MSD - battery drain

[Gaz64]

Originally Posted by undy

The professional way to do it would be to again disconnect the box's ground wires with the car turned completely off, in the battery drain mode. Place a voltmeter between the ground wire(s) and actual ground. If you've a battery drain you'll read a voltage. The amount of volts read will be proportionate to the amount of drain.

Originally Posted by Gaz64

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Placing a voltmeter in the circuit will not work, you need a ammeter to measure current.

Originally Posted by undy

Not true... Your'e looking for a voltage potential difference. It'll tell you what you need to know.

Please reread your above statement.

You're telling me you have a voltmeter connected between your disconnected ground wires and ground.

Of course you'll read something, that's not how you look for voltage drops.

[undy]

Originally Posted by Gaz64

Please reread your above statement.

You're telling me you have a voltmeter connected between your disconnected ground wires and ground.

Of course you'll read something, that's not how you look for voltage drops.

That's exactly what I'm telling you. You could also check it the same way on the 2 hot leads going to the box, the switched and unswitched power supplies. Place the voltmeter in series

...not looking for voltage drops (that's not what we're doing anyway), looking for MSD box power drains through the box's ground circuit(s). If everything's off ... no, you shouldn't read voltage.

[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.

[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.