Chevelles.com banner

1 - 14 of 14 Posts

·
Registered
Joined
·
875 Posts
Discussion Starter #1
While inspecting my dash harness, I noticed a burned wire coming from my headlamp switch connection. The insulation is burned/melted for as long as I can trace the wire before it disappears into the harness wrap. The wire is, according to the schematic in my assembly manual, 20 gauge brown and runs forward thru the fuse block to the parking lamp/direction signals and/or rearward to the rear parking lamps/directional signals. As best I can tell, this is the only wire that is damaged. My forward lamp harness and engine harness are new, so those wires offer no additional clues/information. My question is, what caused this wire to burn and what precautions can I take to prevent it happening again?
Many thanks for the help...

For what its worth... The guy I bought this car from had crammed foil into several of the fuse holders. Maybe this somehow contributed? My ignorance of automotive electrical circuits is evident I suppose. I bought the car sans the engine and tranny and have spent the last four years and countless thousands of dollars getting it ready to breath again.I'm hoping I don't need to buy a new dash harness as well. While I am anxious to fire it up, so to speak, I really don't want any surprises.
Patrick
 

·
Registered
Joined
·
3,091 Posts
Patrick, buy a dash harness, that thing is trash. The foil is a "circut breaker" that doesn't ever trip, it just burns the harness down.Why chance it with all the time and $$$ invested.
 

·
Registered
Joined
·
7,762 Posts
The brown wire is an output wire from the switch. It is powered when the switch is on and runs the running lights (tail lights) on the car. Runs both the front and rear bulbs. I would look around near the connector that the intermediate harness plugs in. This is the harness that plugs into the dash harness above the brake pedal area. The harness runs under the carpet and provides power to the rear bulbs. If the brown dash harness wire is burnt as bad as you describe, suspecting that the brown wire, in the intermediate harness, is also damaged IF the problem was caused by a shorted REAR bulb.
If the intermediate harness isn’t damaged, perhaps you already found the problem. Was the forward lamp harness brown wire damaged before you changed it out?
At a minimum, the dash harness needs to come out and be examined. The brown wire and any other damaged wire needs to be replaced. Good chance the contacts in the headlight switch are fried.
 

·
Registered
Joined
·
875 Posts
Discussion Starter #4
Thanks John...
I will examine the intermediate harness this evening. I removed and replaced the forward lamp harness earlier because it had been hacked and modified to allow the use of a passenger side alternator. I did not notice any burned wires at that time, but then, I was not looking for burned wires either. Since I still have that old hacked harness, I'll examine it more closely this evening.

When you say that a shorted rear bulb may have been the culprit for the burned wire, what exactly does that mean? Is a short the same as an open circuit or closed circuit? Again, my electrical naivety is evident. Is the fix, a new bulb, a new socket, a new wire, or a possible combination? Will a continuity check verify the presence or absence of a short?
In any case, I'll heed your advice and unwrap the dash harness for a thorough once over. Thanks again...

Patrick
 

·
Premium Member
Joined
·
6,843 Posts
[Is a short the same as an open circuit or closed circuit? Will a continuity check verify the presence or absence of a short?]
A short is a short. A complete circuit goes from battery +, through a load, then returns to battery -. An open circuit is usually a broken or unplugged wire or component--the current leaves the battery, but doesn't get to or through the load to get back to battery -. A short happens when the current leaves the battery, but then goes directly to ground (-) without going through the load. This creates heat, smoke, and all sorts of mayhem. You can check for a short with a continuity tester. Disconnect the battery cables. Disconnect the wire you wish to test from its load (switch, bulb, etc.). Attach one end of the tester to the wire, and touch the other end to ground (body or black wire). If you show continuity, then you have a short, (if it's a power wire and not a ground) if it shows open circuit, then you don't. This is a VERY simplified explanation, but it should help you understand circuits better.

BL
 

·
Registered
Joined
·
7,762 Posts
Adding just a little more:
Normally when a short occurs, the worst burn area is closest to where the wire shorted. Everyone is guessing around what happened but you are the only one that can see.
If the switch shorted, the burnt wire will be worse there and start to look better as you look down the wire. If the short is further away from the switch, the worst burn area is somewhere else.
If my first guess is the rear bulbs or sockets the wire will be burnt all the way back.
Dennis had a point about a ruined harness. However, it might be savable if you take it out and examine it. Good chance the brown wire got hot and damaged other wires in the bundle.
 

·
Registered
Joined
·
875 Posts
Discussion Starter #7
Guys,
I checked the old hacked forward light harness last night and found no burned wires. The intermediate harness shows no burned wires and and a continuity check verifys the integrity of the wire to the back lights. I also unwrapped the dash harness and found no burning/melting of the brown wire beyond eight inches or so into the wrap. Having done this and accepting John's summation that a short normally burns a wire worst at the source, is it reasonable to assume that I have a bad switch? Anything else I should check?

John and Bill... thanks for the Circuits Primer 101
 

·
Registered
Joined
·
7,762 Posts
If it's only 8 inches down the wire the short occurred along that length.
It could have been a bare spot on the wire itself.
It could have been something touching the headlight switch connector.
It could have been an internal short inside the headlight switch. The frame of the headlight switch is grounded, so anything is possible.
If you are planning a repair there is no 100% guarantee that you will find the original problem after the first repair. It might pop fuses until the real problem is found.
If you go this route:
1, Replace the headlight switch.
2, Examine all wires next to the burnt brown and replace accordingly.
3, Replace the burnt brown wire.
4, Use the proper fuse in the fuseblock.
Good luck with it.
 

·
Registered
Joined
·
1,413 Posts
It could also have been a bad connection where the spade terminal connects to the light switch. That could have got the wire so hot it burned down the wire a ways. Copper conducts heat quite well.

Peter
 

·
Registered
Joined
·
7,762 Posts
Originally posted by Peter F.:
It could also have been a bad connection where the spade terminal connects to the light switch. That could have got the wire so hot it burned down the wire a ways. Copper conducts heat quite well.

Peter
Quite true Peter but that wouldn't have popped the fuse. If I read things correctly, the smurf that owned the car before was foil wrapping the fuse.
 

·
Lifetime Founding Member
Joined
·
1,960 Posts
If this is the brown wire that turns into a doubble brown wire doesnt it also go to the gas sending unit?
I have a 69 and it has the same brown wire burnt. I replaced it and have found no other burnt or bad wires. (yet)
I have taken all of the switches apart cleaned lubercated and reassembled them and found no problems.
If you find a cause please let me know, I dont want the added expence of new harness either.
T.C.
 

·
Registered
Joined
·
7,762 Posts
Originally posted by TC:
If this is the brown wire that turns into a doubble brown wire doesnt it also go to the gas sending unit?
No he was asking about the brown that goes to the running lights. It the rear stop lights you will see it along with black, green and yellow.
Fuel gage sender wire, from the tank, is tan.
 
1 - 14 of 14 Posts
Top