Rick's Free Auto Repair Advice

Engine Overheats – GM vehicles

Engine Overheats, no radiator fans on GM vehicles, Chevrolet, Impala, Pontiac, Buick, Cadillac

GM uses a pretty nifty technique to control the radiator fans on late model vehicles. The system consists of two fans and three relays. When the powertrain control module (PCM) detects the need for cooling from the engine coolant temperature sensor (ECT) at 223 degrees or refrigerant pressure above 190-psi., it provides a ground signal to the low speed fan relay. Power flows to one fan, and the ground for that fan flows to the second fan. So both fans run, but they run at a low speed due to splitting the voltage and an end result of 6-volts to each fan. If that doesn’t provide enough cooling to bring down engine temperature or refrigerant pressures, the PCM provides ground to the three relays, thereby providing a full 12-volts to each fan so they can run on high.
Obviously, if you run into a situation where the fans don’t run or the engine overheats, you should check the fuses and the relays. But GM has issued a service bulletin PIC3045B to alert technicians to a possible cause of good relays and fans not powering up. The body control module always monitors system voltage. It’s programmed to shut down power hungry accessories if it determines that the vehicle is not providing enough power to keep the battery at minimum voltage. This is an electrical “limp home” feature. It’s designed to get you enough power to get home or to the shop.
In some cases, the BCM can falsely detect a low voltage situation and send a digital signal to the PCM to shut down the radiator fans. If you don’t have a scan tool to check BCM status, this could drive you crazy and cause you to start replacing good parts.
So if you’ve checked out the relays, wiring, and fans and still don’t get activation, get your mitts on a scan tool and check out the BCM. If the load management status indicates “ACTICE” it is in power management mode and that’s what’s causing the problem. You’ll have to replace the BCM.

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© 2012 Rick Muscoplat

Posted on by Rick Muscoplat


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