Odd voltages

Charlville

Charlville

Senior Member
Now, as many of you will deduce from this post I’m not an electrical genius. But I’d like to get an answer to this one as it’s stumping me :)

1929 Sport Coupe
Points ignition
12v Negative Earth with alternator
Kill switch in the battery to chassis line (edit negative post thanks Paul)
1 x engine to chassis earth
1 x gearbox to chassis earth
All kill switch on voltage is correct
Charging fine

Kill switch off and main fuse in and pressing horn switch I get around 6 volts at the horn.

Same process but with the main fuse out the voltage is 0.2 volts i.e. negligible.

Can I assume that perhaps the voltage is being held by something such as the condenser or even the alternator (Wild guesses :))

The reason I’d like the car to be “totally dead” is that it’s in my house garage.

Any thoughts are welcome and be gentle with me please :)
 
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Negative ground would also mean ground post (for clarity). Some later horns in the early V8 era, have a capacitor but Model A horns don't since they are a relatively simple electric motor drive. Later ones are a ******** coil type.

If your horn is getting any voltage at all then the battery cut off is not 100% effective. No voltage should be detected on any component. If you have an ohm meter, isolate the cut of switch and see if it has any continuity from one side of the switch to the other. It should have none. Ground from engine to frame or from transmission to frame should have no effect.

Another thing to check would be the alternator. If a diode in the internal bridge of it goes bad, it may get some connection to ground through it. The alternator would have to be disconnected or isolated from the power wire to check this.
 
If the kill switch is truly disconnecting as it should, then the bayyery is isolated out of the circuit and there will be no power anywhere, sounds like you have a resistive disconnect switch.
 
The condenser is blocked by the ignition switch so that's not it, (even if it stored and voltage the act of measuring it would discharge the cap.) Same with the alt, there is nothing to generate voltage when still or to store voltage that would not go away when measured.


You have a bad switch. With the switch off it is the same as removing the battery cable.
 
eagle said:
... sounds like you have a resistive disconnect switch.
Click to expand...

I agree. Remove the fuse and measure between battery post and chassis with an ohmmeter. It's probably something in K-ohms; if so, not to worry about any hazard. Could also be conductive surface contamination on the switch or battery case.
 
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