Diagnosis and Understanding- Voltage Drop

[Music] greetings to percenters we're going to do another diagnosis and understanding series for the playlist the diagnosis and understanding series comprehensively covers Automotive Concepts and component function and things like that and today we're going to talk about one that many of you guys may not be very familiar with probably heard of we're going to talk about voltage drop and why as an advanced do-it-yourselfer it's it's going to be critical for you to know what voltage drop is how to test for it and most importantly of course how to understand voltage drop so that you don't have to rely on flowcharts or instructions you'll be able to come up with your own voltage drop tests because after this video you'll Master the concept of voltage drop so let's start off with why especially for many of you guys who have been doing your own auto repair and you're very good at it you know how to replace things without help you are well familiar with how to use your dvom and do electrical testing and you've gotten by all this time without ever having to do a voltage drop test well uh let's go ahead and put that to the test let's do the simplest diagnosis that there possibly is on a car let's look at a no cranking no start and see how our scientific diagnostic testing thinking ability holds up if our skills do not include voltage drop knowledge so so here I have a very very basic starter setup here with a ignition switch that I made so this would of course be in the car we would say and we turn the key or hit the ignition switch and nothing happens at all now of course for your 98 percenter this is about the easiest diagnosis you can do you replace the battery and then when that doesn't fix it battery works car don't start you need a starter but I actually got the idea for this video from I was at an auto part store and as I was in line to buy my parts a guy had bought a battery and a starter and then later in the day when I was returning the core from what I replaced the same guy was actually there at the store trying to return his battery and starter because his car still didn't start now of course that would never happen with us because we are advanced diagnosticians and because we are more advanced do-e selfers and we actually are one of the 2% of people that would spend time on learning about a starter system the components and how to diagnose exactly what might be wrong we would come up of course with a list of all the possible variables that could explain this situation other than the battery and starter as you see there are quite a few of them and most of you guys are certainly familiar with these so let's go ahead and use our logical reasoning scientific approach and go ahead and Rule these out and see where our no start problem is and then let's see where we go wrong with this cuz this looks like it's going to be pretty comprehensive so if your first instinct is not to test the battery then you are on the wrong Channel but obviously in a car this would be the easiest thing we would do all of you guys should know of course how to test a battery so let's go ahead and check we've got uh a little bit low on the battery but of course that might be expected with a starting problem so we've got our 12 volts at the battery there and of course most of you guys are well aware that just because we have 12 volts at the battery that does not mean the battery is good when the battery is put under load we could very well expect the drain on the battery so we are going to go ahead and test for that with the starter so let's turn the starter on and we see we Dro voltage down a little bit so there is definitely some activity there but it's not very much not as much as we would expect maybe that's a clue but whatever the case is we also could do some other things we could turn the headlights on uh we could take it to a auto part store have it load tested but whatever case whatever we do uh we are going to rule out the battery as the problem here okay so battery is rolled out the next thing we're going to see obviously we know any problems with loose connections or dirty terminals or things could cause that so let's take a look and of course this would mean going through doing all the basic stuff of cleaning the terminals making sure everything's clean and tight some people might even be inclined to maybe go ahead and verify that we've got good continuity here with our terminals we've got no resistance there we might check other places and see that we've got continuity so we have eliminated Terminals and bad connections all right the next thing of course it could be that we have some type of ignition switch problem so various ways we could check an ignition switch one way I like to do it is to just jump the solenoid and bypass the ignition if we get the starter to turn over then then we know that there was some control issue ignition issue we could also of course do a continuity test and then turn the ignition key and we see that we get continuity then but turn the key off and we don't um we could check for voltage there in fact so all you guys know that stuff but we could do all kinds of tests to verify and rule out that there is ignition control problem and by grounding out the solenoid we can we get a little bit of a spark there but we can see the starter does not move this uh rules out just the fact that we can do this this rules out pretty much that we have any kind of ignition problem I guess we could have an ignition problem and this issue but the fact that we're seeing some electricity there uh this along with our other ignition tests we've ruled out ignition all right so at this point it is definitely looking like it's going to be the starter because if if you saw the Sparks there and everything it does indicate