Service Engine Light on! [Archive] - Forum


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03-25-2009, 09:41 PM
I've had this on going issue with my 01' 3400 gagt.
The fault thats being set is my thermostat coolant temp sensor signal implausable or no signal. I reset the pcm with a scanner and/or pulling the 10amp fuse, yet only works from 50 to 200 miles and comes back again.
My cluster temp gauge drops to a default at 100' F and a few miles later the ses light is on.

Things I've checked or done:
*Replaced coolant temp sensor.
*Checked connector at temp sensor to be ok.
*Checked signal voltage to the temp sensor from the pcm to be at norm. 5v.
*Checked ground to temp sensor connector to be ok.
*Checked voltage drop across the connector at temp sensor when plugged in to be at around 3.5v.
*Checked connector which is behind alt. to be ok.
*Checked continuity for signal and ground wireds from temp sensor connector to the connector behind the alt to be ok.
*Checked continuity from the temp sensor connector to the pins at the pcm to be good.
*Conducted a wiggle test of all the connectors and including the ground splice that is shared by the map sensor which is under the upper manifold while monitoring the voltage signal at the pcm and watching the temp gauge, nothing changes.
*Switched between my DHP pcm and factory pcm and nothing changes.
*Draged tested the female pins on the connectors at the pcm with a male pin I got from a dealer which felt normal from all the other female sockets on both connectors.
*Checked battery with a load tester, and charging system to be good.

If I drive it for a while with the ses light on, it eventually goes off after a few hundrend miles and the temp sensor reads normally. But history repeats it self after another hundred miles or so.

Anyone work at a GM/Pontiac dealer that could help me with this?
Or just any help would be great.

04-04-2009, 10:40 AM
Humm that was a lot of help. Is there no real technicians on here or just shadetree mechanics?

04-04-2009, 12:00 PM
have you replaced the thermostat, when they stop opening or opening all the way you get a gauge that reads 100 and usually the cooling fans turn on, but not necessarily. If your stumped a new thermostat is 10 bucks and labor is 30min to 60 mins in your garage.

04-04-2009, 12:21 PM
Humm that was a lot of help. Is there no real technicians on here or just shadetree mechanics?

nope..your just like the rest of us, all shadetree mechanics

04-04-2009, 01:51 PM
A specific DTC would be a help here. Furthermore, monitoring the PCM's interpretation of current ECT value (with a proper scan tool) would help confirm if this is an intermittant problem or not, and would indicate whether said signal is absent - 0v - or too high; your description of 'implausible or no signal' tends to suggest either an open circuit or a short to power (possibly in excess of the reference voltage). PCM pin 13 feeds reference on the orange/black wire to the ECT, PCM pin 26 receives signal on the yellow wire. If this truly is an 'implausible or no signal' condition, find out what voltage is on the yellow wire at the time of code trigger, and go from there.