Parasitic Drain

hi I'm marking room with garage gurus today we're going to show you a technique which can save time in diagnosis of a parasitic green parasitic drain is when devices or modules are drawing too much amperage from the battery with the key off generally anything above 25 milliamps is considered too much as you read through the manufacturers diagnostic process they always tell you to pull fuses in order to isolate circuits once you pull a fuse that feeds the module causing the problem then you can look at the wiring diagram and isolate the problem further unfortunately when you pull all these fuses number one there's a lot of fuses on a late model vehicle and it's going to be time consuming and the other issue is that when you pull these fuses sometimes that can wake up a module and once awake it can stay awake causing excess current for as long as 45 minutes to an hour so if you can avoid pulling fuses unnecessarily it saves you time in a number of different ways so what this technique consists of is measuring the voltage drop across the fuse while it's still plugged in the meter on the right is reading voltage the one on the left reads amperage I've set up just a simple circuit on this electrical diagnosis board and with no current flowing through the circuit if I measure voltage across the fuse the meter goes to zero with the width it disconnected you show a small transient voltage that just has to do with the internal circuits of the meter but connected to a fuse with no current you show dead zero now when I turn on the circuit now the voltage drop goes to 4.3 millivolts and the current flow through the circuit is 0.24 4 amps I turn it off everything goes back to zero again the voltage drop is caused by a very slight resistance in the fuse itself it's perfectly normal now if I switch to a low current circuit this resistor if again I measure the voltage drop with no current flow no voltage drop but when I energize this now with only 12 milliamps of current flow I'm showing a voltage drop of 0.2 millivolts so that proves to me that this views does have current flow so to summarize if I go through and measure voltage drop across fuses any fuse that has no voltage drop there's no reason to pull it any fuse that does show a voltage drop I should pull as part of my diagnostic process because that could be the source of my draw now let's go use this technique on a vehicle okay we're on the car now checking our parasitic drain I'm using an inductive aunt meter which is the best tool for you to use for this because you don't have to break any connections in order to measure current draw out the battery you can see from our reading that we're averaging 25 to 28 milliamps which is just over the specification required of 25 so first thing you want to do as you read through the instructions in the service manual is check for anything being left on or anything plugged in in the vehicle I'll give a quick check I found that our customer had left a cellphone charger plugged in by disconnecting that you can see that our draw has now dropped well under the specification of 25 milli amps now we're good but just for demonstration purposes let me show you a couple of other effects on this process one thing I mentioned is that when you pull fuses to try to isolate the problem sometimes you accidentally wake up other modules so if I start pulling fuses I'll just start down here that one had no effect I'll try this one we have no effect there oh boy that one when I pulled it the amperage went to 300 milliamps now it's jumped up to 1.6 and it's holding for right now so I accidentally woke up some modules by disconnecting that fuse if you wait a while sometimes the current will drop down fairly quickly sometimes it may hat take as long as 45 minutes to an hour so as we watch this well there it's back down to 30 32 milliamps already so it's almost within spec if we give it enough time it should drop on down to be within specification again but that's one of the issues you run into pulling fuses if I did have a large draw in the system there's another technique we can use just for demonstration I've turned on a dome light to cause a high current draw to isolate which fuse is feeding those dome lights rather than going through and pulling all the fuses until the draw drops down I'm just gonna measure voltage drops so looking at the meter on the right on this fuse I'm showing zero volts flickering back and forth to point one Villa millivolts I'll check this next fuse that one's dead zero I'll check this one this one gives me dead zero zero over here there I'm showing 0.2 volts point 1 there I have a fairly high draw of point 8 no bolts so I've isolated this down to two fuses that have a fairly high voltage drop those should be the ones that are carrying the majority of the current so I'll go ahead and pull them now you see the amperage dropped some now the amperage dropped back down almost within specification again but these two fuses were identified using voltage drop I didn't have to pull every fuse on the panel I can just measure voltage drop to identify the ones that need to be pulled as part of the isolation process you can see how this can save you a lot of time in diagnosing a parasitic drain I'm Mark Ingram with garage gurus thank you

Channel: Garage Gurus

Views: 112,467

Keywords: Garage Gurus, Federal-Mogul, Federal-Mogul Motorparts, FMMP, Parasitic, drain, diagnosis, vehicle, electronics, fuse, parasitic drain, late model vehicle, circuit, how to automotive, automotive education, technician, training, auto repair, auto care

Id: tX3t4SrSOt8

Length: 8min 39sec (519 seconds)

Published: Tue Nov 15 2016