Help Locating a Dead Short

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Joined
Dec 17, 2012
Messages
40
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1
Location
Dallas, TX
My Car
1973 Mustang Mach 1
Bright Red
I'm looking for any recommendations from anyone who might have been in a similar situation.

I recently got my car back from the shop after having new wiring installed throughout. When trying to install the center gauges and realized that the circuit they are on blows a fuse when turning the ignition key. To arrive at this conclusion, below are the order of recent work/diagnosis after picking up the car:
  1. Put in a new radiator & thermostat. After installation, ran the engine for a bit to get it up to temp and make sure the radiator wasn't leaking and thermostat was opening to allow coolant flow. During this test, I heard a very loud high pitch whining noise coming from the alternator and just thought it was the belt slipping at the time. It went away on it's own after about 30 seconds or so. I tightened the belt after and have not heard the noise since.
  2. When I hooked up the temp center gauge to the new wiring, it was not functioning properly. The temp needle was just fluttering and never raised with temp.
  3. I checked the fuse box and found that the fuse for the circuit that center gauges are on was blown. I replaced fuse (#11) and then found that it blows every time the ignition key is turned, leading me to believe there is a short somewhere. This also happens to be the same circuit that the electric choke is run from, so that is not functioning either. I noticed the car has not been starting as quickly/effortlessly as it did in the shop or immediately after picking up over the past couple months so now I understand why--no electric choke.
  4. I searched all of the under dash wiring and connectors and everything seemed to be fine and could not find any shorted wiring or bad grounds. The only harness connector that eliminated the short was the ignition switch wire connector that bridges between old and new wiring--see attached photo. The wires upstream of this connector are:
    • Neutral safety switch
    • ignition switch accessory
    • 12V ignition
    • Brake light/switch
    • Ground
    • 12V battery
  5. I then shifted attention to the alternator/main 12V feed. Here I saw evidence of major resistance at the mega fuse bussbar--see attached photo of melted covers. I also noticed that the alternator output nut was loose.
  6. Last thing I checked is the voltage of the ignition 12v circuit and it appears to be fluctuating quite a bit, leading me to believe that there could possibly be an issue with the voltage regulator in the alternator (Wondering if maybe this is the source of the loud noise that I was hearing earlier?) Either that or my digital multimeter is struggling with the points firing on the distributor. I've heard that you really need to use analog volt meters on these old cars?
So a couple items I'm considering trying next, and wanted to see if y'all agree or have any other items you think I should check first. My hope is that this is something simple and it's just a matter of finding the source and that someone out there may have run into a similar issue.
  1. Install a new NSS--it's on order. The shop did not replace this, but when I looked at it everything it seemed to be connected properly and since the car was starting ok didn't think it was an immediate concern. My gut says that this is likely causing the issue but won't be able to confirm until this weekend when I dig into it. The NSS is on the same circuit as the center gauges.
  2. Double-check all grounds.
  3. Try a new alternator and see if that does anything. Wondering if that loud noise I heard from the alternator was the diode going bad and now there is an issue with the voltage regulator when I turn the key?
Thanks.
 

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You need to go to the top of this page and click on 7173 Wiki and then download the wiring diagram for your car. You're going to need to put on your detective hat and start tracing wires. The gauges receive their power on a black-light green dot wire from the instrument voltage regulator (constant voltage unit) which receives it power on the black-light green hash wire, which receives its power directly from the ignition switch. If yours is losing power due to a blown fuse someone has rewired the gauge circuit. In any case, you'll need to find out why the fuse is getting blown. The wiring diagram has a fuse diagram that shows what the various fuses power. I would start by looking at the wire to the electric choke. The original wiring for the electric choke came directly from the stator terminal on the alternator via a white-black stripe wire.

A volt-ohm meter is a critical tool for tracing wiring.
 
The loud whining from the alternator makes me think you may have a problem with the alternator and/or voltage regulator. The fact that it quit happening makes me think something failed. A failed regulator can allow the voltage to go several volts high, which can damage electrical components.

