Thread: Battery or Alternator bad?

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Originally Posted by Sniper

Nope, I'm not goonin ya! Ok, the test light your using, is it an incandescent bulb? ( Can't use an LED type for this type of test. )Did you have the door open at the time of the test? That would cause a draw, because the interior light would be on. How about the hood, have a light that comes on when it's open, or the trunk lid, same thing would happen. Everything has to be off, that would normally be off, including the key switch. Any of these would cause a closed circuit and make your test light come on. The one thing that may be getting you, is the plug on the alt that plugs into the regulator. Unplug it, to eliminate it, while you do your test. If everything is off, and pulling the fuses won't make the light go out, you have one of three things happening. 1. The circuit that is involved isn't fused, or 2. you have a short on the positive side somewhere, ( pinched or wire rubbed bare ) or 3. an item isn't being shut off with the key switch. ( Electric fuel pump? Stuck brake light switch? Glove box light? Radio-tape-CD player? ) With the test light hooked up inline ( in series ) on the positive cable, any, and all power flow will have to go through it. It will only light up if there is flow. Just have to keep hunting, feel and smell for anything getting hot. Now isn't being a mechanic fun. I've been doing this stuff for a lot of years, and most times these things are simple and stupid. Well, they are once you find them. Sniper

I have seen some cars with keep alive circuits that fade to a very small draw (clock for instance) when the battery is actually connected but will still light up the test light. Some stereos will do that too.
Very often I find the draw on a GM to be inside the alternator, shorted internal regulator I suppose. They can even still charge ok and do that.

Originally Posted by willowbilly3

I have seen some cars with keep alive circuits that fade to a very small draw (clock for instance) when the battery is actually connected but will still light up the test light. Some stereos will do that too.
Very often I find the draw on a GM to be inside the alternator, shorted internal regulator I suppose. They can even still charge ok and do that.

Hi Willowbilly, Yup your right, an alt can show a draw for a couple of reasons, and if it does, it would or should, be very small. At least not large enough to kill a battery under normal use. But that is why I mentioned unplugging it so it wouldn't interfer with the test. Also, if a meter is used along with the test light, you can determine just how heavy the draw is. The one draw that gets a lot of people is when a diode on the trio goes bad. If you have an indicator light instead of a gauge in the dash you might catch it. It will glow real dim, so dim in fact that you can't see it in day light, but it will show if it's dark out. That one, can and will drain a battery, if a car sits for a few days. But the darn thing will allow the alt to appear to charge in a normal manner. As long as you drive it enough, you cover up the problem. Until it's parked at the airport for a week or two, or stored for a bit, and then it's, "hey, what happened, it was ok when I left it here?" This stuff pretty well has to be eliminated step by step, if you hop scotch around the system your more likely to miss it, if you do find it this way, it was likely pure luck. Not that there's anything wrong with being lucky, it just doesn't happen often enough. Sniper