How to find shorts

Cougar Community Classic Cougar Maintenance Restoration and Repair
1

If your Cougar is repeatedly blowing a fuse it can be extremely frustrating. The temptation is to keep sticking bigger fuses in until the car catches on fire and you no longer need to worry about the short.



There is a better way. Many years ago I made a simple troubleshooting tool that can keep that smoke inside the wires where it belongs. The idea is to replace the fuse with something that can dissipate the power instead of burning up countless fuses and or wires. Light bulbs do exactly that. So you replace the fuse with a light bulb on the end of an extension cord.



This is a medicine bottle with an 1157 tail light bulb inside. The leads are about three feet long. long enough to reach under the dash to the fuse box and then place the medicine bottle in clear view. The trick part is what is at the end of the leads. Both leads are terminated in .250 insulated female spade connectors. Then the ends of an old dead fuse are soldered to two add-a-tap connectors.



Because they plug in you can also replace them with the ones used for ATC style fuses for newer cars. I store this inside the medicine bottle when not in use.



You do need to Solder the leads to the tail lamp bulb.



It’s really a very simple device.



2

Once you have short finder plugged in the bulb will illuminate very brightly as long as the circuit is shorted out. A short means that a wire is grounded. That means all the power is going straight to ground creating heat in the fuse or the wire seeing which one burns up first. The bulb dissipates the power in the form of light and heat.

To find the short the first thing you do is unplug what ever the fuse is designed to protect. If the bulb goes out you have found the problem is in the device. Keep in mind that a properly working circuit will pull some current so the bulb will still glow some if it is in the circuit. With a dead short the bulb will glow brightly. If the bulb is still glowing brightly you need to follow the wire supplying the protected device looking for a place where it has been pinched or the insulation at a connector is missing allowing the wire to contact metal. Many times wiggling the wire will make the bulb flash or go out showing you that you are in the neighborhood.

1 Like
3

Much easier than the old test light method.

4

Dang I was hoping it was these kinds of shorts :

Just kidding - an excellent ( and easy to make ) custom made short detector ! Many thanks.

5

Still working on a device that simplifies that…

6

Nice!
Just a thought, would it help to connect the two fuse clip ends with a piece of wood dowel stock so that it will plug in easier (especially when your hanging upside down under the dash). I do realize that you might need more than one length since a few of the fuses are shorter.

7

Originally, I had several of the fuse ends made up using blown fuses with the glass still intact. As it turns out it was actually easier to plug in the ends one at a time, at least for me. The single hardest part of making this is getting a mechanically strong solder on the fuse part. If I make another one I think I will try to make a better “handle” for sticking the fuse parts into the fuse box.

8

Interesting. Here’s what I provide all of my customers to checking shorts prior to installing a new (refurbished) underdash harness:

First, always install electrical wiring with the battery disconnected.

Physically install the underdash harness with fuses. Connect all underhood and taillight harnesses.

When ready to check out the harness, close all doors, turn off all lights, ignition key in the OFF position, accessories off, etc. Make sure both battery connectors are disconnected. Connect the positive battery cable to the starter solenoid and the positive side of the battery. Leave the negative terminal disconnected from the battery. Then use a digital volt meter to measure the current draw across the battery. Place one probe of the DVM on the negative battery cable terminal, and the other probe on the negative battery post. Be sure that the DVM is set to amps, DC. If there are no shorts in the wiring connections, the reading should be on the order of 30 milliamps or less. Anything above 1 amp indicates some accessory is on; readings above 10 amps (may blow the fuse on the DVM) indicates a dead short.

If you have a short, disconnect all other wiring from the starter solenoid post where the battery connects and re-measure the current. If current drops, the source is the underdash wiring harness or the harness from the starter solenoid to the firewall. Reconnect the wiring to the starter post and then disconnect the underhood harness from the firewall and test again to isolate the source of the short.

If the short is not in the underhood or underdash harness, the problem lies in the alternator or voltage regulator. Disconnect each of these in turn to isolate the source.

If the short or high current comes from the underdash region, keep the battery negative side disconnected and remove one fuse at a time. Measure the current as above. If you now see low current, there is either a short or an item on that particular fuse circuit is on. Turn off that item and continue checking.

Eventually, you’ll have low current readings with all fuses installed and all wiring connected. This series of tests checks all of the battery directly powered systems. To check the Accessory systems, now repeat all of the tests above with the ignition key in the ACC position. The current with the key in the ACC position may be up to 1 amp or so. When all of the tests are done and no anomalous current it noted, at that time, and only at that time is it safe to connect the negative battery cable to the battery.

9

That is a great home made tool anyone can build themselves, nice idea!

I have a commercially available option I have used for years that has a reset-able circuit breaker/buzzer. Alot of modern cars have multiple fuse boxes and being able to hear it is helpful when your not in line of sight

10

Excellent cheapskate tool, Bill. Here’s an idea- is there enough metal to roll the ends of the add-a-tap connectors into the shape of the fuse ends to eliminate the solder joint?

11

Sadly there is not. What I really want for this is a part that you squeeze, insert and then release to make it super easy to insert. I think this might make a good product, but it is so cheap and easy to make that I’d rather just show people how to make one.

12

This is a great topic. My car recently grew what I think is a dead short, and I plan to use Midlife’s procedure to track it down. But I’m pretty certain I’m going to blow fuses in my DVM as Midlife mentions in his post. At the risk of hijacking the thread, here’s what my car is doing.

Ignition key off, battery fully charged and cables disconnected, voltage between terminals is a little over 12V. Reconnect the cables, key still in the off position and voltage immediately drops to 3 or 4V and continues to decrease quickly. It takes about 30 seconds for it to get to less than 1V. My initial thought was something inside the alternator, but I unplugged it, retested and got the same results.

So, I’m pretty confident that if I connect my DVM between the negative post and cable, this will blow the DVM fuse. Is there a workaround or maybe another likely culprit I should check first? Thanks in advance.

13

You have a bad battery. Take it to Auto Zone to verify. A bad battery with no load will measure normal voltage and then collapse when asked to deliver current

14

Darn! It’s practically new. At least I should have some good warranty left.

Thanks for the quick reply!

15

Well, the way I find dead shorts or intermittent shorts that blow fuses is to use an alarm siren in place of the fuse. As long as that siren is blasting, you have a short. It’s a lot easier and you don’t have to watch a light bulb. As far as an intermittent short, just wiggle wires/harness until the siren blares at ya! It can be a bit loud, but it gets your immediate attention.

16

Those are the hardest ones to find!

17

And they tend to be the most expensive when you do find them.