The BEST Way TO Perform a Parasitic Draw Test

hey everybody has travelled from humble mechanic common today I'm going to show you the easy way to measure parasitic current drop a while back I had shot a video talking a bit about parasitic current draw but I thought it would be really important to follow that up and show you guys exactly how I do parasitic current draws this is one of those tests where there's multiple ways to do it and while none of the ways are going to really be wrong there are certain ways and techniques to do a better job more accurately faster than others this also can really depend on the age of the vehicle in the equipment if we're talking about a 98 GT I like my GTI right over here you can use a different strategy than you would on say a 2015 toerag the way I'm going to show you is the way I do it at the shop on the most modern vehicles and this way is great because you don't have to open up any circus you don't have to disconnect your battery to get your meter hooked up to it you don't have to fiddle around with one of these amp clamps which is really challenging to get calibrated right or break out the big test equipment in order to measure like I mentioned in that other video pulling fuses and disrupting circuits in any way even with the key off can cause the draw to go away temporarily and lead to you not finding a problem with the car at that moment when there's actually something going on the only tools that we're going to really need for this is going to be our multimeter as well as a chart showing a relationship between voltage drop and current draw I got these charts from the power probe website because they're really nice their color charts you can download them you can print them and it shows what style of fuse that they're talking about which is important there are other resources as well but I highly recommend the power probe one and I'll be sure to put a link down in the description so you can check that out before we get started on this test we want to make sure that we check our multimeter leads simply turn your multimeter to ohms and touch the leads together you should get about 0.1 or 0.2 ohms if you get anything higher than that or if you wiggle the leads around numbers jumping all over the place it's time to get new meter leads I can't tell you guys how many times I've seen other people get burned by this and been burned myself so now anytime my multimeter gets turned on the very first thing I do is check the leads and if I'm even concerned about them being bad at all I get new ones there's no way to rely on accurate readings when you're not a hundred percent sure that your test equipment is good we're going to be using my Passat for this test the fuse panel is in a really great location it makes it really easy to shoot a video your car could be different fuse panel maybe in a different location but the testing of this really is exactly the same no matter what kind of car you have so with that let's head out and check for a current draw alright so before getting started on the current draw we're going to just do a quick check and make sure the battery is good we're going to turn our meter on and turn it to DC volts now you'll notice that this voltage is a bit low to accurately do this test we would want to make sure we charge this battery and had it somewhere around twelve point six volts if your battery has a weak state of charge it can affect the reading of the current draw test we also want to load the battery by starting the car and make sure that it doesn't drop below 10 volts this is also going to be a perfect time to check the charging voltage you should see something in the neighborhood of 14 volts but be sure that all the loads are off if you have the air conditioning on the heated seats the defroster the radio booming and while that's not abnormal we want to make sure we're getting to around 14 volts unloaded after we've confirmed the battery is good we want to make sure we fix any known issues if we have a door latch that's not locking or any faults stored in any of the vehicles computers we want to make sure we take care of that first there's no sense in trying to find a potential problem while ignoring something we know is wrong that goes for issues in the starting charging system as well next we want to begin to prep the car want to start with latching the hood and all the doors now the reason we don't want to simply just shut all of the doors is we're going to need to get access to this fuse panel so we shut the door we're going to have to unlock the car and open it back up and that's going to defeat our current draw test so it's always better to go ahead and latch all the doors the hood and the trunk then lock the doors and put the car to sleep it may also be a good idea to mark the doors in some way that's very obvious so that you don't accidentally walk by and just slam the door shut it's almost second nature to walk by a door and want to shut it putting some tape on the latch or the handle may help remind you not to shut the door another thing that I like to do is just double check in the instrument cluster make sure it doesn't show any of the doors the hood or the trunk still open if it does you might have a problem with one of the latches and that could be your draw problem too next we're going to lock our car we're going to give it about an hour to two hours in order for the vehicle to go fully to sleep once the car is fully asleep we're going to take our other multimeter and we're going to put it on millivolts next we're going to measure the voltage drop across the fuse that's as simple as putting a meter lead on each side of the contact as you do that you're going to get a reading and then it should drop down to zero if it doesn't drop down to zero we know we might have a problem so normally the way I do it is I measure each fuse and I look at my meter to see the drop it takes about 3 to 5 seconds per fuse for it to drop all the way down