some electrical but remember uh we need to be absolutely certain when we put a starter in because maybe you guys have the same policy that I do when I do work for people if I am wrong about the starter I buy the new starter I have to pay for my mistakes so I want to be absolutely sure um relays would be next we have a video on the diagnosis and understanding series of how to test relays so you can watch that video and then ruling out if a relay issue another thing could cause this too remember this system that we've got set up would be in the car maybe the engine won't turn over maybe the engine has a spun bearing uh maybe there's some type of accessory issue uh where the accessories are binding the serpentine belt not likely here we don't even hear the solenoid click really indicative that we've got a bad starter but even so uh you might be inclined to go ahead get a ratchet turn the engine over by hand we can verify that there is no mechanical interference issue we are down now to wiring or the starter so being fairly confident that this is a complete waste of time we're going to go ahead and look for voltage at the starter and of course this for many of you would have been the second or third thing in a row but I'm kind of doing this on purpose let's go ahead and check our wiring now and uh we can go to a ground uh here and here let's go ahead and keep the leads in order here just for Simplicity the yellow wire here is a ground of course I made it yellow so that it would stand out against the dark backdrop here let's go ahead and check ground and let's go ahead and check voltage and check that out we have battery voltage at the starter so we know that our wiring is good maybe we weren't so convinced of that so let's make another mistake uh let's go ahead and check for continuity um we know there's continuity there but let's let's go ahead and check for resistance so we go ahead just to be safe and granted the cable's a little bit old here but let's test for resistance and we see that we have uh just a couple ten of an OHM of resistance not anything to be concerned about pretty much actually it's it's almost a tenth so virtually no resistance at all we're not very worried about that uh even a new cable might have that let's go ahead and also just to be safe go ahead and check our continuity and resistance on the ground wire and we can see uh on the ground wire we have just about the same so uh no resistance issues we have pretty much ruled this thing out it looks like haven't we let's go ahead and look at the starter so through our systematic approach we have deduced that the only possibility left is the starter we are very confident on that but just to be absolutely sure before we go ahead and replace the starter let's go ahead and pull the starter out of the car bench test it verify that the starter doesn't operate so after calling the parts store and verifying they have a starter in stock let's just make sure the starter doesn't run and uh-oh well we've got a problem here the starter does actually that's not really a problem so we of conclude that the starter of course has to be bad it's the only possibility but there's a massive difference between just free spinning the starter outside of the engine with 12 volts to it like we just did versus having the starter have the extra load of actually turning over the engine so the starter must be bad it's just when it's under the load of turning over the engine it can't work but when it's free spinning outside of the engine of course it turns but if there was any load on it it wouldn't so we conclude that despite that bench test the starter has to be bad just to be safe let's go ahead and verify it's not an issue with any kind of resistance or anything um and that little tiny bit of resistance in the battery cables isn't an issue so let's measure the resistance on our jumper cables that we use that we know work so just to be scientific about it we go ahead and see if we've got any resistance here and look at that we actually by that reading have more resistance in the jumper cables that worked then in the battery cables verifying without question the only possibility we have a bad starter and we go ahead and buy a starter and put it in the vehicle all right new starter in the car maybe if this is a customer you've already called them while you were installing it to give them a head start to get over and pick up their car early and we go ahead and hit the ignition switch and no way no way let's double check our connections everything's good wow um that's a problem where did we go wrong so how is it possible that using our scientific methodical approach we ended up no better than 98% of the people that would have just guessed a starter from the beginning and we both ended up putting a starter in the car when it didn't need it our mistake was in ruling out the wiring we actually did not properly Ru out the wiring because we did not consider for voltage drop and actually that is where the problem is the problem is with bad wiring in this starter motor there is high resistance in the positive battery cable but there can't be we measured 12 volts at the starter so there can't be bad resistance there furthermore we even did a continuity test and measured no Ohms on The Wire actually less if anything than there was on the jumper cables that worked so how is it it possible there could be high resistance in The Wire it's because we don't understand the concept of voltage drop but that is indeed what the problem is and in order to really understand it I could show you how to do a voltage drop test in 2 minutes but what we're going to do is understand the mechanism of voltage drop so that you will understand the concept you won't have to look up how to do a voltage drop test for the rest of your life you