750mV from battery to ground, Tracing the short: any usual suspects? Options

[Prospectfarms]

Key off. Red terminal to body gives somewhere between 1V and 750mV. (IMG:style_emoticons/default/sad.gif) To me that seems like a bare wire short to ground but I'm not an expert.

Likely spots? Anyplace I could test first before tracing the whole electrical system?

How about a short (N.P.I.) list of always-hot wires in the car. Doesn't have to be comprehensive -- just looking for a starting point.

Thanks.

[stugray]

What?

Does the car run?
You should have ~12-13 Volts between the Positive terminal and the car chassis regardless of the switch position.
Same as between the + and - of the battery itself.

To find a short, you need to have the meter in series with the battery and measuring current not voltage.

Stu

[Valy]

1St, your battery is dead.
Disconnect it and measure resistance between red wire and ground(body). If it's below 1k ohm start looking for shorts.

[underthetire]

To check ghost drains

Remove red wire from battery with key off
Make sure doors are closed, lights off, etc.
Place one meter lead on battery, the other on red wire
Place meter leads in meter to measure amps
Put meter in amp measuring range.

50MA or so would be normal for back up on stereo etc., possible more if you have an amp.

[Spoke]

Here's some basic voltages to check.

1) Measure battery voltage (on each battery terminal, not clamp)

2) Measure battery positive terminal (not clamp), to chassis ground.

3) Measure battery negative terminal (not clamp), to chassis ground.

Let me know what you find.

[Prospectfarms]

What?

Does the car run?
You should have ~12-13 Volts between the Positive terminal and the car chassis regardless of the switch position.
Same as between the + and - of the battery itself.

To find a short, you need to have the meter in series with the battery and measuring current not voltage.

Stu

Thanks, Car runs. With the key off and the + terminal removed from the battery. I get a little less than 1V. between the positive clamp and positive battery post.

[Prospectfarms]

1St, your battery is dead.
Disconnect it and measure resistance between red wire and ground(body). If it's below 1k ohm start looking for shorts.

Shoot. I went inside tonight and forgot to do the resistance test you described.

I spent all day and part of this evening replacing a connecting rod bearing on a tractor -- in the field -- and only spent a few minutes searching for the short on my car.

Will check tomorrow.

[Prospectfarms]

To check ghost drains

Remove red wire from battery with key off
Make sure doors are closed, lights off, etc.
Place one meter lead on battery, the other on red wire
Place meter leads in meter to measure amps
Put meter in amp measuring range.

50MA or so would be normal for back up on stereo etc., possible more if you have an amp.

I used to have a decent mulitmeter that did that, now I only have junk VAC/VDC/Ohms, etc. May get another good one to perform your test. Thanks.

[Prospectfarms]

Here's some basic voltages to check.

1) Measure battery voltage (on each battery terminal, not clamp)

2) Measure battery positive terminal (not clamp), to chassis ground.

3) Measure battery negative terminal (not clamp), to chassis ground.

Let me know what you find.

Sorry it took so long to respond Jerry. I'll have to perform the test tomorrow, but will let you know. Thanks very much.

[ape914]

What?

Does the car run?
You should have ~12-13 Volts between the Positive terminal and the car chassis regardless of the switch position.
Same as between the + and - of the battery itself.

To find a short, you need to have the meter in series with the battery and measuring current not voltage.

Stu

Thanks, Car runs. With the key off and the + terminal removed from the battery. I get a little less than 1V. between the positive clamp and positive battery post.

the one volt drop is between the battery post and the batter clamp? that is telling me that you have a high resistance clamp. remove the clamp and clean it and the post, re-install and see if the voltage drop goes away. the drop on that connection should be near zero.

The next thing to ask is what is draining the current when every thing is supposed to be off. but for sure if you have a 1 volt drop across the clamp, you got a high resistance clamp / post connection and since this is happening with all accessories off, you got some current draw that should not be happening.

Fix the post / clamp issue first, then figure which of the positive terminal wires has the leakage, by hooking up each of the wires that go to the positive post one at a time , one of them has a leakage thru it, else you wouldn't see any drop across the clamp/post. So you really have two issues to deal with.

[Valy]

What?

Does the car run?
You should have ~12-13 Volts between the Positive terminal and the car chassis regardless of the switch position.
Same as between the + and - of the battery itself.

To find a short, you need to have the meter in series with the battery and measuring current not voltage.

