Finding Electrical short

[n5eba]

I was given a 1971 240Z from my nephew who had not driven it for over a year. He said there was a short somewhere and the battery would go dead. I have a new battery and took the altanator off and had it checked and it is OK. Any suggestions on how to trace down a short in the system?

[geezer]

Hmm, is there anything not working on the car? Any aftermarket add ons installed or removed? Try removing all the fuses and then use a test light with a long lead connected to your battery 12V post. Use the probe on the test light on both sides of the contacts in the fuse box where you removed the fuses. If the test light lights up while doing this, you have found the defective circuit and it will be a simple matter to find the short to ground.

[MikeW]

Get a multimeter and set it to the maximum amps setting; for most cheap meters that will be 10A. With everything switched off remove a battery cable and measure the drain between the cable and the battery. That will give you the total drain; the only thing that should be drawing current at this point is the clock but if there's another draw you'll be able to measure it. To figure out which specific circuit is the problem put the battery cable back and go to the fuse box. Pull each fuse out one at a time and replace it with the leads from the ammeter. The total draw through each of the fuses should add up to what you saw previously at the battery.

[RogerZ]

an easier (in mho) is to leave the meter betwixt the batt cable and batt post and start pulling fuses until the drain goes away. course first time you might want to have a 8 amp fuse between the meter and the battery in case total draw is more than that. most meters got internal fuse but safer to have another.

[Nissanman]

...or just insert a 12V globe in series with the Ground or Negative cable.

It will glow if there is a load, won't rupture any fuses and will extinguish when you remove the load.

Don't try and crank the engine with either the multimeter or the globe in the circuit :finger:

Typical culprits for running a battery down between starts are: -

1. alarm systems

2. rear window demisters

3. HI-FI head units with a constant 12v feed for memory etc.

4. anything else you can think of:(

[MikeW]

...or just insert a 12V globe in series with the Ground or Negative cable.

It will glow if there is a load, won't rupture any fuses and will extinguish when you remove the load.

Does the clock pull enough current to cause this test light to glow?

[dalee]

Gee n5eba must have read my mind, I found the same problem with my 71 240 last weekend. She drained a new battery (600cc) in 3 days, originally I thought it was the alternator not charging so I replaced that with a reco one.

May be I wired it up incorrectly:stupid:

NB : Everying on my car works fine and has done for the last 12 months. I pulled all the aftermarket crap off when I first bought it.

I'll try those ideas and see if I can track down the culprit.

[Nissanman]

Does the clock pull enough current to cause this test light to glow?

It can do but it depends on the wattage of the globe you use.

A smaller wattage will glow brighter for small leakage currents and vice versa.

Conversely, a high wattage globe such as a headlamp, say 60W, will allow most loads to function normally when used, bar the starter. So if you suspect an intermittent short you can use the high power globe and switch everything on and rest assured that the maximum current draw will be limited to 60/12 or 5A.

The idea is to give you an instant visual when the short occurs and not blow any fuses or burn any wiring:classic:

[SteveJ]

A bad voltage regulator can cause the battery to drain. It's easy to check, too. Just look in any repair manual: Haynes, FSM, etc.

[peng155]

Question... How do you know there is a short??... If you have a short you should be blowing a fuse. Also how long does it take to drain the battery down?