will know how to do it and you'll also be be able to come up with your own test methods so let's go ahead and look at what voltage drop is and then let's look back at our wiring test and then it'll be very apparent where we went wrong and before we do that just to verify this is indeed a wiring problem I'm going to remove that bad battery cable we're going to install this good battery cable and we can see that the starter works so obviously the bad battery cable was the issue and just to show you our resistance measurement with the OM meter shows the same as the cable that didn't work all right conclusive proof we somehow made a mistake on our diagnosis that the wiring was not an issue when it absolutely was so as you guys probably know if you follow the channel uh electricity is by far my weakest attribute and for many of you I know it is too and for many of you it is not I know I have several electrical Engineers on the channel we are not going to go into things like OHS law and stuff like that but um the way that I get by electrical issues and understanding electrical problems and coming up with ways to test them is by sort of using an analogy with water that's kind of how I do it and that's what I'm going to show you it's not exactly a physically accurate analogy to electricity but it will get you by for pretty much 99.9% of anything you're going to run into an automotive electric electrical including what we ran into here so uh let's go ahead and draw a very simple circuit here all right so let's start with a very oversimplified diagram of our starter circuit ignoring the solenoid and by the way some Vehicles even have firewall mounted solenoids that you would also need to check in a diagnosis but um obviously it seemed very obvious that our problem was with the starter we verified good ground we measured 12 volts at the starter um between the power and ground we measured continuity and no resistance with the cables um obviously you you all well know the ground actually goes back to the battery and actually just for Simplicity we're going to go with kind of a conventional electrical Theory here um because as most of you do know electrons actually do leave from the negative terminal on a battery but uh we're going to go with the the positive flow just because I want you to master the voltage drop concept here again this is not an engineering class so we're going to go with this system that most of you are similar are more familiar with and then my way when I think of electrical I actually think of it in terms of water and pressure particularly with voltage so if we look at a similar setup here with water we would have of course water flowing through this hose here and of course the water does some type of work maybe there's a sprinkler at the end of the hose um I tried to draw a water wheel here but pretty much a similar thing as what we would have here where you've got a flow of electrons Through the Wire and it does work at the starter now the mistake that we made was when we looked at continuity of this system you have to have the system underload there has to be flow through here why well again it's easier to think of it in terms of water so at our battery we of course have 12 VTS and we can think of this as a water pressure so just like at our water tap here with our water flowing through the hose we have say 12 volts of pressure here now anywhere along this hose we should have 12 VTS of pressure anywhere along it until it actually gets to doing the work and that would be the same thing here as well we should be measuring 12 volts anywhere if we were to to a voltmeter between the negative and anywhere along this positive wire we should have 12 V volts up to the starter at any given time while the starter is working if we do not have the same voltage as battery voltage that is known as voltage drop now what is the cause of voltage drop well let's look at our hose example let's say that we put a kink in this hose and we kind of step on the hose or pinch the hose so the Kink is going to restrict the flow and you can kind of think of it if you put your thumb over the end of a garden hose if you let your thumb off a little bit of course the water sprays out but you much more pressure at your thumb than you do after your thumb obviously so think of it as a kink in the hose the same thing can happen in an electrical circuit and in an electrical circuit we would have some type of I don't know some type of not so much a kink in the line but some type of resistance corrosion something like that where we would have a drop in the voltage after the resistance or the restriction and of course that drop in voltage may not be enough to do the work to activate the starter and that is in fact what happened in our starter example here so here's the thing about this is that this only applies when there is flow through this hose let's do this for example let's go ahead and cap off the end of that hose so the end of the hose is closed off do you see now I hope that the water is going to go through the resistance but build up the same pressure on both sides of the blockage because we've capped off the end of the hose and that was our first mistake we made when we measured the voltage on the battery terminal what was the first thing we did we did not have the starter operational we have some type of resistance here but we measured 12 Vols at the starter why because it's capped off there is no flow because we were not operating the starter at the time to see the effects of a restriction along a system you must have flow through it just like with water and that was our first mistake now there was a second mistake that we made and it seems to be like we didn't make a mistake we measured for resistance in this wire using the ohm meter and we found no resistance