Truck Trend Garage: Chevy 4x4 1500 Series Light's Out
Question: I have a Chevy 4x4 1500 Series with the 5.3-liter engine and an automatic transmission. When I shift from Park to Drive, the tow/haul light comes on. When I go back to Park then into Drive, the tow/haul light goes out. When I press the button on the end of the shifter, the tow/haul light will not work. The check-engine light isn't on, and when I used a Snap-on scanner, there were no trouble codes.
Answer: The tow/haul switch applies a momentary ground to reference voltage coming from the Body Control Module. When the BCM sees this signal for the first time since the ignition was turned on, it sends a signal to the Powertrain Control Module, telling it to activate the tow/haul mode at the transmission and turn on the tow/haul indicator at the instrument panel. When you hit the tow/haul button a second time, it momentarily grounds the voltage from the BCM again and turns everything off. By default, tow/haul is off when you start the truck; the light coming on when you put the transmission in Drive is a bit strange. There may very well be a fault that doesn't turn on the check-engine light or set a trouble code. First thing to do is attach a scanner and view the tow/haul switch input to verify the correct ground from the switch, along with proper system activation. There's a detailed diagnostic chart in the service manual that will take you through a complete wiring check procedure. It could be the switch, the BCM, PCM, poor connection within involved circuits to each, or possibly an incorrect sensor input to the BCM, PCM, or instrument panel. I would check the tow/haul switch first. It may be partially applied and supplying a varying ground to the voltage from the BCM. A quick check would be to disconnect the switch, and see if the problem still exists when shifting into Drive.
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