Interpreting DC Voltage Reading on Multimeter

[Lester Lugnut]

Am tracking down a parasitic drain on one of my cars. Using a test lead that has a 1 ohm 10 watt resistor in the middle of the lead as demonstrated in this video:

This is a key-off test - engine not running of course. My meter is a Craftsman with auto-ranging. Set to the DCV scale, I see the following:

037.8

If it read 3.78 I would interpret that to read 3.78 volts. I can't make sense of the 037.8 reading?

Thoughts appreciated.

Thanks.

 

[Trey T]

I would do current test on ciruits for fuses less than the DMM fuse first. This easy to do and quick to eliminate a lot of the circuit.

 

[Lester Lugnut]

Trey:

I take that to mean pull each fuse individually, set meter to DCA scale then place leads at each end of fuse holder?

Thanks.

 

[Bruce Amacker]

You're probably doing your testing correctly and reading 37MV, which in your testing equates to 37mA which is not going to hurt your battery. Turn on your dome light, or open the door, and your reading should go to approximately 1-3 amps, or 1.000-3.000v as read on your meter. Many Cman meters will read amperage up to 10 amps, in which case I recommend you test it that way. If it has a third port on the front of the meter with a 10a designation it will do amperage to that level.

The typical industry guidelines for parasitic drain are in the 50mA area or less. Very few mfr's actually publish a number because the drain varies widely depending on what options are on the vehicle. GM published a TSB a few years ago on Trailblazers with a number about 30ma IIRC. If you do the math, a normal battery with a 30-50ma draw will last 6+ months before going dead.

Good Luck!

 

[Trey T]

Yep, exactly. It worked for me but I'm no expert; I diagnosed two cars with the issue you described.

I remember one car I tested was the auto domelight circuit. I didn't fix it because the owner didn't care for it and it would be a lot of effort to track down the problem like door switches and such. The owner just pulled the fuse and disable that whole circuit.

Trey:

I take that to mean pull each fuse individually, set meter to DCA scale then place leads at each end of fuse holder?

Thanks.

 

[Bruce Amacker]

The fuse method works if the circuit in question has a fuse, but many don't. For example, a common source of battery slow drain is a shorted alternator diode which has no fuse and won't show up in that test.

 

[MrMark]

Easiest way to do this is to let the car go to sleep (wait 45 minutes after shutdown) and doors closed - you will need some way to make the car think the door is shut if the door has to be open to get at the fuse boxes. Once the car is asleep, measure the voltage ACROSS the top of each fuse. You are measuring the voltage drop across the fuse. Compare this value to table values available online for each fuse and you will very quickly as least find the circuit's fuse that has the draw. You also need to know what are the acceptable parasitic draws for this car.