Blown stop lights/hazard flasher fuse

[French demon]

Hi all

My stop lights/hazard flasher 20 amp fuse burns frequently (he did it twice in two days). Other than this, the car works perfectly and I have 13.8 volts on the voltmeter while driving.
What can be the trouble?


François

 

[halifaxhops]

You have to trouble shoot the wiring sounds like something is shorting to ground. My mopar has the wiring diagrams if needed,.

 

[Dartswinger70]

Yup a short somewhere check sockets also

 

[French demon]

I will check... but the fuse doesn't burn instantanely as it would do with a short circuit.
The first time it burned, I replaced it and my stop lights were back.
But the new fuse burned the day after...
It's why I find this breakdown a little bit strange.

 

[67Dart273]

First thing we need is the make/ model/ year of the car. Do you have a factory shop manual? The manual lists "what all" is on each fuse. In most manuals, this is in the electrical (chapter 8) right before the diagrams begin.

First thing I'd do physically, is simply open the trunk and examine the harness all the way from the lamps to the point it leaves the trunk. Then pull the rear seat and see if it is pinching the harness. Then pull the left rocker, and follow the harness through there for sharp sheet metal cuts, etc.

Not sure, the cigarette lighter may be on that fuse.

 

[French demon]

It's my '71 Demon. Yes, I have the factory manual and all wiring diagrams.
The dashboard wiring has been replaced last year.

I will check the stop lights wiring and sockets.

Speaking about the cigarette lighter is a good idea : I use it as an USB charging plug for my phone and my Bose wireless speaker. I didn't noticed if the USB worked after that the fuse did blow.
Maybe that the problem comes from the USB plug... too much amperage asked.
I will try to replace the fuse (not done yet as I don't have glass fuses anymore) and avoid using the USB plug
Thank you

 

[cudamark]

Remove the fuse, attach an ammeter inline in it's place to see what amperage it's actually drawing. If it's too high for that circuit and fuse, start disconnecting the items using that circuit, one at a time until you find the one drawing more than it should. Then trace that circuit wiring from source to load to find where the problem is.

 

[French demon]

@cudamark : I don't have an ammeter

Today, before replacing the fuse, I checked my USB plug and it works.
So, the cigarette lighter isn't on the same fuse.
It seems that on '71 Dart/Demon, this fuse only protect the stop lights, the hazard flasher and the dome light.

I replaced the fuse.
Everything is working normally, but for how long?

 

[Dartswinger70]

I'd check the sockets they get corroded high resistance condition etc.

 

[67Dart273]

Dome light circuit is an absolute suspect, then, as well. It also runs back there with the rear seat and get crimped, and other places where it goes. Always hot. (Powered).

 

[French demon]

I found the problem.

There is another "always hot" circuit protected by this fuse : the trunk lamp.
Today, as I was checking the stop lamp socket, I noticed that the trunk switch made some sparks. And the fuse blown.
The little "L" shape switch is old and visibly corroded.
In fact, the fuse blown each time I opened the trunk.

I disconnected the wire, and so far, the fuse didn't blow anymore

 

[67Dart273]

Thanks for coming back and letting us know. This is also why manuals are important, and finding out "what all" is on each fuse.

 

[French demon]

I didn't drive my Demon since the repair, so I can't be 100% sure that the problem is solved. But I found this scheme showing that the trunk lamp is connected in parallel on the dome lamp circuit. And as I'm encountering trouble with the "stop / dome lamps" 20A fuse, I think that there is a good chance that I have identified the source of the problem.

Have a nice day
François

 

[French demon]

Hi
Just to confirm that the fuse trouble came from the trunk light switch.
Since its deconnection, no burned stop lamps fuse anymore.

And this grounded light switch has also been the cause of a main circuit failure :
Fuses issue
I remember that this fuse failure came after opening/closing the trunk...