MSD - battery drain

[Gaz64]

Undy, I've been a mechanic for 30 years, you don't use a voltmeter in series.

You're measuring CURRENT draw for which you use an Ammeter (AMP meter).

Amps - series, Volts - parallel.

[undy]

Originally Posted by Gaz64

Undy, I've been a mechanic for 30 years, you don't use a voltmeter in series.

You're measuring CURRENT draw for which you use an Ammeter (AMP meter).

Amps - series, Volts - parallel.

I won't debate this any more...

FWIW, I've got you by 10 years and have a degree in Automotive technologies.

[PANAVIA]

Another idea would be to check overall harness resistance at the battery cables. --

with the car , off , cold, key removed etc.

-- undo both battery cables, -

Measure resistance in OHMs for the harness (one lead on the - bat cable one lead on the + , NO BATTERY TOUCHING .

-= , it should be just about open.

if you want to find out how much your MSD leads are drawing, you can remove the MSD battery lead from the solenoid and observe the difference.

if the issue is somewhere else , you can isolate by pulling and replacing fuses 1 by 1 until the resistance goes away.

There are some items that require constant voltage. - clocks, computers with memory, radios with memory, alarm systems.

but in a simple car like the Cobra, there should just be about NO draw at the *all-off* point.

[PANAVIA]

Another idea would be to check overall harness resistance at the battery cables. --

with the car , off , cold, key removed etc.

-- undo both battery cables, -

Measure resistance in OHMs for the harness (one lead on the - bat cable one lead on the + , NO BATTERY TOUCHING .

-= , it should be just about open.

if you want to find out how much your MSD leads are drawing, you can remove the MSD battery lead from the solenoid and observe the difference.

if the issue is somewhere else , you can isolate by pulling and replacing fuses 1 by 1 until the resistance goes away.

There are some items that require constant voltage. - clocks, computers with memory, radios with memory, alarm systems.

but in a simple car like the Cobra, there should just be about NO draw at the *all-off* point.

[PDUB]

Originally Posted by undy

I won't debate this any more...

FWIW, I've got you by 10 years and have a degree in Automotive technologies.

From a testing perspective y'all are arguing about nothing, as either way works effectively the same. The good news is everyone is correct! (Don't ya love it when that happens!

A VOM (for volt-ohm-milliampere) meter can be used to measure the open circuit (CKT) voltage when the ground wires are lifted (though this is not intuitive). The reason this works is because the ground lead on the box floats up to the source potential if it is powered on and attempting to draw currect, i.e. to operate (recall that you need a voltage potential to be present in order for current to flow). If you read no voltage potential then the unit is not on nor trying to draw down the battery. At this point, though, you might see spurious readings, i.e. the meter might pick up electrical noise... and no, the volt meter will not get cooked.

Alternatively, you can measure the current draw directly in the ammeter mode (but make sure that you are on a high enough range setting so as not to fry the meter). Of course, if you measure current then the box is on and draining the battery.

That said, I am not familiar with the MSD box. From what I have gathered in this thread, it appears that there are two power leads to it, one switched, and one that is always on??? If that is the case, then either measurement technique should reveal some power usage by the box, though it might be quite small when in standby mode, i.e. engine not running and ignition off. Sorry if I misunderstand how the box works.

Good luck!

[Gaz64]

I'll stand corrected, I'll try it myself one day, but I still prefer to measure current draw, after all that's we are looking for in this case.

[undy]

I'm well aware of checking by a measured amp drain. As others have said, there's more than one way to skin a cat though.

[PDUB]

Interestingly, most ammeters measure current by actually measuring the voltage drop across a very low precision resistance (so as not to load the CKT you are testing) and then doing a conversion (using Ohm's law). This is why it is easy to fry the ammeter section of your VOM if you aren't careful.

The voltmeter section of the VOM is a very high impedance CKT (also as not to load what you are testing) and thus uses negligible current. This is why you might see some noise on the meter if the box is a high impedance too...