we actually found less resistance than in a known open hose or wire so how come that happened well that actually is more with understanding how your volt ohm meter works but basically what happened in that situation so the reason that our ohm meter test did not show resistance on a wire that clearly has a ton of resistance is because that wire that that battery cable is kind of more like a drainage pipe than a garden hose so if we can think of an obstruction in this drainage pipe and the way that your volt ohm meter works is it's kind of like again using the water analogy instead of flooding water through here and looking if we had a massive amount of water through here then obviously we would see that drop because of the Restriction at the end of it but that's not the way that your dvom does it the way your dvom does it is it's almost like taking this drainage pipe with the blockage and then just pouring out maybe like a half a cup of water into it and then just checking to see that we get water coming out on the other side well obviously with a very low trickle of water that's going to have no problem making it through and we easily detect it on the other side so again we're not really loading the circuit we're just sending a trickle through there and if there's any kind of Integrity through that corrosion there will be full continuity measured and that is why our continuity test failed us so our first mistake obviously we Now understand we were measuring a tapped off system initially because without running any current through the starter we basically took our dvom here we measured from negative to positive at the starter and of course we measured 12 volts but we understand now that even after a restriction without flow in the system because this is capped off there's no flow we measured the 12 volts the same way we would have measured Meed it with a kink in the hose so now we know that we need to have flow through the system but the other thing that we need to do is we have to understand that the way a dvom works is by measuring difference in potential so what we need to do is activate the starter cause the actual flow to occur which creates that voltage drop but move our lead to battery positive so that we can measure that potential difference between the start of the flow and after any obstruction when we do this we would see 3 volts on that meter representing the amount of voltage lost because of the resistance or corrosion in the wiring notice by the way that you always have to have everything add up to your Source voltage so just to test our knowledge if we were to measure here and here just before the resistance what would our meter measure well it would of course show zero there should be no voltage drop in the good system so there's no change in potential so we should measure zero there if we were to measure here and here then of course we would see the difference in potential we would see that voltage drop so there's various places where we could measure if we wanted we could even do this we could measure from the battery and then we could take this lead and move it to various points along this cable until we found the voltage drop that if we were interested would allow us to find a bad region in the cable if there was one often times the cable is completely completely corroded and as a matter of fact sometimes on cables you will read Ohms on the actual cable itself but not always as we just saw in this example here so there's various ways and I'll even show you another way but now that we understand this concept let's go ahead and repeat our tests on the wiring and see if we can catch that voltage drop all right so let's go ahead and set up again and repeat our initial mist stake so we of course if we measure negative to positive we see we've got our 12 volts there but what should it be now if we try a voltage drop test and we know that this cable is bad of course what should our voltage be when we set over to the positive terminal and it should read pretty much zero because the system is capped off so even whever the resistance is whether there's one point of resistance or the whole wire has resistance because there is no flow through the system it's like a closed off garden hose any point we measure there's going to be the same 12 volts so we do not have a difference in potential along the wire now when we go ahead and apply actual current flow by hitting the switch let's go ahead and see what happens and we can see that we read about 9 .4 volts a tremendous amount of voltage drop so we have a significant loss of voltage because of resistance in The Wire because everything's going to have to add up to the Battery Source voltage we know that the reason this starter is not working is because it's only getting approximately 3 volts and another indication is as I leave this on um the wire gets quite hot so it's a little bit different than our obstruction example one of the reasons for the resistance is because of generation of heat which of course does not apply to a water analogy now that we understand the concept of voltage drop let's see if we can use a more methodological approach and come up with a easier way to test the system and there's there's a few ways to do it but one way would it not be easier rather than running our leads through the car around the engine making sure we were clear of the serpentine belt and all that just in case vehicle started let's think about this if we apply current through this system then that would mean that we would actually have only 3 volts coming to the starter because we have 12 volts originally 9 volts lost through the voltage drop so if we were to measure from here to the ground which might be easier to do because that way we just stay under the car and not have to run our wiring everywhere