When Should You Conduct Parasitic Current Draw Testing?

When Should You Conduct Parasitic Current Draw Testing?

July 19, 2019 3 By Bernardo Ryan



we want to leave you with a parting tech tip here on when to do parasitic current draw testing now nothing's worse than installing a new battery in a customer's vehicle only to have them come back in a couple weeks and say that new battery you sold me is not working out batteries went dead twice I've had to jump it then you check this charging system and that's testing just fine you know passing all tests you could imagine so we assume we have a parasitic current draw but as you well know unlike years ago when they pretty much would not do much coming and going there and mostly there now they come and go a lot modules can wake up and stay awake and not go to sleep like they're supposed to because of a software error or something of that nature and run the battery down but Murphy's Law comes into your shop and you check for parasitic current draw and there's non present well first off the procedure for checking a parasitic current drawl can be a bit tricky it can either require a very expensive very fragile and an accurate high-resolution inductive amp clamp with a big opening the jaws to get around the battery cable or cables or be you've got to go in series with the battery cable and your ammeter which most of us is what we do and it required maybe a knife switch or one of these parasitic current drawl switches like we have here a shutoff switch so you basically take your ammeter hook it to one side and the other lead of the ammeter goes the other side and you open up the switch and now you don't actually disconnect the battery you simply change the path of the parasitic current from going through the switch to going through now the ammeter we're not going to do that on this vehicle what we are going to do is show you what leads up to doing the test like this now the problem with all that work of hooking up the parasitic current draw test switch the knife switch or the rotary switch is it it often requires several minutes maybe a half hour hour waiting for all the modules to go back to sleep because you disconnected the battery to put all this stuff in series with the battery cable and sometimes you only find the problems not present so you have to check it again tomorrow what I wanted to do is give you a tip on how to find if it's if you were meaning to do the extra mile and hook up all of the parasitic current draw testing type of apparatus so let's walk over to this Mini Cooper and see what we can do here will already have took a booster box up a minute ago powered it on did the override so it would work without a battery present and took the battery cable off the negative now I connected right here through these two leads right here an ammeter in series with the battery cable and I made sure I had a good fuse in that ammeter all right and so now we can see I am drawing about 12 milliamps of current now if you're wondering what is a good current draw reading what's the max value well it's all over the place but after typically a half hour to an hour and these are very big generalities we're looking for somewhere between 30 to 50 milliamps now it could be higher it could be lower in this case it's a lot lower time is of the essence it has to take a while for the modules to finally all go to sleep so ideally you want to keep the modules all I've put a substitute power source like I did over here at this junction box turn it on and disconnect that battery so we not allowed the computers to go to sleep they're being powered up through my jump box over here and hook the ammeter in series if you do it right common over here the 10m scale here good fuse in here and now I can turn that jump box off now all the current that's powering up those radios and PCMs and abs modules the little couple milliamps here a couple of minutes there finally they all calm down and we're about 12 milliamps now you may be wondering oh by the way you also have a high-dollar high-resolution very delicate instrument this is a nice 0 to 40 milli amp amp clamp inductive and it's fairly accurate not as accurate as going in series with the ammeter but the next best thing it's showing about two or three milliamps we know that's not accurate because this is the gold standard an actual meter in series it's 12 milliamps but it gives us an idea earlier it was showing 180 200 milliamps and finally the modules went to sleep so now why what am i doing you may be asking with the second meter to the right so this meter right here as you see it's got some leads coming out there those leads by the way the black lead is coming over here to the negative battery cable a known good ground okay the red lead is coming down here to a braided ground strap another known good ground they're both good grounds therefore the ohms should see zero point one or zero point zero five zero Demeter right I've got a perfectly good ground cable going back to this cable over here not necessarily is it going to read that if there's current flowing through the circuit so in this case I have very little current flow 12 milliamps what is my resistance reading here the real McCoy resistance of what a good ground strap should read less than an ohm it's 0.16 so sixteenth hundreds of an ohm that's pretty good now if I didn't want to do all this work if I just wanted to see do I need to go snooping in my parasitic current draw testing using methods like this obviously if I don't have one of these high-dollar and clamps I'm kind of sunk with having to take the cable off try to keep the power live yadda yadda a bunch of work if the problems not present so what I want to show you is how the ammeter responds to abnormal amounts of parasitic current draw I'm sorry the ohm meter the meter on the right will start wigging out when there is more than just let's say 50 milliamps of current draw which would be normal 15-under remember that's normal if it's over 50 if it's one hundred three hundred five hundred that old meter is going to show a pull of false positive what I mean by that it'll show resistance that's not there why is that because it's putting out a tiny bit of voltage maybe four tenths of a volt and it looks to see how much he gets back what can't do that if there's current flowing through a live circuit I think everybody knows it's watching you cannot I mean you cannot use an ohmmeter accurately on a live circuit it's got to be a dead so it is the vehicle with battery cables hooked up whether through an ammeter or just hooked up post on the terminal is the vehicle hooked up even with normal parasitic current draw a dead circuit know how about if the doors open even less of a dead circuit every electron that goes through those red wires is coming back these black wires so let's go ahead hit the the keyless entry and you want you to watch the current draw 12 milliamps and the amount of resistance that we perceive is in that known good ground wire can also watch that meter right there and see how a budget follows this one over here I'm gonna hit the pop the trunk the hatch on this Mini Cooper and we'll see what happens to my o meter and my ammeter now I'm drawing a lot of current 3 amps and what did my own meter start reading it started reading a false positive so we'll do that again it's about 1.2 3 ohms I'm gonna make this a little bit more auto-ranging here there we go do that again there we go so as we see we can open the door up now in about seven-and-a-half amps now my meter that's less accurate at lower parasitic current values is very accurate agreeing with this meter and I'm showing a false positive on the o meter what I mean by that is you can see that two point seven ohms there's not two point seven ohms in there there's less of one tenth or 2/10 of a know them in there in reality it's showing more resistance than is real so here's the thing to remember a lot of work to get in there in series with an ammeter to do accurate parasitic current draw testing a lot of work only to find the problems not present right now wouldn't it be nice to have a quick and easy test to determine do I need to go doing this so I'm gonna use this little little jingle if you don't have an OL meter that's goofing up there's not time to go parasitic snoopin so if the meter ain't goofin no need to go parasitic draw snoopin so if my meter goes 20.1 I don't need to unhook all this stuff I don't need to get the fancy and clamp out I don't have a problem so get a known good ground wire look at the ALMS between the post of the battery negative and that ground wire and if you see something that's abnormal resistance it's time to go parasitic snooping but if you don't don't bother you're gonna waste your time you have it intermittent