1998 Ford F-150 R rear brake light doesnt come on unless I h

Tiny
MCCOTTER
  • 1998 FORD F-150

Brakes problem
1998 Ford F150 V8 Two Wheel Drive Automatic 100000 miles

I've noticed while connecting my boat trailer lights that my left brake light wasn't coming on on both my truck and trailer. Couldn't be my brake light switchh because left light comes on but my right rear brake light will comeon when I have left blinker on. What am I not considering?

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Friday, May 14th, 2010 AT 1:33 PM

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Tiny
CARADIODOC
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Hi mccotter. Welcome to the forum. First you said the left light didn't come on, next you said it was the right one. Just a typo I suspect? Disconnect the trailer wiring and have a helper press the brake pedal, then turn on the turn signals. I think you're going to find the brake light bulb is different from the signal bulb. Most manufacturers have done this and it really complicates connecting a trailer. You will need to install an adapter harness, otherwise, the common bulb on the trailer will connect the left signal and brake bulbs together electrically, and that will connect the left and right brake bulbs to the right brake bulb. Some vehicles, especially trucks, have relays in the front specifically for "trailer tow" packages. You need to find those wires to connect to the trailer rather than splicing into the wires in the tail light lenses.

Caradiodoc

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Friday, May 14th, 2010 AT 2:24 PM
Tiny
MCCOTTER
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Boy did I make that question confusing! I shouldn't have mentioned trailer. Truck has tow package set up. Never had problems before. Sure if I can just get truck brake light (right) to come on problem solved. When I replaced right bulb plug it seemed to resolve matter until next day driving down road and wife following me noticed it had gone out again when applying brake. Could it be a connection going bad?

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Friday, May 14th, 2010 AT 3:16 PM
Tiny
CARADIODOC
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Yup. Brake lights are the easiest to diagnose because you have two sides to look at that are connected together. When one side works, you know the brake light switch, fuse, and wiring up to that point are good.

If you can catch the problem while it's acting up use a stick to hold the brake pedal down a little, (or use a helper to push the pedal), then use a test light or cheap digital voltmeter to measure the voltages on the dead bulb. If you have the newer bulbs with the plastic base, you will have to back-probe the socket through the rubber seal. I really hate it when people pierce the wire insulation to take a reading. That leads to corrosion in the future. There must be 12 volts on one wire and 0 volts on the ground wire. If the ground wire is broken, both the test light and the brake light will be half brightness when you touch the test light to it. The test light will complete the circuit so the brake light will work, but it will also add extra resistance so the brake light will be dim. That won't work the same way if you're using a voltmeter because they don't pass nearly enough current. If the ground wire is broken, the voltmeter will read around 12 volts on both the feed and ground wires.

If the voltages appear ok, check for corroded connections inside the socket.

Caradiodoc

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Friday, May 14th, 2010 AT 4:01 PM
Tiny
CARADIODOC
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Have to leave for a few hours. I'll check for any replies when I get back, so don't panic, I don't mean to leave you hanging.

Caradiodoc

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Friday, May 14th, 2010 AT 4:03 PM

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