Bad switch could make dome light misbehave
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 firstname.lastname@example.org; he cannot make personal replies.
Show commenting policy
TribLive commenting policy
You are solely responsible for your comments and by using TribLive.com you agree to our Terms of Service.
We moderate comments. Our goal is to provide substantive commentary for a general readership. By screening submissions, we provide a space where readers can share intelligent and informed commentary that enhances the quality of our news and information.
While most comments will be posted if they are on-topic and not abusive, moderating decisions are subjective. We will make them as carefully and consistently as we can. Because of the volume of reader comments, we cannot review individual moderation decisions with readers.
We value thoughtful comments representing a range of views that make their point quickly and politely. We make an effort to protect discussions from repeated comments either by the same reader or different readers.
We follow the same standards for taste as the daily newspaper. A few things we won't tolerate: personal attacks, obscenity, vulgarity, profanity (including expletives and letters followed by dashes), commercial promotion, impersonations, incoherence, proselytizing and SHOUTING. Don't include URLs to Web sites.
We do not edit comments. They are either approved or deleted. We reserve the right to edit a comment that is quoted or excerpted in an article. In this case, we may fix spelling and punctuation.
We welcome strong opinions and criticism of our work, but we don't want comments to become bogged down with discussions of our policies and we will moderate accordingly.
We appreciate it when readers and people quoted in articles or blog posts point out errors of fact or emphasis and will investigate all assertions. But these suggestions should be sent via e-mail. To avoid distracting other readers, we won't publish comments that suggest a correction. Instead, corrections will be made in a blog post or in an article.
- Education tech firm Acrobatiq does software to supplement college learning
- Tesla investors leery as shares, targets plummet
- Chesapeake Energy appoints Brad Martin chairman of the board
- CMU showcases its lengthy list of fledgling companies at venture event
- Budweiser brewer AB InBev wants to take over SABMiller for $108.2B
- As craft fades, personal touch helps Northway Shoes & Repair thrive
- Kombucha producers resist call to indicate alcohol content on labels
- UAW locals compact Fiat Chrysler voting to 2 days
- Safety of credit cards up to banks
- Wabtec buying Australian sensor maker Track IQ
- Class action lawsuit in California seeks Volkswagen buyback