[Steve B]

Am changing to an internally regulated alternator for my '70 and I am looking pretty good in the wiring department, but in checking a few things out with my twenty year old old Radio Shack analog VOM, I have noticed something really weird that I
thought someone might have an explanation for.

The only change to the electrical system is an aftermarket volt meter. I have
had no trouble with the electrical system, and as such I have left well enough alone. The reason I am upgrading the alternator is the typical complaint of low out put of the original.

Here is the deal though. With the negative battery cable disconnected (it has been disconnected for over a week now) if I put my voltmeter across from the battery side of the solinoid to ground, or from the end of the alternator batt wire to ground, I get around 6 volts. Where is this coming from? If I switch on the heater blower, or push in the cigar lighter, or turn on the wipers, these all make the reading go to zero. There is no effect if I turn on the lights. Now remember- I say I am switching these accessories on- but the negative battery cable is disconnected. If I disconnect the positive battery cable too, the reading is also zero.

Is that weird or what? Like I said above- I have made no other changes except for the alternator. It makes no difference to the mystery 6 volts with the alternator connected or not.

 

[Schurkey]

Yep, this is a stumper! It might help if you told me where you are connecting the leads for your Radio Shaft VOM.

I'm gonna take a WAG. (Wild Ass Guess)

You are connecting the VOM to a handy chunk of iron in the engine compartment, not to the battery negative terminal itself. The + post on the starter should show battery voltage if the meter is connected to the - post of the battery.

The negative battery cable is not the only way to connect the battery to ground. I bet you have a film of conductive material on the battery, road residue, spilled electrolyte, Elk snot, or whatever, that provides a (poor) ground path for a TINY bit of current. A 12 volt battery with a terrible ground path like that might only show a 6 volt reading on a low impedance meter. It would be like having a BIG resistor in the circuit.

Why does the voltage disappear when you turn on certain accessories? Ummmmmmmmmm... The electrons get so dizzy going through the wiper switch and motor that they can't find their way back to your meter???

How about: The other accessories ground through a different section of the steel body, (and then to the battery tray and up the side of the battery on the contaminant trail) than what you are connecting the negative lead on your VOM to. Therefore the VOM shows no voltage when those accessories provide a ground path for your stray voltage.

Other opinions welcome!!!!!

Please E-mail me if you clean your battery and the stray voltage reading goes away. I'm all kinds of curious about this.

 

[Steve B]

I had the same suspicion regarding the slimy battery. Even though the posts and clamps looked good, the battery or tray must have been the culprit. After I removed the battery, and really cleaned the tray, the battery, the clamps, etc, the voltage went away! The battery will be 6 years old this summer, so it has been awhile. Makes a good case for cleaning the battery every year or so. I guess the reason the volts went away after switching on an accessory, is cause there was so little current to start with, the device would draw it down to zero volts.
Thanks! SB

 

[Schurkey]

Glad to hear the problem is fixed. Lots of batteries get scrapped because they "won't hold a charge". In fact, they are just fine...except they are covered with Rhino Poop, and discharge through the conductive film over the course of a weekend or whatever.



[Note: This message has been edited by John_Muha]