Parasitic Draw Testing - What am I doing wrong?

Posted by el_spidermonkey@reddit | askcarguys | View on Reddit | 2 comments

I have a 2005 Saab 9-3 that I suspect has a parasitic draw (it's killed my battery twice).

I have a multimeter, waited 30 min until the car was fully asleep, and tested to confirm the battery was good to go. Battery load test gave me a reading of around 12.3 V.

Next, I performed the actual parasitic draw test by:

switched the multimeter to the DC 10 Amp setting
disconnected the negative battery cable
attached the positive (red) lead to the negative battery cable
then I connected the negative (black) lead to the negative battery post

Doing the above gave me a reading of 0.00 on the multimeter, so I then changed the setting to DC mA. According to my multimeter instruction guide, this setting measures ranges from 40mA to 400 mA. I then got a reading of -01.36 mA on the multimeter (please see picture here).

The negative seems incorrect, but it was the first actual reading I got, so I rolled with it.

After obtaining a pair of pliers, so I could start removing fuses I tried to get another reading with the same setup. This time, however, no matter what I did, I received no reading (0.00) on the multimeter.

I did also check to see if I blew a fuse in the multimeter by switching to continuity mode & connecting the two leads. This resulted in a 0.00 reading & a buzzer sounding (per the instruction manual this indicates that there is continuity).

I also did another battery load test which gave me a similar reading as the first time.

What am I doing wrong here?

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Gunk_Olgidar@reddit

Negative amperage just means you have the probes reversed. Switch them and it will read positive.

Search up a video on "How to test for parasitic drain using fuse voltage drop" or just do the steps below.

How to:

Reconnect the battery. Let car go to sleep.
Click here and select the appropriate chart for your fuses.
Set multimeter to millivolts.
Then put the two probes on the two little metal contacts on the top of each fuse, one fuse at a time. Read the voltage drop across the fuse.
Find the appropriate cell on the chart, that's your drain in mA.

Keep probing all the fuses until you find the circuit(s) with the high draw.

Theory of operation: Ohm's Law. Each unique fuse as a specific internal resistance, you measure the voltage drop and by V/R=I you can determine current. The charts just make it easy and do the math for you. Just make sure you use the correct chart for the fuses in question.

el_spidermonkey@reddit (OP)

thanks for the reply. Much appreciated.