Here's why I'm thinking in this direction. When I was in college in the mid 1970s, I drove a '69 Dodge Coronet. The mechanical relay-type voltage regulator had a bad habit of getting stuck, and when it did, the alternator would sing like a bird and the system voltage would spike up to over 18 volts. If I didn't bring the engine RPM down to idle very quickly, I could smell wiring overheating. Never had a fire (luckily!). I had to go under the hood and whack the regulator with a wrench to knock the contacts free. I replaced that damned regulator again and again and still kept having the same problem. Finally I replaced the regulator with a solid-state unit from a newer MoPar car and never had the problem again.

If you've converted the car to use a single-wire alternator with built-in regulator, I'd start there. I've read on the Ford truck chat groups where people have done this conversion on their trucks and experienced all kinds of problems.
 
Thanks guys. I’ll dig into this a bit more this weekend and hopefully uncover the source. I do have a new single-wire alternator with built in regulator. If I end up trying to replace that, are there any recommendations for make/part number? Seems like I wouldn’t want to just use the same unit again
 
You can buy "poppers". They are basically resettable fuses you use for diagnosing these sort of issues. The only store I know of that sells them locally is the HVAC supply store.

It saves you having to go to every auto parts store in town to buy up all their fuses so you can keep retrying.

Once you have a popper in place of the fuse, you can keep disconnecting things and retrying over and over till you find the one thing that you disconnected and the problem stopped happening.

This guy shows off using one called "short pro". It lights up when the fuse would have blown.
 
I'm looking for any recommendations from anyone who might have been in a similar situation.

I recently got my car back from the shop after having new wiring installed throughout. When trying to install the center gauges and realized that the circuit they are on blows a fuse when turning the ignition key. To arrive at this conclusion, below are the order of recent work/diagnosis after picking up the car:
  1. Put in a new radiator & thermostat. After installation, ran the engine for a bit to get it up to temp and make sure the radiator wasn't leaking and thermostat was opening to allow coolant flow. During this test, I heard a very loud high pitch whining noise coming from the alternator and just thought it was the belt slipping at the time. It went away on it's own after about 30 seconds or so. I tightened the belt after and have not heard the noise since.
  2. When I hooked up the temp center gauge to the new wiring, it was not functioning properly. The temp needle was just fluttering and never raised with temp.
  3. I checked the fuse box and found that the fuse for the circuit that center gauges are on was blown. I replaced fuse (#11) and then found that it blows every time the ignition key is turned, leading me to believe there is a short somewhere. This also happens to be the same circuit that the electric choke is run from, so that is not functioning either. I noticed the car has not been starting as quickly/effortlessly as it did in the shop or immediately after picking up over the past couple months so now I understand why--no electric choke.
  4. I searched all of the under dash wiring and connectors and everything seemed to be fine and could not find any shorted wiring or bad grounds. The only harness connector that eliminated the short was the ignition switch wire connector that bridges between old and new wiring--see attached photo. The wires upstream of this connector are:
    • Neutral safety switch
    • ignition switch accessory
    • 12V ignition
    • Brake light/switch
    • Ground
    • 12V battery
  5. I then shifted attention to the alternator/main 12V feed. Here I saw evidence of major resistance at the mega fuse bussbar--see attached photo of melted covers. I also noticed that the alternator output nut was loose.
  6. Last thing I checked is the voltage of the ignition 12v circuit and it appears to be fluctuating quite a bit, leading me to believe that there could possibly be an issue with the voltage regulator in the alternator (Wondering if maybe this is the source of the loud noise that I was hearing earlier?) Either that or my digital multimeter is struggling with the points firing on the distributor. I've heard that you really need to use analog volt meters on these old cars?
So a couple items I'm considering trying next, and wanted to see if y'all agree or have any other items you think I should check first. My hope is that this is something simple and it's just a matter of finding the source and that someone out there may have run into a similar issue.
  1. Install a new NSS--it's on order. The shop did not replace this, but when I looked at it everything it seemed to be connected properly and since the car was starting ok didn't think it was an immediate concern. My gut says that this is likely causing the issue but won't be able to confirm until this weekend when I dig into it. The NSS is on the same circuit as the center gauges.
  2. Double-check all grounds.
  3. Try a new alternator and see if that does anything. Wondering if that loud noise I heard from the alternator was the diode going bad and now there is an issue with the voltage regulator when I turn the key?
Thanks.
Did you find that short yet? If not this is what I would do.
Disconnect the positive side of the battery. Turn your meter to OHMS not the diode checker. That setting pushes more current through the leads to break a PN junction of a diode and will give you a false reading for small resistance I.E. 1 to 10 OHMS.
Touch the meter leads together and note that reading, for my Fluke it is .3 ohms.
Probe the connector opposite the ignition connector you disconnected, original wiring, and check them off on the wiring schematics If there is no problem there turn your attention to the new connector, I have seen after market wiring pinned incorrectly on other vehicles.

