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Nuts & Bolts: Wiring Woes

Posted in How To: Tech Qa on May 10, 2017
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Photographers: Trenton McGee

Wiring Woes I am wondering if you could help with my 1982 Ford F-150. It has the 300ci straight-six and a four-speed manual. My problem is with the electrical. I have power to items like the heater, wipers, and so on, but when it comes to lights I have almost none. I have a couple of dash lights, and with the high beams on I have headlights, although the signal lights also come on and stay on without flashing. Occasionally I will have taillights but more often I have nothing in back, and I never have anything when the dims are on. I have checked and I have full voltage at the light switches and at the taillights. I have gone over all the wiring multiple times looking for bad wires and have found none. I checked all the fuses and they are all good. I have been told to look for a bad or missing ground, but I haven't found it. I am not sure where to go next and it is giving me fits.

Isaac
Via nuts@4wor.com

It sounds like you are probably fighting multiple problems. Some of what you describe could be caused by one or more shorts, some by bad switches, and some by bad grounds. Start by trying and chase down one problem at a time. For example, with the headlights, make sure there is 12 volts at the switch, then turn the headlights on and check the dimmer switch for power in and out. If there is power there, then check each headlight. If power stops at some point, you should now be able to back-trace the problem.

Beyond the above technique, are you absolutely certain that the wiring is solid? Critters, age, and hack jobs all take their toll on wiring. Did the truck have any kind of aftermarket alarm system in it, or has it had a bunch of different radios in it? Both of those things could cause some of the problems you describe. If the wiring is 100 percent solid, check the grounds. There should be a ground near each headlight and one or more for the taillights. Make sure they are clean, and be aware that the taillight housings themselves may be the ground so their mounting points should be clean.

If all of that checks out, start looking at and replacing switches. Look closely at the headlight switch and dimmer switch. It’s possible to check them for proper operation, but if you are not sure how to do that, just replace them. The headlight switch for your truck was under $15 and the dimmer is under $8 at our local parts store. Dimmer switches are especially susceptible to problems because of their position on the floor. Both could easily cause your headlight problems, while the turn signal switch could be the source of your parking light and taillight problems. Remember that while wiring problems can be frustrating, a methodical approach to diagnosing each individual problem is the best plan for success.

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