Green Monster
1/2 ton status
Help! 76 K5 with 350/350/205. Brake light fuse is blowing when I shift into reverse. I have a wiring diagram but can’t read it. Any ideas? Thanks.
CK5
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Brake light fuse popping when shifting to reverse
- Thread starter Green Monster
- Start date
Green Monster
1/2 ton status
That’s Bent77. The reverse lights work fine. Would they still work if I had a pinched or grounded wire?
Green Monster
1/2 ton status
I meant thanks. Looking at the diagram it looks like the brake lights are on a different circuit. They are “constant on” and the reverse lights are on when the key is on.
Shouldn’t, but obviously something is causing a higher amp draw
Only hits the fuse when both are activated?
do either one of the other lights come on when the other is activated
Green Monster
1/2 ton status
Yes the fuse blows immediately when shifting into reverse. The reverse lights work fine, fuse doesn’t blow. No other lights are affected. Just checked again, and the truck has to be running to blow the brake light fuse. When the key is on, but truck not running, brake lights work fine, reverse lights don’t come on. But there is a noticeable flash (off) of the brake lights when shifting into reverse, and you can hear a draw of power. Strange that the reverse lights only work when the truck is running, right?
rampage
3/4 ton status
Is it possible that the PO made some splices in the wiring near the bumper to hook up wires for a trailer?
Green Monster
1/2 ton status
Yes he did and I put everything back together. Maybe I have a wire crossed?
Dig in there and let us know
I am guessing that the voltage is higher when the truck is running, but I'm not quite sure how it would cause the amperage to get high enough to pop one fuse and make the reverse lights work.
I would start by disconnecting the rear harness at the plug just ahead of the rear bumper. (This is the one @Bent77 mentioned) Then if you pull the bulbs out of the tail lights and reverse lights, you can use an ohmmeter to check for shorts. You should also not see any continuity on the reverse light and brake light wires coming from the front of the truck, but there will be some reading on the taillight wire.
Maybe you will find a culprit this way.
I would start by disconnecting the rear harness at the plug just ahead of the rear bumper. (This is the one @Bent77 mentioned) Then if you pull the bulbs out of the tail lights and reverse lights, you can use an ohmmeter to check for shorts. You should also not see any continuity on the reverse light and brake light wires coming from the front of the truck, but there will be some reading on the taillight wire.
Maybe you will find a culprit this way.
Green Monster
1/2 ton status
Well I disconnected the plug on the right side, applied the brakes and put into reverse and the fuse remained intact. Plugged the left side back in and disconnected the right side. Brake light but no reverse light on left side. I only had one fuse left so I plugged the right side back in so everything was hooked up again, and removed the reverse light bulb on the right and left sides. Lost the brake light on the left side. So in summary, reverse bulbs removed and one right brake light working. It’s possessed.
So you need to find out where the problem is on the left side between the plug and the light housing, correct?
I wonder if the vibration of the engine running causes the problem area to short more than engine off. Maybe? I would say that you are to a point of pulling the wiring out or just partially out to examine it. Also look into the bulb sockets closely in case there is a short inside one, by the spring base which holds the terminals.
I wonder if the vibration of the engine running causes the problem area to short more than engine off. Maybe? I would say that you are to a point of pulling the wiring out or just partially out to examine it. Also look into the bulb sockets closely in case there is a short inside one, by the spring base which holds the terminals.
Green Monster
1/2 ton status
Yeah I’m to the point where I may just run all new wires.
I guess that you could do that, but I would caution you to check the bulb socket first if you plan to reuse it. The problem may be in there and you could spend time to install new wires and still have the problem. I personally don't like to add connections by splicing wires in, unless I have to. Repairing one wire would seem to be easier than replacing 3 or 4. And I like to actually find the problem, then decide a course of action. I feel like it helps me prevent the problem from returning.
Just a thought.
