Electrical - is this normal?

[Swngaxl]

This may be a basic, simple question but it has me confused. Please help me understand this phenomena. Tested two 66 Monzas, both do this:

1. Everything turned off, car not running.
2. disconnect negative cable from battery, leave positive in place.
3. touch a test light from the bolt where the negative cable attaches to the head and to the negative terminal, it lights up.
4. Voltage reads 12V between the grounding bolt and the negative terminal.

i can see how the test lamp will complete the circuit by taking the place of the negative battery cable, but:
How can their be current flow to light the test lamp if everything is turned off?
I can understand 12V from the positive terminal to the ground bolt, but the negative terminal to ground?

Thanks, hoping to understand how this stuff works.

[66vairguy]

Your process is the first step to determine if "something" is using power and running down the battery while the car is not running.

I'm not sure how much amperage is needed to turn on your test lamp. The first question is - does your battery run down if the car is unused for a couple of weeks, assuming the battery is in good condition? If it does NOT run down, then the current would be very small and could simply be the alternator if you are using an electronic voltage regulator, or have converted to the internally regulated alternator. The amperage draw would be very small and a good battery would not be affected by it unless the car sat for many months. BTW - a battery should always be re-charged monthly, even when disconnected.

The next possible issues are things that are always connected to electrical power - Clock, cigarette lighter, brake lights, dome lamp, and don't forget the often overlooked GLOVE BOX LIGHT.

BTW - All my old cars have NEGATIVE battery terminal disconnect switches. Helps avoid dead batteries and possible fires, not to mention making it more difficult for thieves.

[bbodie52]

One more possibility — the installation of an aftermarket electronic radio head unit. Modern radio systems often have two power wire connections. The low-current, continuous power lead is used to power circuits that run continuously, like an internal clock, station preferences stored in memory, etc. The power drawn by these memory circuits is minimal, but needed to retain settings. The other radio power lead is the main (primary) power that powers the unit only when it is turned on.

If you have installed an aftermarket radio system, a modern electronic digtal or analog clock, etc. it would account for the results you are seeing with a test light that shows an active device that is connected to a voltage source and is continually drawing a small amount of current. Reestablishing the connection between chassis ground and the battery negative terminal with a trouble light would cause the light to show an active connection somewhere in the car between the positive and negative terminals of the battery.

[Jerry Whitt]

One more possibility. If there is a bad diode in the alternator, in effect an electrical leak, this might happen.

[terribleted]

Yet another possibility is there a clock?

[64powerglide]

He did not say anything about draining the battery!!!!!!!!!!! His test light is coming on when he touches the disconnected negative post of the battery to the frame, the light is acting as the ground wire. He's simply completing a circuit & the test light comes on. That's the way I interpret his question anyway.

[Swngaxl]

Thanks for the input, now for a few answers. First car: yes, the battery drains. yes, there is an aftermarket radio. No clock. Yes the test light is simply completing the circuit in place of the negative battery strap.

At first, the test light would glow bright then dim, disconnect, repeat. As if a charge were building up for some reason. So we pulled one fuse at a time, and checked each time. Then we noticed when the fuse for the tail light, dome light, etc fuse was pulled, completing the circuit caused something under the dash to click. I thought it was the turn signal relay, disconnected it (I t hink) but no change. Then we reconnected the battery cable, the click went away and would not come back. Never did figure that one out.

So, we did the same test on car #2, which is basically in storage. The negative cable is not connected on it, but the test light glows strong when touched negative terminal to ground.

So I don't know, this may be completely normal and is just something we never tried before. Or it could be a problem. I know modern radios have a parasite load to maintain channel settings on the radio. Think this particular car would drain the battery in about two weeks of sitting. It has a disconnect now where the cable attaches to the negative terminal, just want to make sure whether we have a problem or not.

[joelsplace]

Unless you have a clock or a radio or something that should draw power all the time you have a problem. The light going bright and then off sounds like a factory clock winding up.

[64powerglide]

At least you question is better explained, could also be bad batteries. I would have the alternator & batteries checked first.

[joelsplace]

Bad battery won't cause the test light on symptom he has.

[Jerry Whitt]

Next step would be to stop by a local Autozone and ask for a battery, starter and alternator test. The test is free. Test results should point in the right direction.


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[bbodie52]

The positive cable provides a voltage source to all electrical/electronic devices in the car. Some, like the clock, dome light, horn relay, etc. have voltage applied continuously, but only electronic clocks, electronic radio memory circuits, etc. draw a small amount of current continuously and would cause a continuous path from the positive battery terminal to ground. This would be indicated by an active test light when that device provides a connection from chassis ground to the battery negative terminal. The original electro-mechanical clock was really a "wind-up clock" that only draws power briefly when the internal clock spring unwinds enough to cause the motor relay contacts to close, powering the motor to wind the clock spring. Once the spring cycle has been momentarily activated to wind the clock, it stops drawing electricity until the clock spring unwinds again. Electronic clocks draw a small amount of power to maintain the time, but if it has an illuminated digital display the display may remain off unless the key is ON.

If you suspect a shorted diode in the alternator, you can simply disconnect the alternator temporarily to see if disconnecting the alternator eliminates the test light power indication. Also make sure the doors are closed to ensure the dome or interior lights are off, and confirm that the glove box light is also off. Only an active circuit will permit electrical continuity from positive to chassis ground, which would be indicated by an active test light when completing the circuit to the battery negative terminal. Substituting a multimeter to measure current would provide an actual current reading in amps or milliamps to indicate the measured battery drainage caused by the active circuit.

[66vairguy]

O.K. One battery sounds suspect. If it is over three years old it may need replacing. The ONLY battery I've used in my old cars that stayed healthy after three years was the OPTIMA. Yes they are expensive, but my experience is they last twice a long as other name brand batteries. BTW - my experience with parts stores testing batteries and alternators has been less than encouraging. Recently a club member took a regular 10DN Corvair alternator (external regulator) to a parts place and was told "Those are too old and we can't test them on our setup".

A word of advice, find someone in a local club that knows old car electrics, or can recommend a good shop.

NOTE: That AAA has been notifying clients that an unusually high number of "dead battery" calls has come from clients that parked their cars while on vacation trips for two to three weeks after which the cars would not start. AAA did some tests and found the combination of small batteries and a large number of electronics that draw electrical power in "standby" was the problem. So if you have modern electronics like a stereo system, then that could be a problem. I've seen folks wire up sub amps incorrectly causing them to stay on when the radio is off, etc.

At this point it's a matter of disconnecting things until the test light goes out and then determining if the load is drawing excessive electricity. A good electrical person can use a high capacity AMP meter, or a load and DVM to determine that.

Good luck.