Blowing fuses

[tanman722001]

I keep blowing the clearance/brake fuse.I checks the wiring under the truck and cant find anything wrong.Could it be the light switch causeing it.It only happens when I have the lights on.If I dont have the lights on it doesnt blow any fuses.Let me know Tony

[VetteVet]

If you are blowing the fuse only when the light switch is on it indicates that you have a short in the brown wire that goes to the tail lights or the license plate light or one of the marker lights. If you have the amber clearance lights on the cab then one of those may also be grounded. The parking lights or the marker lights in the front are part of that circuit as well and one of those may be shorted to ground.


All of these lights are fed by an orange wire from the fuse panel and run to the headlight switch. When the headlight switch is turned on it switches power to all of these lights.You can eliminate the dome and brake light circuits because they have power all the time without the head light switch being on. I don't think it matters if just the parking lights are turned on or if the headlight switch is on all the way.

If you can disconnect the connector on the firewall just behind the accelerator pivot it will isolate the rear half of the circuit and if you don't blow the fuse then the short is in that section. If it still blows then the short is in the front end of the truck. If you use an ohmmeter and check between the brown wire that goes to the rear of the truck and a good ground and you get very low or zero ohms then you have found the grounded wire and you just need to find the spot that's grounded. It may be along the frame or in one of the taillight sockets.

I've included a diagram of the circuits both back and front and the marker lights to help with the trouble shooting.
You can see the brown wire that feeds all the tail and marker lights plus the license plate light.



Below is the diagram for the front section. The purple wires in the front lights and the side marker light wires if grounded will blow the fuse.



I will have to look for the roof lights diagram so check what I've said so far and get back and let us know what you have found.

[VetteVet]

OK here is the diagram of the roof lights and as you can see they are also fed by the brown wires off the headlight switch. They are the 5 round lights at the front of the diagram. If you don't find your short in these steps then it's possible that the headlight switch terminal where the taillight wire connects may be grounded.


here is the headlight switch for our trucks after 1968. If terminal 3 or 5 were grounded then the second you turn on the headlights the fuse would blow.
BTW does the fuse blow immediately or at some point after you turn on the lights?

[tanman722001]

It blows some time after I turn the switch on.I can drive for a few mile before ity blows.I will start traceing wire and let you know what I found.Let the fun begin LOL

[mrein3]

I don't recommend this but an old-school buddy of mine says this is how you figure out what fuse is blowing:
1. Put in a fuse that is way larger than what is called for.
2. Turn on the circuit.
3. Replace the melted wire then put in the correct fuse.

I prefer to the less fire hazzard way VetteVet is talking about.

[raycow]

Originally Posted by mrein3

I prefer to the less fire hazzard way VetteVet is talking about.

So do I, but sometimes the smoke test is the only way you can find an intermittent short, which is what I think tanman has. Notice he says that he has to drive a few miles before the fuse blows.

Ray

[cdowns]

if it's blowing while driving
i'd get under the truck and look real carefully inch by inch at the wiring especially the area where it is held in place on the frame by a couple of clips,and also right near the transmission linkage

[crushermechanic]

mine did a similar thing and i found the harness column harness rubbed behind the panel

[sleepertruck72]

If you take a headlamp and long enough wires, hook two terminals onto the back of the headlamp, and the other end of the wire plug into the fuse spades, turn your truck on, turn your lights on since that is causing the blown fuse and wiggle the harness where you think the short may be. The headlamp will dim when you rub the bare wire(s). It works great and does not blow fuses

[haminator]

I had a similiar problem on my 69 a while back. I didn't have the dome light in and the taillight fuse would blow if the dome light switch happened to be on. Turns out a teminal to the dome light was grounding out. I have also seen tail light harnessses pinched between the bed a nd frame.