1956 - determining an electrical short

[DiIanni]

I seem to has discharged two brand new batteries and am thinking that I must have a short somewhere within the car.
If so, how might I go about determining it?

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[sre]

This is my way. I pull the negative battery cable, if there is a spark and EVERYTHING IS OFF, you have a short. Now finding it. I disconnect 1 circuit at a time then check for spark. If no spark go to the next circuit. You should have 2 people, it's a lot quicker. Does everything work exactly as it should, check for frayed wires It could be 5 min. or 5 hours.

 

[DiIanni]

This is my way. I pull the negative battery cable, if there is a spark and EVERYTHING IS OFF, you have a short. Now finding it. I disconnect 1 circuit at a time then check for spark. If no spark go to the next circuit. You should have 2 people, it's a lot quicker. Does everything work exactly as it should, check for frayed wires It could be 5 min. or 5 hours.

What do you mean when you say "pull the negative battery cable".

 

[sre]

You need to pull the negative terminal off the battery. Then touch the cable to the post. There should be NO spark at all. A spark indicates something somewhere is drawing voltage. It may be a very small spark, look closely. A big spark will useally indicate a direct short.

 

[74 Harley]

A spark test is not the answer. Some draws are not big enough to spark, and sparks around a battery can be dangerous.
Yes, disconnect the ground and use a volt meter in between the cable and battery. It shouldn't have a reading unless something is on or drawing current.
Volt meters are cheap.

 

[Tom Alpers]

I seem to has discharged two brand new batteries and am thinking that I must have a short somewhere within the car.
If so, how might I go about determining it?

The classic way is to wire up a light bulb between the negative battery terminal and and the negative battery cable. If you have a reasonably large power draw it will light up the light bulb. If it doesn't light the bulb you may have to use an ammeter to see the smaller current draw. Then start disconnecting things and see if you can find something that when you disconnect it the bulb turns off. If the "short" is a frayed wire touching the metal floor buried under the carpet it will be more difficult to track down. You may have to isolate sections of the wiring harness to find the section with the problem. Some people have found their clocks to be drawing too much. So start with the easy stuff that you can disconnect, the clock, heater blower, headlights.. all those wires in the blocks on the fenders that connect to various lights, the wires on the voltage regulator.. Some people have found their voltage regulator has sticking points and that's draining the battery.

 

[zeroroadkill]

A spark test is not the answer. Some draws are not big enough to spark, and sparks around a battery can be dangerous.
Yes, disconnect the ground and use a volt meter in between the cable and battery. It shouldn't have a reading unless something is on or drawing current.
Volt meters are cheap.

I believe you are meaning an multimeter on the amp setting. A volt meter in series will not detect voltage. An amp meter in series will show if there is current draw. Set the amp setting on the highest setting first. If there is a spark when tapping the battery terminal to the battery post then you better use a shunt or you can damage the meter for all the current draw flows through the meter.

 

[Tom Alpers]

I believe you are meaning an multimeter on the amp setting. A volt meter in series will not detect voltage. An amp meter in series will show if there is current draw. Set the amp setting on the highest setting first. If there is a spark when tapping the battery terminal to the battery post then you better use a shunt or you can damage the meter for all the current draw flows through the meter.

Good points. To the OP, most inexpensive meters which do amps are only rated for 5 or 10 amps. For parasitic draw that should be enough but keep in mind that when you have the meter in series for this testing don't go around turning on headlights and blowers and things. That may draw too much for the meter and burn it out. Even stepping on the brake with old fashioned incandescent bulbs may put you near the limit for a 5 amp meter.

 

[zeroroadkill]

Good points. To the OP, most inexpensive meters which do amps are only rated for 5 or 10 amps. For parasitic draw that should be enough but keep in mind that when you have the meter in series for this testing don't go around turning on headlights and blowers and things. That may draw too much for the meter and burn it out. Even stepping on the brake with old fashioned incandescent bulbs may put you near the limit for a 5 amp meter.

Exactly! Actually most cheap multimeters can’t handle much more than a couple of amps which is 2000 milliamps. In fact we’re talking milliamps for most DVOM’s. 12watts@12vdc=1amp! Ever wonder why your battery cables are so thick? Starters can draw >250A+ and even more for 6VDC. So be careful with your cheap plastic multimeter!

 

[DiIanni]

About a year ago, I started to re-wire all elements at the rear of the car. For whatever reason, I stopped "mid-stream"
so to speak. Unfortunately, I had left some unshielded wires lying on the metal beneath the carpeted area. I recently
went back to the project and much to my surprise, discovered what had to be the cause of destroying two new
batteries. Needless to say it was an expensive lesson well learned. Short Found!!

 

[4VE]

The clock draws some and is always on. So your spark theory is inaccurate. AND sparking near a battery can cause an explosion. I recently experienced this at a Pilot gas station in NC. When the guy one pump over hit the key, his battery exploded. It sounded like an M-80 went off and the top of the battery ripped open like the cover of a book. If you are experiencing a short, this is a symptom of things to come. Rewire the thing. You can get entire harnesses ready to install at a lot of the suppliers. A better and safer technique is to use a volt meter (multi-meter) and measure the drain between the - post and the vehicle frame. (positive ground systems) There is no fuse panel to make it easy so good luck.