Parking Brake switch - trying to set me on fire... and the car

[DJF]

Hello Everyone,

I have a 1970 B body charger, everything in the car has been working fine for years, now this year I take the car out, to take my friends little sister to her school prom. All of a sudden smoke pours out from under the dash and I smell burning. I frantically pull all the wires out from under the dash, burn my hand in the process but the problem goes away. Only to brake down 3 miles down the road in traffic as the battery is out of power, probably due to some short that happened. Eclectic fans were running and the motor would turn over but not start. I don't know if anyone has experienced this but it is HELL!!!!! You feel like suck a loser with your nice muscle car there broken down. I get the car towed home and start to have an investigate.

The black cable going to the parking brake light switch is melted and broke, so my pulling on everything didn't solve the burning I guess the cable became its own fuse and that was that. After 5 minutes of being home I had the car running, now it had cooled off a little and the battery a some time to recover it was running. No parking brake switch connected, I remove the bulb from the warning light as that was of course constantly on.

Nothing can be too wrong here I think. I assumed the black cable had shorted out on the car body somewhere and caused this to happen. Today I took out the dash, the melted length of cable, all the way back, un-taped the loom to make sure there was no other cables damaged. I replaced the black cable taped it all back up, before I put the dash back in I think, I will check this out. Boom!! same thing happens again melted cable, the pone I just replaced.

What can be wrong?

I'm struggling to work out how this system works. I would think the switch is grounded to the body the black wire into it is a positive cable. When the parking brake is released this brakes the circuit and stops the light from glowing. What could be going so wrong that this cable is setting fire? A short somewhere it must be I think. But I don't know where. I wondered if it could be the switch but if it was the switch the light would just always stay on regardless of the position of the brake pedal. But I changed the switch as well just to make sure there wasn't some voodoo going on there that I hadn't understood still the same.

Any help is greatly appreciated.

[John_Kunkel]

The black wire supplies a ground for the warning light and, besides the parking brake switch, it is also hooked in parallel to the brake warning switch in the brake distribution block under the hood. Since this wire gets grounded by one of the two switches it should never have enough current flowing through it to catch fire when grounded.   

[DJF]

Thanks for the info. I checked the earth cable to the switch, it has no voltage going through it. This means there is not a short or connection there that should not be. I didn't put the dash in this time I held the two wires that go to the amp meter together and, what would you know everything is behaving as it should. No hot wires. The only other thing I had connected last time were the other lights in the respective places and also the wiper switch. Tomorrow I will try just connecting the amp meter and seeing if it works ok from there.

[nascarxx29]

Got pictures of what most fried in the wiring .I can examine it see what set it off.Usually a bad amp gauge will cause a dash harness fire.But the wire you find to been the hottest will have no melted plastic covering left to it.Heres a link to online 70 charger wiring    http://www.mymopar.com/index.php?pid=27

[nascarxx29]

I think I have some good clean undamaged 70 charger harness .New repos are 450-UP.Ive done my share worst case

[Dino]

That's scary stuff!  We've all been on the side of the road though, don't worry about that part. 

Ammeter comes to mind first as nascar mentioned but since something happened, twice in a row, there's only one thing to do and that is to take it all apart and find the root cause.  That harness you were offered may be a good idea if not a necessity. 

You have the cluster out and you can remove some more things to uncover more wiring but if it looks like a mess under there then just replace the entire harness.  If you only find a concentrated problem then you can trace it back and hopefully find the cause.

Any damage to the bulkhead connector?  Fusible link still intact?

[DJF]

Big thank you to everyone for the Doge wisdom. Thank you Nascar, seems you have been here or a similar place before, I didn't want to rewire the whole car, it seems like quite a job but a job I may well need to do. I have been at it all day, nothing to report really. I cant reproduce the problem today. I did play with the bulb holder yesterday and since then all has been well. I ran yet another new wire to the switch and its all been ok. My thoughts are perhaps it is was the bulb holder shorting out. How else would a ground wire get that hot, and only with the bulb installed? I have taken pics which I will post tomorrow. I'm unsettled that I cant put my finger on the issue. I just want this to be helpful to someone else. Also in my playing I must say, I hate the AMP meter, I want it because it is how it should be but the negative cable has a lot of heat in it. I can see how this is weak link in the wiring of these cars.

[nascarxx29]

Hope pictures and your post will solve this mystery issue and help the next guy.Ive done all my own cars..And a lot of local cars all makes .40 to 45 year old wiring is going to have its gremlins..If its isolated to that brake warning wire examine the whole circuit from where it gets its power and where it branches off

[nascarxx29]

black wire that goes to emergency brake switch also intersects with blue white wire.  low brake warning lamp and switch on brake metering block head light relay and opt key buzzer.To ballast resistor and alt field wire.Check for downline damage at these points that share this connection