Sorry if this is a long post, but I want to get as much info out there as possible because I am stumped.



My alternator went out and I had to drive about 50 miles on the battery and minimal output. The truck stayed above 10 volts most of the way home, but dipped to about 8-9 volts by the time I made it home. The check engine light came on about halfway home and I thought it had to do with the charging problem.



After I changed out the alternator the code didn’t clear, so I read it with my scanner. P0118 ECT High input voltage. Ok so I change out the sensor and start the truck and the code is still there. Clearing it does no good. It comes back immediately.



In addition, my temp gauge quit reading as well. The truck runs great. There is a minor idle issue that may be related. I also changed the IAC valve.



I am thinking the problem might be in the ECM, but don’t want to replace that without some confidence that it will fix the problem. Would resetting the ECM make a difference or not?



I am at my wits end here and would appreciate any and all help.



Thanks

Steve

 

What are you driving? Engine? Trans? Year?

 

Sorry in such a rush to get so much info out, I forgot to get the basics in there.

1998 EC 4WD 5.2 Auto.

 

Disconnect the ECT, and with the ignition on, see what voltages you get on both wires. (check them individually to ground.) Should be five volts on one wire, and nothing on the other.

 

Before you do anything, make sure the battery is fully charged and don't try to charge it with the new alternator. An alternator's job is to generate enough power to run the vehicle, not charge a dead battery. You'll kill the new alternator making it try to do both. Running a battery down like that, unless it's a deep discharge battery designed to be drained, will damage the battery internals so it might be worthwhile to replace the battery with a new one as well.

 

1) Alternator died

2) Battery Died

3) Low voltage caused a high amp draw, damaging an internal 5 volt regulator

With that said, #3 is assuming the ECT uses a different 5V regulator from the throttle body sensors.

As mentioned, volt probe both ECT wires with engine running. Should get 5V and 0V.

 

I measured the voltages with the engine running and got 0V on the tan wire, 2.5-3V on the other wire (light blue I think).

The truck runs great except that it will occasionally die coming back to idle.

Steve

 

Input voltage is too low, which seems really odd given the code you are getting. Have a scanner? Plug in and see what tempt the PCM thinks the engine is.

 

On a ten mile trip to town the scanner ECT value NEVER changed from (-40F).

 

That's with wires unplugged from the sensor, correct? The reading you are getting is typical of a connected sensor wire.

I don't have my FSM handy, but you've got me thinking something popped in the PCM.