Stu

Thanks, Car runs. With the key off and the + terminal removed from the battery. I get a little less than 1V. between the positive clamp and positive battery post.

Ok, cool down.
You're not measuring correctly.
Measure between battery terminals and will get 12v or a bit more.
what you did is measured the voltage drop in your voltmeter. Switch it to mA and repeat your initial test - should get few mA with the key off. Dont measure when crancking the engine or you will fry your multimeter.

[Prospectfarms]

If I understand what your saying, if there was only a short, and no voltage drop, I'd see battery voltage (@13.11V.) between clamp and post ? (key off, no accessories, clamp not connected to post)

I tried to test the clamp for voltage drop yesterday. With engine running I can stab the heavy red wire with a probe and get the same voltage to ground as I get measuring from the post to ground. Not a good way to test?

[Prospectfarms]

What?

Does the car run?
You should have ~12-13 Volts between the Positive terminal and the car chassis regardless of the switch position.
Same as between the + and - of the battery itself.

To find a short, you need to have the meter in series with the battery and measuring current not voltage.

Stu

Thanks, Car runs. With the key off and the + terminal removed from the battery. I get a little less than 1V. between the positive clamp and positive battery post.

Ok, cool down.
You're not measuring correctly.
Measure between battery terminals and will get 12v or a bit more.
what you did is measured the voltage drop in your voltmeter. Switch it to mA and repeat your initial test - should get few mA with the key off. Dont measure when crancking the engine or you will fry your multimeter.

I know your right but it feels wrong. I'm visualizing the ignition as the master switch for the entire circuit between the car and battery and with the key off the circuit should be "open." But my meter showed juice when there shouldn't have been any. Maybe if I do the test you suggested It will help me "get it." Thanks a lot. I'll post back tomorrow.

[Mike Bellis]

The voltage you are reading could be capacitance from the ECU, coil or radio. Really the way you are testing will tell you nothing. You need to do a current test is you are worried about drain. 750mV is nothing and as long as you have electrical loads in the car, your test will tell you no valuable information. Remove the switch to test it properly. If you had a real direct short, your voltage reading would be 12V and not 1V.

I think you are worried for nothing.

20+ years of electrical troubleshooting experience.

[Prospectfarms]

The voltage you are reading could be capacitance from the ECU, coil or radio. Really the way you are testing will tell you nothing. You need to do a current test is you are worried about drain. 750mV is nothing and as long as you have electrical loads in the car, your test will tell you no valuable information. Remove the switch to test it properly. If you had a real direct short, your voltage reading would be 12V and not 1V.

I think you are worried for nothing.

20+ years of electrical troubleshooting experience.

O.K., its all becoming clear now. The coils, capacitors and such on that car act like small batteries, in a way, so their voltage will come across. A short, on the other hand will read the same voltage as the supply (battery) even it it's a tiny cut on the thinnest wire in the harness. I saw 1V, not 12V, so probably no short.

If a bare wire is finding ground I'd see 12V. The severity of the short, if any, would be read on the mA setting of the meter. That would tell me how much current is coming across. mA is "milliamps" or thousandth of an amp, no?

It is so fun when the haze clears and a little understanding sets in.
A lot of nice people with huge expertise have chimed in. Thank you all so much!

I'll keep testing...

[rhodyguy]

try taking the inline fuse for the stereo out and a measure again. any other electronic stuff other than stock that would require?

[Mike Bellis]

You want to measure amperage. You want to see less than 300mA with the car in a normal state (all fuses in place). Anything above 500mA will kill your battery in a couple days.

[Prospectfarms]

Dugout my real mulitmeter. Replaced fuse and batteries. Seems to work.

test results

Key off:

post to post: 14.6V

Pos post to ground: 14.4V.

Pos clamp to Pos post: 1V.

Pos clamp to Pos post: 25mA

Neg clamp to Neg post: 1V.

I guess that's within spec. I am grateful for all the generous help. Thank you.

[Valy]

You are good although 25mA for the stereo is a bit high. Is anything else always on?
And again, those 1V measurements only measure your multimeter internal resistance - in your case it shows a not so good result but be careful as you can fry your tester this way.

[Prospectfarms]

You are good although 25mA for the stereo is a bit high. Is anything else always on?
And again, those 1V measurements only measure your multimeter internal resistance - in your case it shows a not so good result but be careful as you can fry your tester this way.

(IMG:style_emoticons/default/confused24.gif) What stereo? No accessories . Uh, oh....