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I am trying to help a friend of mine with her truck. It has 168,000 miles. About three weeks ago she reported that her cooling fans stayed on and ran her battery down. Battery would not hold a charge so she bought a new one, which I installed. I did not find any codes at that time, and the cooling fans came on and shut down at the proper times, all seemed fine after I tested it multiple times.
Fast forward three weeks to today. She reported that her truck's cooling fan again stayed on after engine shutdown. She pulled the two 40A fan fuses to get the fans to shut down, and then messaged me hat this intermittent problem had cropped up again. By the time I arrived the truck had cooled down again and engine was cool to the touch. P0128, P0480, and P0691 all stored in PCM and displayed on my scan tool. I installed the 40A fan fuses again and started the truck. Both fans came on full blast despite the engine being cool and the temp gauge pegged low (<160F). My scan tool showed the Cooling Fan command value to be 100%. Fans stayed on full blast through warm-up and at engine operating temp.
I shut the engine down, removed the fuses, and let the engine cool down a little bit but not all the way down. This time the cooling fans did not come on despite driving around for > 20 minutes and bringing engine back to operating temp (>210 degrees). Thermostat apparently opened when I was driving highway and temp gauge dipped down before rising again to 210. Cooling fan never did come back on. With engine at 210F I checked the voltage on the back-probed ECT signal wire and that measured .44v.
This is as far as I got with the time I had before I had to leave.
Any ideas on where I go from here with this truck diagnosis?
Did you check the fan control wire for a short to ground. Was the fuse reinstalled properly after removal? It is odd that that the fans don't come on after reinstalling the fuses when the ECT sensor shows 110 deg F.
P0480 and p0691 are fan circuit codes and may have been set by pulling the fuses to stop the fans.
P0128 indicates the engine took too long to reach temperature.
When this code sets on GM models, the fan defaults to high and the temperature gauge may stay at it's lowest position.
Assuming the coolant is full, this code generally sets when the thermostat needs to be replaced.
I like to disconnect the coolant temp sensor and make sure the voltage PID goes to 5v to rule out resistance on the temp sensor signal wire, then change the thermostat.
Thanks for all the great ideas, guys! I just got the truck today after the owner left on vacation for the week so I’ll be spending more time with it. Driving it today, the fans never did come on with an hour of driving. When stopped at long red lights temp gauge would go to 210, driving at highway speeds temp would decrease to the next lower hash mark which I assume to be about 185. So there is a “fans not coming on” problem, as well.
When stopped at long red lights temp gauge would go to 210, driving at highway speeds temp would decrease to the next lower hash mark which I assume to be about 185. So there is a “fans not coming on” problem, as well.
Reaching 210F wouldn't be that unusual for this engine, in my experience. Keep in mind that the OE thermostat temp is 207-212F. If anything, that's just another indication that the thermostat is failing.
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