Struggling to find an electrical short

[spamtank]

Working on my 70 SE Challenger with a rallye dash.  Had some screwy electrical problems off and on, and they were intermittent.  Fortunately, now I have a hard short (or nearly so).

I've separated the steering column connectors, and I measure ~2 ohms to ground on the Key Buzzer Pin, wire 13A-18R on the harness side, not the column side. 

On the schematic I've traced this to the headlight warning buzzer (diode on a connector pigtail), and then back to the key-in buzzer.

I believe the Key In buzzer is located on the right side of the glove box. 

1) Is there a separate Headlight Warning Buzzer?  I don't think so, I believe they are just referring to the circuit that energizes the Key-in Buzzer if the lights are left on with the key in.

2) Where is the Headlight Warning Buzzer diode located relative to the dashboard?  I haven't been able to locate it.

When I attempted to connect the battery, I was getting a healthy arc, since it was trying to draw ~6 amps at 2 ohms. 

Thanks for looking and any comments you may have.

[DocMel]

I have used this this to at least narrow down where a short might be by removing one fuse at a time until I located the circuit where the short is. 

Prob is that this technique might help narrow it down, the one fuse might cover allot of wiring and accesories

[spamtank]

Hey, thanks DocMel.

That's what I ended up doing. 

Turned out that the bug I was chasing was real (a bad key in switch), but a red herring.

I have isolated the problem to the Tail Light Fuse circuit.

So I'm focusing my effort on that.  But as you pointed out, there are other things hanging off of that, including the Cigar Lighter, the clock, and an accessory feed.

I feel like I'm getting closer.

[dodj]

I have isolated the problem to the Tail Light Fuse circuit.

So I'm focusing my effort on that.  But as you pointed out, there are other things hanging off of that, including the Cigar Lighter, the clock, and an accessory feed.

There is a connector in the driver's side kick panel. Pull that apart and see if you still have the ground. I expect it will go away. The harness routing along the floor and up over the rear wheel well to the trunk has several places that could cause a grounded wire.

[734406pk]

Consider replacing the tail light harness if this is the local cause. Year One sells the exact replacement to change out that 40+ year old copper. I've had a similar issue, a phantom short. Replaced the harness/bulbs and it all good. Just try to narrow down the circuit as best as possible. The wiring diagram is worth it's weight in gold!

[spamtank]

Hey guys, thanks for the reply.

It's very funny, I just pulled that plug apart 15 minutes before reading this, and you guys were spot on.

I also noticed that this harness was very brittle.  So, I'll be checking year one out.

Thanks again for confirming my suspicion. 

Boy, I just had to keep unplugging things.  I still want to find exactly where it's at, that should happen tomorrow.

[734406pk]

I'm happy that you have found your electrical issue! 

[dodj]

I still want to find exactly where it's at, that should happen tomorrow.

Just a matter of examining the wire routing along the floor and into the trunk now. Of course that means removing the sill plate and rear seat and interior 1/4  panel, so it will take more than 5minutes. But hey, it's a hobby right? 

[734406pk]

Good luck finding the short! It would be nice to know where it happened. What is the condition of the rest of the wiring in the car? Especially the bulkhead connector/dash harness.

[spamtank]

Well, after separating the connector, I pulled the harness out of the cable chase by the rocker panel.  I inspected every inch from the front to the trunk.  I was unable to find any sign of damaged insulation.  The harness was actually in pretty good shape.  The overbraid was dried out and brittle, and in some areas a bit crumbly.  I used some flex tubing in areas where the harness passed over or around edges, or where the over braid crumbled.

I went back and looked at the connector, and low and behold, I found a layer of corrosion between the tail light supply and the two adjacent blades. 

I cleaned that up, and the short was gone.

The bulkhead connector is in reasonable shape,  The dash harness hasn't been hacked, except for the radio connection, and in general is in good shape with one exception.  The headlight switch plug shows evidence of thermal trauma at some point before I owned the car. 

So I'm not exactly convinced I've found the root cause, since the circuit I was tracing routes through the headlight switch.  I find it hard to believe that the corrosive residue I cleaned up could have conducted enough current to blow the 20 amp fuse.  One the other hand, depending on how much all of the bulbs take, maybe a few extra amps is enough to take the fuse over the edge.   

Now that I have one short cleaned up, I'm going to go back to the front of the car to look at that portion of the harness a second time.  I looked pretty closely the first time.  I was monitoring resistance while I moved the harness around.  But saw no change.

It's always something with an old car.

[734406pk]

Unfortunately the headlight circuit is one of the largest draws in the car. My suggestion is to narrow things down between the parking light circuit and the actual head lights. You can install a relay kit for the headlights to take the bulb amp demand off of the entire headlight circuit(highly recommended). I have also expirienced random shorts in light bulbs themselves for an unknown reason. I agree with your findings regarding the corrosion. Low voltage ...yes. Short circuit ... No. Hope you find something obvious!

[ShelbyDogg]

Next time you have a dead short and worried about keeping it powered on, use a big light bulb and or buzzer hooked in series with the battery terminal. The bigger the short, the brighter the light glows. Start unplugging fuses or connectors till the light goes off. You can then have a bulb soldered across a bad fuse to power that circuit.
A small light bulb can be used for finding any power drains that kill a battery over time.

The buzzer is for working by yourself so someone doesn't have to help watch the light or hear the buzzer go on and off as you shake wires.

 Too many want to help, but who do you blame if a bolt gets sheared off? Yourself

[spamtank]

Thanks for the feedback and moral support guys.

Seems every week brings a new thread to pull on.

I'm still looking for the location of the Headlight Buzzer Diode.  I'm sure I have it, I have the A01 package.  Any help would be appreciated.

[ShelbyDogg]

Just a 2 pin connector with 2 wires sticking out that look like they are cut off and taped up.

[spamtank]

Just a 2 pin connector with 2 wires sticking out that look like they are cut off and taped up.

Thanks Rob.

Which side of the dash is it located, passenger side (near the key-in buzzer), drivers side, or sort of in the middle?  I'm crawling around under the dash looking for it, and have yet to find it.