Proper Current Draw Test

[rattmann316]

working in an install shop for many years, one of the most frequent complaints i used to get was "that (whatever we installed within the last 5 years) drained my battery!" or "i took it to the dealership and they said your (whatever we installed within the last 5 years) killed the battery. they wont touch it until you pull it out. wheres your manager...etc" anyway, so i figured i have had to explain a current draw test so many times, i would do a write up on it so no one will ask me again. pictures to come soon, i just had to write this all down.

1. make sure vehicle is off and the key is out of its cylinder. all doors should be shut and if there is a light under the hood disconnect the harness running to it. your goal here is to make sure no interior lights are on (or any other convenience feature for that matter) so that you can accurately reproduce a "parked overnight" scenario.

2. you need a multimeter that can measure amps (current). most can do this but you have to switch one of your leads to go into the slot with a big "A" on it. this will all depend on your meter so make sure you know how to use one.


3. set your meter to measure milli-amps. depending on your meter (some autorange) you will have to look for a symbol like this: mA. if yours autoranges, then just setting it to "A" should suffice.

4. to measure current, the meter must be physically in line with the source thats being measured. this is unlike measuring voltage which is done in parallel with the circuit. basically you need to take off the positive terminal from the battery and attach one of your meter's leads to the terminal post on the battery and the other to the wire you just removed from said battery. so now, essentially your meter is completing the circuit.


5. now you will see a measurement of the vehicles idle current draw.

0-14mA.....normal
15-29mA.....above normal. this will more than likely cause the battery to fail prematurely
30 or more.....this battery will be dead in 24-48hours if left untreated.

6. if your measurement is above 15mA, you need to begin troubleshooting your electrical system. obviously, the first things to check would be ANY aftermarket electrical modifications you have done to the vehicle (amps, alarms, even gauges, etc). you do this by literally pulling one fuse at a time (and then putting that fuse back after you get a reading on your meter) until the current reading drops. once that happens, youve isolated at least one of the problems (there may be more depending on your situation...ie something electric failed causing something else electric to fail etc....).
recent work on an m3:

before:

after:


so here i have isolated the problem to fuse 21. i read the manual and this was for clock, radio, dome lights, glove box, etc. etc....took forever to isolate the actual problem.

7. in the rare instance you have a reading of 150mA or more, you should be pulling relays and not fuses because one is probably stuck open. 150mA is about the standard current draw for a 12v bosch style relay when energized so that would be a pretty easy diagnosis (usually).

hope this helps everyone. if there is anything i forgot or something that needs to be clarified, let me know.

[vtecduck]

good stuff

[zexaccord]

very nice writeup indeed

[92sickw/it]

yea VERy nice thanks!

[Mixelplix]

Good information, and easy to understand.

-Des!

[EJ1 SCRAPPIN]

Nice Man!

[rattmann316]

thanks fellas. im hopin this will get stickied just so people can say "check out the thread on current tests" instead of reposting what a current test is.

[Printz]

Originally Posted by rattmann316

thanks fellas. im hopin this will get stickied just so people can say "check out the thread on current tests" instead of reposting what a current test is.

Word, I vote sticky!

[rattmann316]

Originally Posted by Printz

Word, I vote sticky!

thanks...maybe i need pictures or something. too dark now tho

[woody]

Good information. I have a question about some remote starts that I have had problems with. Have had several that have had park neutral position switch codes.

[eastbobdam]

Never thought about doing this, but good write up.

Also, great GIF in your sig...where's that from (movie)?

[bluehatch]

Make sure your interior light is switched off before you open your door to start pulling fuses! You could blow your meter's fuse if you're on the milliAmp setting. On my meter they are like $6 and you have to take the meter all apart. I put an inline on one of my leads but it's still a pain to blow one.

[D2ZBOY]

I remember how to do that in UTI.