54 Century, burnt 10 gauge wire from underhood junction block to ammeter

[joelj]

So my project took a different direction today, what started as a horn contact replacement turned into a wiring problem that I didn't anticipate. Bare wire in several places the worst of witch seems to be the black 10 gauge that runs from the underhood junction to the ammeter. I also noticed that the 10 gauge red feed to the headlight  switch had burnt insulation. Apparently, I am going to have to unwrap and inspect the underdash harness for further damage. I am wondering what the root cause may have been. Can a defective ammeter cause a short that can cook an entire harness? Or do you think that the short may have originated elsewhere. I am surprised that almost everything seemed to work up to this point. Can't say as much for the amp gauge, I wish I paid closer attention to it given the current situation. 

Thanks, your advice is always appreciated.

[Bloo]

3 hours ago, joelj said:

Can a defective ammeter cause a short that can cook an entire harness?

Maybe, but...

3 hours ago, joelj said:

Or do you think that the short may have originated elsewhere.

Offhand, yes, but more information is needed.

 

What exactly were you doing that triggered this?

 

An ammeter is just a flow gauge, and is *entirely* on the hot side. No ground, ever. If something happened that grounded it, fireworks would happen.

 

One side of the ammeter is connected to the generator output, and the other side to the battery. On a 54 Buick, the generator output is technically the "BAT" terminal on the voltage regulator. On most GM cars, the connection to the "battery" side is usually done where the battery cable connects to the starter. The ammeter measures flow, in or out of the battery. How? Well, most things are connected to the generator side of the ammeter, so any current they draw is "siphoned off" (so to speak) from the generator before the ammeter has a chance to measure it. Any current going through the ammeter is actually current charging the battery. If the generator is not charging, such as when the engine is off, current from the battery has to flow *backwards* through the ammeter to get to any accessories that are drawing current. Either way, you ore only seeing current flowing in or out of the battery, and that is what you want.

 

On a typical GM car though, there is an exception: the horn. GM typically connects the horn to the "wrong" side of the ammeter, in other words the battery side. I don't know why, but I have a theory. A stuck horn is effectively a dead short. Maybe they think the generator is only capable of 40 amps or so, but the battery is capable of several hundred amps, and in the case of a stuck horn 40 amps wouldn't be enough to burn out the ammeter, at least not immediately, but several hundred amps sure would.

 

By connecting the horn to the "wrong" side, they prevent battery current from ever having to go through the ammeter to get to the horn. Generator current would go through the ammeter to get to the horn if the engine is running, but there's only about 40 amps of that available.

 

Is the horn stuck? If it was, and the wire up the column or the steering wheel contact was also shorted so the horn relay was in constant "honk" mode, I guess that could have caused a meltdown.

 

The wire up the column to the contact is on the ground side of the horn relay, and any shorts there would normally cause the horn to harmlessly honk.

 

Edited November 26, 2023 by Bloo (see edit history)

[joelj]

I baught this car in September. I guess I am trying to figure out what  may have happened many years ago.  When I started looking at this problem I had a non working horn ring, and someone had wired in an aftermarket under dash switch. I disconnected the aftermarket switch and touched it to the factory connector on the column and the horn blew. Ok, I thought the contact was just shorted to ground on the column shaft so it would have to be replaced. It was only when I  began the process of disconnecting the signal wires from the underdash fuse block that I noticed burnt wires. Tracing the worst wire back through the firewall, I found that it went to the underhood junction block on the same terminal as the positive battery cable. It was wrapped in electric tape , but was bare wire under the tape. Looking at the factory wiring diagram in the shop manual, it looked like the wire went directly to the ammeter. Feeling around the connection at the ammeter I can feel the burnt disconnected end. The more I looked, the more I noticed, it must have gotten pretty hot at some point in time. Probably came close to catching the whole thing on fire. The wire cooking off the ammeter probably saved the car. Probably a separate problem from the horn contact. That 10 gauge black wire definitely looks like it cooked more than anything else.

Edited November 26, 2023 by joelj (see edit history)

[Bloo]

Was the wire out to the generator regulator "BAT" terminal also toasted? And the one from the GEN terminal down to the generator?

