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February 3, 2013

Bad switch could make dome light misbehave

I have a problem with the dome light on my 2008 Ford Explorer. It keeps coming on while I’m driving. It usually stays on for a few minutes then goes off. I had it to the garage several times, but they can’t find the problem.

Your dome and courtesy lights are controlled by the Explorer’s Smart Junction Box, or SJB, also called a generic electronic module. This very smart box receives inputs from the door ajar switches (one in each door latch), the lift gate and lift gate glass ajar switches, and instrument panel dimmer switch. Within the SJB, the interior lamp arbitrator can initiate either basic interior lighting, lighting delay, entry or exit lighting, alarm flash, and battery saver modes.

My hunch is you have a flaky switch in one of the door latches or lift gate that is seen as a request for interior illumination. Determining which switch would be easy, using a manufacturer-specific or high-end aftermarket scan tool.

These differ from the inexpensive OBD-II generic scan tools found at auto parts stores, as they’re capable of communicating with modules all over the vehicle and performing some neat tricks. These scan tools can look at a dizzying amount of input information, control many functions — rolling up and down windows, sweeping gauges, etc. — and obtain clear diagnostic trouble codes.

Since the Explorer uses normally closed-door and lift gate ajar switches, meaning each switch completes a circuit when the door or lift gate is fully closed, a faulty electrical connection somewhere could be interpreted as an ajar signal.

This would be more difficult to locate, but the scan tool input data can narrow the search to a particular area. The reason the interior lamps may come and go when driving, besides the faulty input, is the SJB will cancel the dome and courtesy lamps when vehicle speed exceeds 9 miles per hour.

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