Your Temp gauge sending unit is a variable resister, it may be shorted out or part of the shorted wire problem you have.

Keep in mind that, I don't know what wiring throughout is, what wires the shop replaced. Also fuse 11? Is that an after market fuse block?
As for your mega fuse...for that to melt the cover but not discolor the wires to me tells that there is a high resistance connection there. 75 amps can do a lot of damage before it blows.
 
Did you find that short yet? If not this is what I would do.
Disconnect the positive side of the battery. Turn your meter to OHMS not the diode checker. That setting pushes more current through the leads to break a PN junction of a diode and will give you a false reading for small resistance I.E. 1 to 10 OHMS.
Touch the meter leads together and note that reading, for my Fluke it is .3 ohms.
Probe the connector opposite the ignition connector you disconnected, original wiring, and check them off on the wiring schematics If there is no problem there turn your attention to the new connector, I have seen after market wiring pinned incorrectly on other vehicles.

Your Temp gauge sending unit is a variable resister, it may be shorted out or part of the shorted wire problem you have.

Keep in mind that, I don't know what wiring throughout is, what wires the shop replaced. Also fuse 11? Is that an after market fuse block?
As for your mega fuse...for that to melt the cover but not discolor the wires to me tells that there is a high resistance connection there. 75 amps can do a lot of damage before it blows.
Whoa! Disconnect the positive side of the battery? Never do this until the negative side has been disconnected!!! If you inadvertently have your wrench touch the car body, you'll have a dead short and start welding immediately. Never, ever, disconnect the positive side of the battery before the negative side.
 
Whoa! Disconnect the positive side of the battery? Never do this until the negative side has been disconnected!!! If you inadvertently have your wrench touch the car body, you'll have a dead short and start welding immediately. Never, ever, disconnect the positive side of the battery before the negative side.
Just don't forget to hook the negative side back up when you are looking for a hot wire touching ground.
 
Ok update: I located the shirt at the fuel sending unit. Shop had wired it up backwards. Now I’m having the issue of dancing gauges. When ignition is in either the accessory on or run position, center gauges dance at some sort of consistent rhythm. Fuel gauge reads as 1/4 tank past full. Voltage coming from battery & starter relay reads a consistent 14+/- volts.

Video below for reference. Any thoughts on what to look at next? Alternator? I have the American Autowire upgrade kit with single wire alternator.

View attachment IMG_9050.mov
 
The gauges appear to respond instantaneously with the constant voltage regulator, which actually switches between 12 and 0 V. Most gauges react slowly. Change over to the electronic CVR?
 
The gauges appear to respond instantaneously with the constant voltage regulator, which actually switches between 12 and 0 V. Most gauges react slowly. Change over to the electronic CVR?
So are you saying that even though my voltmeter is reading a consistent 14-ish volts at battery & starter relay it’s actually fluctuating between 12 and 0 V?

I have not been able to find any sort of instrument voltage regulator behind the dash. Any idea where this is typically located?

I’m unclear what you mean by changing over to electronic CVR?

Are you talking about something like this?
https://eclassics.com/eclassics-196...MIwvvu7NaRhwMV525_AB1XWgeDEAQYBCABEgLtjPD_BwE
 
Looks like it is almost in sync with a flasher. Turn your lights on next time and see if the dim at the same rate. Just a process of elimination to find the problem. I also noticed you don't have a ground connected, black wire next to your gauges.
 
Update for anyone who has this issue in the future. Bottom line is the issue spawned from the combination of factory and aftermarket gauges in the American Autowire harness. Factory fuel gauge needs 5V and thus the CVR, however the aftermarket gauges require 12V and must bypass the CVR. I tapped into the "aftermarket gauges" wire in my American Autowire harness and ran it to the new gauges and all is running as should now.

Now I need to find someone who can rebuild my factory tach to work with new wiring. So far it sounds like this outfit is my best option:
https://www.bobsspeedometer.com/
 

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