ECM temperature spike

Hi KASEKENNY1. Thanks so much for your advice. Before I bite the bullet for a replacement ECM, I decided to do some more diagnostic work today. I used a potentiometer in place of the CTS, so I could "manually" send resistance readings to the ECM. If I was real careful (the potentiometer is very sensitive), I could actually dial in resistance readings that translated to temperatures the ECM wasn't normally reading. I found that a resistance that translates to about 229 degrees turned on the cooling fan, and if I dialed it back to 220 degrees, the fan turned off. I also checked signal voltage to the CTS and got a reading of 4.85v, which seems to me to be about where it should be. My next step is to carefully inspect connections/condition of the ECM itself for any obvious compromises. I'll keep you posted.

I wanted to follow-up on this topic. I removed the ECM, opened the case, removed the MEMCAL chip, and the ECM circuit board. I inspected the chip, the circuit board and connectors and found no obvious areas of concern, so I reassembled and reinstalled. I then used my potentiometer (KOEO) to "walk" up and down the temperature scale several times up to about 260 degrees. Next I removed the potentiometer, reconnected the CTS, started the car, and amazingly, it now behaves normally (as far as controlling the cooling fan). It starts near ambient, heats up (steadily) at a normal rate until it gets to 229 degrees. The cooling fan then comes on, and runs until it brings the temperature down to 218 degrees. I let it cycle several times and tested again today from a cold start and all seems good. I guess I either had a poor ECM connection or I just had to reteach the ECM how to progress steadily through temperature readings. Many thanks again for your helpful insight.