Well in the midst of our snowstorm, I went to start the 97 GMC. Dead battery. I bought a new one but found I am getting sparking at the negative cable when I go to connect. I hooked up my voltmeter and can measure battery voltage between the negative terminal and disconnected cable. This is still the case when I pull ALL the fuses and disconnect alternator. Any suggestions? BTW - I used the volt meter instead of a test light but seems the same to me in theory. Any pointers greatly appreciated!

Rick, I don't have a good answer for you. Just a question..... Is the POS cable still on the battery when you put the test leads on the NEG post and NEG cable end?
The multimeter is just completing the circuit if it is. Could that be where your meter reading is coming from?

I always get sparks when setting the NEG cable back on the post of the battery. Same goes for when I'm hooking up jumper cables. That's on a good battery, by the way, and the POS is already hooked up.

There is a big difference between a meter and a test light. The test light is a self regulating current sensor of sorts. The volt meter doesn't pass hardly any current at all. It's like the difference between a 10 ohm resistor and a 10 megohm resistor. Use the light for this.

You can use the current setting on the meter, but I would advise against it. For one thing, if the current is too high, you'll blow the fuse in your meter if not the meter itself. Secondly, it's just a matter of time before you forget to switch the leads from reading amperage to voltage before you place it accross a live circuit and blow your meter.

I know test lamps seem low tech, but it really is the best thing for what you are trying to do.

Live and learn. If I measure resistance across the disconnected cables, what's a good reading to look for as I repull all the fuses? I would think if the system is OK I should see 100K ohms or so (?). I'm just concerned that a new battery will soon go dead if that wasn't the problem. BTW - it was the original 97 battery so I don't feel bad about buying a new one.

I always check for parisitic drain with an ammeter.Make sure ALL lights are off,and the key is not in the ignition.On your GMC you should press the dome overide button so you can have the door open to pull fuses.I cant remember the exact spec but I think all you should have is about 40 milliamps.
And do NOT try to start it with an ammetet hooked up or you will see smoke from the meter. :eek:

Originally posted by RickD:
Live and learn. If I measure resistance across the disconnected cables, what's a good reading to look for as I repull all the fuses? I would think if the system is OK I should see 100K ohms or so (?). I'm just concerned that a new battery will soon go dead if that wasn't the problem. BTW - it was the original 97 battery so I don't feel bad about buying a new one. Are you an electrical engineer? It depends on a lot of things. For one, system resistances change with voltage - such as transistors in aftermarket radios, amplifiers etc. If everything is disconnected I would expect to see about a trig-ohm.

-dnult

dnult gives a good suggestion using the test kight. You should get a very dim glow due to ECM & radio.

Using the ammeter function of your DVM should show you, as Aaron suggested, no more than 50 milliamperes.

A 7 year old battery Delco battery may not hold a charge overnite. Get a new battery and try again in the morning.

You would be surprised at the resistance reading one would get when reading across a pair of battery cables. I have seen as low as 2 ohms, to as high as 20K on other models & makes. There's always going to be the mysterical path, alternator is a good one, usually around 100K ohms, of course, this is disconnected from the rest of the wiring harness.