Question: The low coolant light in my 2000 Jeep Grand Cherokee comes on randomly. I hear the chime go off and then I see the "Low Coolant Level" message on the VIC. Sometimes it only happens for a second, and goes off. I replaced the switch in the overflow tank and it still happens. Is there another switch?
Answer: There's no switch in the radiator. The low coolant level sensor in the coolant expansion tank is intended to be the sole signal that activates the message at the Vehicle Information Center. The low-level sensor circuit itself is simple: two wires, one to ground, and the other to the Body Control Module. When the coolant level is low, the sensor completes the ground circuit to the BCM. But after that it gets more complex. The BCM receives the signal and sends the information though a serial data line to the VIC. I would start with the simple stuff. Disconnect the sensor at the tank. If the message still appears, that eliminates a faulty new sensor. Also carefully trace back the sensor wires looking for any sign of contact with a metallic object. It's very possible the insulation on the wire leading to the BCM has rubbed through, producing an intermittent short to ground, most likely under the hood. If they don't see anything with a visual inspection, I would remove the sensor wire terminal at the BCM connector. If the message stops, we know that wire is shorted to ground -- somewhere -- and the circuit needs to be inspected further. If the message still appears, it's probably going to require a qualified Jeep tech with the proper tools to figure it out.
If you have a technical question regarding your pickup, SUV, or van, feel free to contact Alex, a master technician with the National Institute for Automotive Service Excellence. Send a letter to him in care of Truck Trend Garage, 831 S. Douglas Street, El Segundo, CA 90245, or e-mail us at email@example.com. Please include the VIN with your question. Due to the volume of questions received every month, we cannot guarantee that everyone's question will be personally answered or will appear in the magazine.