2 sensors read accurately KOEO, but then go to max cold (full 5 volts) with KOER?