to give you an example of a circuit with a draw versus a circuit without a draw I'm on the radio fuse here and as you can see we have no volt drop across the fuse but when I turn the radio on you can see we now are reading 3.4 millivolts that doesn't really tell us a whole lot other than we know we have some current going through this circuit once we know the amount of volt drop across our fuse we're going to use the chart on the power probe website to see how much amp draw that volt drop is causing if we're not sure what kind of fuse we have we can click on the chart and right up here in the top it'll show us what kind of fuse they're talking about in this case we have one standard fuse this is the sized fuse that we used for the radio we're going to use the guide at the top to find the amperage and color fuse we had that's a yellow 20 amp fuse then we're going to look at the numbers down here on the left and we're scroll all the way down till we see our reading we had a reading of about three point four which means we had a current draw of about point one and that's about three times more draw than our spec of 35 milli amps while that's not going to cause the battery to die in 15 minutes for an extended period of time that can lead to a dead battery now this is a higher line radio with an amplifier so that's a pretty decent draw but let's look at the kind of draw that will kill the battery in just a few hours this fuse is going to be for the high beams on the vehicle and not both high beams this is actually only the fuse for one side High Beam as I turn the high beams on you can see we have 29.9 blowing on 30 millivolt reading let's look at our chart and see how much current that draws now the Volt drop we were checking on the high beam fuse was a mini fuse that's 10 amps so we're going to stay in this chart here we're going to scroll all the way down that's a big time draw we had a reading of 30 millivolt our chart only goes up to 10 so we're going to multiply that times 3 and that's going to give us a reading over 4 amp I think it's pretty safe to say that if we had a high beam on we would see it but that gives you an idea of how much current one bulb can draw now I know what you're thinking cause this is going to take forever to do a proper current draw this way why can't I simply just pull the fuses out and see if the draw on my meter goes away well like I mentioned earlier that can upset the rest of the car and the computers we're running things on virtual powers modules powering other modules so that may not really be the best way and after you've done this a few times you can run down checking each one of these fuses and maybe only spend about 10 minutes max and it goes really fast if you don't find a problem at all and remember before wrapping it up you want to unlock the car and open all the doors and you can go ahead and remove your tape or whatever way you mark the car to remember not to slam the door so there you have it we have checked for current draw we've seen what a big draw looks like what a minor draw looks like and how to set it up and how to use a multimeter again remember your battery has to be in good condition you're starting charging system has to be in good condition otherwise you may be wasting your time doing this test and not fixing a known problem while finding the circuit that's causing the draw is really easy finding the component on that circuit might be a little bit more challenging you really need to know and understand all the things that are on the circuit and all of these things that are not on the circuit that communicate and talk to the things that are on the circuit so it can get deep and very challenging to find a current draw especially on modern cars you have modules talking to each other over CANbus this module way over here can wake up this module right here so it can get to be a really deep problem but this is a really easy and quick way to check and see if you actually have a draw in the first place all right guys I'm going to wrap it up there you have questions or comments you know what to do you liked this video and got some value from it throw the thumbs up on YouTube I always appreciate that you can also subscribe right here on YouTube don't forget to ding the bell or over on the blog at humble mechanic dot-com if you want to support the show get exclusive content exclusive discounts as well as VW Audi training manuals and a secret Facebook group where we do Facebook live videos just for crew members check out the crew membership program there's a link down in the description you can learn more there you can also follow me on Facebook Twitter Instagram and of course on snapchat alright guys hey thanks so much for watching I hope you never have to do this test ever in your career or in your life but if you do this is the best way to do it

Channel: HumbleMechanic

Views: 2,326,670

Rating: 4.8536396 out of 5

Keywords: PID, current draw, volt drop, millivolt, dmm, dvom, how to find current draw, battery drain, car battery draw, easy way to find current draw, what is volt drop, how to find a volt drop, how to use volts to find current draw, diagnose a current draw, diagnose a battery drain., diagnose a battery draw, mechanic tools

Id: lRcj1fQcWwU

Length: 10min 49sec (649 seconds)

Published: Wed Feb 22 2017

Reddit Comments

[u/Hodgepodge75]

I have a 90 Chevy Silverado which uses a 98 Tahoe wiring harness spliced (by an absolute professional) to the original 1990 computer (long story, the truck is a mess). Over the years as we've swapped parts out and fixed stuff ourselves the truck has developed a nasty power draw.

Since the wiring side of things has never been my strong suit and every mechanic refuses to touch the wiring, we just kind of learned to adapt to the problem with gimmicks like a permanently attached solar powered battery tender. After watching this video I'm fairly confident in my own ability to find the draw, so thank you for posting this!