12 volt reading from Negative terminal

[richardnlambert]

Hi All

I had a short in my wiring and confirmed this with my multimeter I disconnected the negative lead and set my meter to Amps then put one lead to the negative lead and the other to the negative post and got a reading (can’t remember the reading) however I then used a test lamp which lit up and I had 12volts from the negative lead to negative post.
I think I have found the short but still get a reading of 12 volts across the negative lead and negative post but no amps and the test light did not light up

All the above with ignition on or set to park

Have I got another issue
Thanks

[Bryanj]

You are basically measuring the battery voltage

[Sesman]

Assuming the ignition is on, the current you have measured will be the the coil and field winding current. Don’t leave the ignition on for long as I understand it’s possible to overheat and damage your coils. In fact, if you rotate the engine, you will see the current fluctuate as the points open and close. The voltage ‘negative to ground should be virtually zero as the negative post and lead should be at substantially the same potential. You will of course measure the battery voltage from positive to negative or indeed positive to earth, if the negative lead is connected. Remember that voltage is what you call it and is always ‘relative’. The voltage you have measured will be proportional to the the current x the lead impedance. V=IR.

[richardnlambert]

Thanks you’re right about the coils my neighbour had just fitted new coil to his classic car left the ignition on overnight only to find the oil from the coil all over his engine

[McCabe-Thiele (Ted)]

Thanks you’re right about the coils my neighbour had just fitted new coil to his classic car left the ignition on overnight only to find the oil from the coil all over his engine

Nasty,  I've accidentally left the ignition on decades ago without causing a problem - I was told it was proably because the points were either closed or open - forgotten which it is that messes them up.

[deltarider]

When I expect to work long time with IGN ON, I do this (see pic). If I want to check for spark, I have a wire with alligator clip on one end and the other end is just wire. To test spark, I clamp the aligator clip on the points little bolt and I strike the stranded copper wire on the engine, gearbox, whatever ground. The result is a serie of sparks that will give you a far better impression of the spark's quality.

[Sesman]

Good tip👍