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Bad switch could make dome light misbehave

| Saturday, Feb. 2, 2013, 12:01 a.m.

QI 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. — C. May

AYour 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 liftgate and liftgate 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 liftgate 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 liftgate ajar switches, meaning each switch completes a circuit when the door or liftgate 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. 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.

Try this before seeking a heads-up repair shop: Wiggle and tug on each door, individually, while observing the dome or courtesy lamps, in addition to the liftgate and liftgate glass. While you do this, turn the key to on with all doors closed to simulate driving, and leave a window down to prevent accidental lock-out. With luck, you may determine which switch is the offender. The Explorer's door and liftgate ajar switches are attached to the door and liftgate latches, and can be serviced separately after trim panel removal.

NOTE: Remember Jim's Town and Country Minivan with the dragging front brake? Several readers suggested an additional possible cause: a faulty flexible brake hose. Internal degradation or injury could make the hose act like a check valve, holding hydraulic pressure within the brake caliper. Seems crazy, but it can happen. In such a case, loosening the line connection at the brake caliper soon after a brake application would yield a telltale spurt of fluid, as opposed to a few drips. Wear eye protection to play it safe.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.

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