Under the Hood: Check relays when radiator fan acts up | The Republic
QUESTION: Our daughter borrowed our 2000 Jeep Grand Cherokee Laredo and when she returned home in the mountains, she noted that the electric fan by the radiator was still running. Evidently it ran for a couple of hours and eventually discharged the battery. The next day it wouldn't start. The AAA tech that arrived jumped her battery and "reset the code on the computer" and the problem went away.
We've owned the Jeep since new and the fan has never continued to run after the vehicle was shut off. This is possibly because we live in Santa Cruz, Calif., which generally doesn't get that kind of heat, but it's a little puzzling and somewhat disconcerting. The car has functioned perfectly since. I hesitate taking it to an automotive center without a clear problem, and presumably the AAA tech erased any codes, which would leave little to go on. Any suggestions?
—Mike Malbon
ANSWER: Mike, I checked out the wiring diagram for this circuit, which is my favorite way of obtaining a clear overview of what components are involved and who controls what. The Jeep's radiator cooling fan is controlled by the powertrain control module, or PCM, via a relay, based on information taken from the engine temperature sensor. The relay serves as a heavy-duty switch for the large electrical load of the fan. When coolant temperature rises above 217 degrees, the PCM commands the relay to turn on the fan, then turns it off as the temperature falls below 208 degrees.
My first hunch was the fan relay contacts had became stuck in the closed, or "on," position after a period of use, in spite of the appropriate "off" command from the PCM. This is rare but can occur because of deposit buildup on the relay contacts. Upon further research, I found the Grand Cherokee uses a solid state relay, while the base Cherokee continues to employ the electromechanical one, foiling my theory.
I'm thinking your solid state relay may have begun to fail. I've seen power transistors conduct electricity continuously when shorted and the greater duty cycle it endured with mountain driving may have led to excessive heat and erratic operation. I'm focusing on the relay because PCMs typically don't make mistakes, and I don't believe key-off fan operation is part of the game plan. The other likely scenario is the dark blue/pink PCM command wire leading to the relay is intermittently shorting to metal, simulating an "on" command. The PCM would identify this fault (code no. P1491) and illuminate the "Check Engine" light, whereas it might not identify a shorted relay.
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ANSWER: Mike, I checked out the wiring diagram for this circuit, which is my favorite way of obtaining a clear overview of what components are involved and who controls what. The Jeep's radiator cooling fan is controlled by the powertrain control
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