Bizarre Cap Brake Light Issue

Just started happening after perfect operation for around four months.

Background: 2002 2.7 Reg Cab. ARE cap, got used off of Craigslist this summer. Wired the cap's brake light to the left brake taillight. One of the cap's brake light bulbs is blown, but it was that way when I got the cap and I haven't gotten around to fixing it. Drove all around the country from 9/15-11/15, no issues whatsoever.

Here's what is happening:

1. Turn ignition to ON, headlights still off. Press brake pedal, and the cab 3rd light comes on, as well as the cap 3rd brake light. Cool.

2. Turn headlights on, cap 3rd brake light turns stays on with my foot off of the brake. Press the brake and the cap's brake light turns off, and the CAB's brake light goes on.

Whats goin on? I'm going to pull the taillight tomorrow to check the wiring, but it seems bizarre that this problem would appear out of the blue. Any hints would be helpful! Cheers!

 

Can you elaborate on what, if anything, is happening with all your rear lights (tail and brake).

 

Thanks for your reply! All other rear lights function normally, no blown bulbs or anything. I'm about to go take another look at the wiring. The weirdest part for me is that the cap light functions properly before the headlights are turned on. I don't know enough about what changes in the rear electrically when the headlight switch is turned on.

 

Without being there, it is really hard to troubleshoot, but I can point you to problem areas to look for. For DIY-er wiring, a lot of problems occur in incorrect wiring (thinking you are connected correctly but you are not). Another thing to look at is bad connections... this isn't always installers fault and can develop over time due to vibration or pinched wire. Last, and probably the most important but least understood, is grounding issues. Many weird problems can occur if you didn't ground your cap light correctly. Does it go to true chassis ground are is it grounded to a fiberglass cap? Without knowing more or seeing it myself, I would guess that it is a bad ground, just on the mere fact that it doesn't make sense and this is a common source of problems.

 

Is the Cap light LED? Do you have a trailer wiring harness?

Both of these things can lead to issues like this. If the Cap is LED, try unplugging the trailer harness from the tail light sockets.

I had similar issues when I upgraded all my lights to LED and had to get an LED specific trailer harness.

 

Turns out that the taillight cable and brake light cable were shorted together. I separated them and put electrical tape around each to insulate for the time being. The ground is kinda sketchy... its just attached to the ground on one of the bulbs. I couldn't find a good spot inside the bed to ground to... any ideas about that?

I've been wanting to get new housings and put LEDs in for the taillights too. Thanks again fellas!

 

Glad to hear you found it and it's a simple fix. The best way to ground the cap imo would be to run a ground wire with the power wire and run it back to where you tapped into brake light. There should be a bunch of white ground wires there inside light housing. I would splice it in there, that way you are at the same ground as the tail light system. This should ensure you have no further weird lighting problems. Happy trails.

 

I have mine tapped into the white w/black stripe for ground at the left tail-light. no problems for 4 years now. I would try and tap before the tail-light harness in case you need to replace the whole light at some point (broken or otherwise)