ECM- 5 volt reference sensor
 

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.

Key on engine off I can read both temp sensors fine. Start the engine and both ecm reads -40 for both sensors on the scan tool. I have checked all grounds and powers to the ecm. Provided another wire for battery ground at the ecm. Unplugged the coolant temp sender and shorted it out. Reads 304 degrees at that point, but start the engine and immediately drops to -40 again.

Why would these 2 sensors read accurately KOEO, but then go to max cold (full 5 volts) with KOER? Is my ECM shot? Engine runs, but thinks it is COLD out so I never enter closed loop, don't have lockup in 4th, and am running REALLY rich. Just dumping the fuel right now. Any thoughts or ideas to try?

Re: ECM- 5 volt reference sensor
 

Did the above tests cause the ECM to set the CTS hi & lo error codes? It should have, as either an open sensor or a shorted sensor while the engine is running should.

Once the error is noticed by the ECM it usually uses a default value. Usually in the 100* F area. It may be that the default value is set to -40.

RBob.

Re: ECM- 5 volt reference sensor
 

No codes are present in history or current. I think I have determined I have a bad ECM. With engine running ecm maintains a steady voltage to the tps and the manifold air temp, but the voltage bounces going to the coolant temp sensor. I shorted that to ground right at the ecm to bypass the harness and sensor and make it think it was HOT, and it still has issues. I have been so focused on those three sensors I wasn't paying enough attention until tonight, and now I see I actually lose read out to ALL sensors with engine running. IAC, O2, TPS, MAF, MAT, CTS etc. They all read with engine off, but none read with engine running.......