I have an 86 IROC, 305 TPI. My alternator recently shorted out on the valve cover and started a small fire. No prob, new alternator and wires fixed it. Now I am getting a code 15, Low Coolant Temp on my ECU. I checked for an open circuit on the CTS, it checks out good. Voltage side is right where it should be. While checking wires I found that almost all of the wires that should not read anything unless the ECM tells them too are reading .3 volts. I am thinking that my alternator shorting out may have ruined my ECM. Also, when I run a diagnostic check my fan comes on while the jumper is in and doesn't turn off until I remove it. Is this normal, is my ECM bad, or what circuits could cause this?

 

the fan turning on in diagnostic mode is completely normal.

 

Thanks for clearing that up. Any idea on the .3v in the wires that shouldn't read anything according to what I have seen? I was running great until my alternator shorted out.

 

Good to go. If anyone cares, after checking all of my wires and signals with a multimeter and everything else I could think of I found that the problem was at the sensor itself. I put in a new sensor already so that wasn't the problem. I pushed in on the plug and Wa La. The plug is not making good contact. Temporary fix: I took off the rubber seal and pinched the sockets a little with the needle nose. Long term fix: I ordered a new plug.

 

i believe the make replacement pig tail connectors for these.standard,everco etc.congrats on finding your problem!