 

I've not seen it happen in person, but have on two cars had to clean up the mess that is created when a generator gets polarized backwards and somehow manages to start charging. That should never happen, but maybe a particular combination of stuck points and backward polarization.... It burns everything in that circuit from generator windings all the way to the battery or battery cable.

 

If only the wire section from the battery cable to the ammeter is melted, then probably some accessory shorted to ground. and the battery did the rest. It could have been the horn circuit, but probably not the part of the horn circuit having to do with the steering column, more like the heavy wiring to the relay and the relay itself. If there is no major damage to the ammeter that is probably what happened. If it was some other accessory, it should have really toasted the ammeter internals. If you can find another wire that is melted but not just melted because it was laying against a melting wire, that could lead you to the thing that did it.

 

 

[JohnD1956]

Had a 1960 MGA once with a bunch of melted insulation throughout the car. The seller said some one hooked up a battery backwards. That MGA was a positive ground system and the battery had been hooked up as a negative ground.  

This may have been a similar situation for your car in the past. 

[old-tank]

The yellow wire to the vacuum start switch on the carb and the red wire to the horn relay are not fuse protected and if shorted will fry the harness causing new problems.

[joelj]

14 hours ago, Bloo said:

Was the wire out to the generator regulator "BAT" terminal also toasted? And the one from the GEN terminal down to the generator?

 

I've not seen it happen in person, but have on two cars had to clean up the mess that is created when a generator gets polarized backwards and somehow manages to start charging. That should never happen, but maybe a particular combination of stuck points and backward polarization.... It burns everything in that circuit from generator windings all the way to the battery or battery cable.

 

If only the wire section from the battery cable to the ammeter is melted, then probably some accessory shorted to ground. and the battery did the rest. It could have been the horn circuit, but probably not the part of the horn circuit having to do with the steering column, more like the heavy wiring to the relay and the relay itself. If there is no major damage to the ammeter that is probably what happened. If it was some other accessory, it should have really toasted the ammeter internals. If you can find another wire that is melted but not just melted because it was laying against a melting wire, that could lead you to the thing that did it.

 

 

I checked things out under the hood, and everything looks good. The problem seems to be limited to the part of the harness under the dash. An aftermarket accessory that has long since been disconnected and disposed of may have been the culprit. The evidence is there. I am going to  look into a replacement harness tomorrow. 

Thanks for your input.

[joelj]

2 hours ago, old-tank said:

The yellow wire to the vacuum start switch on the carb and the red wire to the horn relay are not fuse protected and if shorted will fry the harness causing new problems.

I checked the wiring at the carburetor  and that seems fine except for aftermarket terminals. The red 10 gauge at the horn relay is fine until you get under the dash. There it is burnt at the headlight switch feed and maybe the ammeter as well. I think that the black 10 gauge from the junction block to the ammeter fried first (most all of the insulation is gone) the red probably shorted once the black melted through. 

[joelj]

Thanks again for everyone's input. The new harness is on order. 10 to 12 weeks out. I have plenty of other projects to do on the car while I wait so it's really not a problem. Meanwhile, I'm back to the horn contact project I started on Saturday.

[Joseph P. Indusi]

I added a fuse to the horn circuit where the voltage supply goes into the horn relay.   This may hopefully prevent major damage if the circuit shorts to ground somewhere.

Joe, BCA 33493

[joelj]

1 hour ago, Joseph P. Indusi said:

I added a fuse to the horn circuit where the voltage supply goes into the horn relay.   This may hopefully prevent major damage if the circuit shorts to ground somewhere.

Joe, BCA 33493

I was thinking that this might be a good idea. Did you add the fuse in the engine compartment on the 12 gage wire that goes to the center post of the horn relay or elsewhere? I am also curious what amp fuse you used? Pardon my ignorance, I know enough about wiring to follow a factory diagram but not enough to make any improvements. Thanks much,

Joel J. BCA 47829

 

[Frank DuVal]

On 11/25/2023 at 8:10 PM, Bloo said:

On a 54 Buick, the generator output is technically the "BAT" terminal on the voltage regulator.

Only when the contact inside the regulator closes upon seeing the generator having output current. 😉

 

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