and of course we see our battery voltage here but when we actually apply the current this time and measure that after the voltage drop we should see about 3 volts and you can see we see about 2.2 volts or so as you can see this is another way that we could measure the voltage drop by putting the current here and measuring that voltage we can now move our terminals up to the battery and we can see our voltage at the battery there in a good running system we would have 12 volts pretty much from the source to the ground even with the current flowing because again a good system will have very very very minimal voltage drop so let's go ahead and verify that by going ahead and putting in the good B battery cable and verifying that we do not have voltage drop all right so now I know I've got a known good cable installed here which actually if anything may have measured more resistance with our ohm meter than the bad cable did but of course we understand that that resistance measurement is completely useless on a battery cable so let's go ahead and repeat our test so let's go ahead and put our lead onto the battery positive here and we know that when we put our lead onto the battery positive down here we will see no difference in potential because it is a closed system it is capped off so regardless of any resistance the pressures would be equal at any point but now when we go ahead and we add flow let's see if we read 9 volts I don't think so and we can see our voltage drop is approximately a tenth of a volt very reasonable generally with voltage drops I look for you know if it's more than half a volt I get a little you know fairly concerned anything under a couple tents of a volt I don't even blink an eye at so we can see we have just very minimal voltage loss oops and that's why the starter is able to run because we are getting 12 volts through the motor so let's go ahead and do this let's do our other way because we know that we should not have any loss of voltage through the wiring if we measure at this location with current flow we should have about the same amount of voltage drop so let's go ahead and see and you can see that we've got about 10 1/ 12 volts let's go ahead and compare that with our soure voltage and we see that we've only got a couple tents of voltage drop our battery is getting very weak by the way from all of this playing around but a total difference now that we've got good continuity with no resistance through the wiring and another thing that we know is that if we go ahead and bypass the switch and we ground the solenoid we would be able to start the engine by grounding the solenoid out because we no longer have the lack of voltage delivered to the starter and yet another way that you could do this test and this is something that I'll often do in the field uh as well is not necessarily to even do the voltage drop measurement but as a way to rule out that your you may have a bad cable or something of course electricity always takes the path the least resistance so by going ahead and hooking up here and using a jumper cable or something to bypass the suspected bad wiring now if I have the person start the engine we can see it runs but without the good wiring in place we do not get the starter to start so this is validation that of course that cable I bypassed must be bad all right well hopefully that was helpful and gave you some more Tools in your toolbox of knowledge and also some new ways of using your dvom that you maybe didn't consider but it is uh definitely hard to remember to consider voltage drop we're so used to checking for 12 volts at an accessory or 12 volts at a bulb or system and we see 12 volts there so we assume that the component must be bad as you can see unless you are actually considering flow of electrons through that system that would be a false conclusion now there are some times when I always consider voltage drop uh as a first thing that I'll consider uh any kind of headlights or tail lights or any kind of Lighting systems that are dim you can see where voltage drop in the wiring would explain that typically you would unplug the suspect bulb you would look for 12 volts at the system there's 12 volts has to be a bad bulb as you can see wiring with even a little bit of voltage drop could cause that bulb to run dimmer so you'll want to actually do a voltage drop test there to consider if this starter were chattering as opposed to a complete no start like that uh that would also be indication possibly some voltage drop one time I always always do voltage drop is the diagnosis on a fuel pump if there is voltage drop on the wiring to a fuel pump or bad ground or something like that that will cause the pump motor to operate slower that could be the cause of the lean condition that I diagnosed as a bad fuel pump or through fuel pressure but before changing the fuel pump I'll always do a voltage drop to test that and further what I'll do is I'll directly apply apply 12 volts directly to the fuel pump see if the pressure increases and if the pressure Remains the Same then I can absolutely diagnose a fuel pump as a matter of fact the second video I ever did I did exactly that but the voltage drop testing didn't show up so well it was before I got HD and all that but uh those are three times when I'll just actually do the voltage drop test maybe first before I even do a regular voltage test so just uh keep that little trick in mind hopefully you found this helpful thanks for watching

Channel: Schrodingers Box

Views: 745,473

Keywords: voltage drop, no crank no start, no crank diagnosis, voltage drop testing, automotive electrical, basic voltage drop, voltage drop test, voltage drop tutorial, how to do voltage drop test, what is voltage drop, starter motor diagnosis, no crank no start diagnosis, diagnose bad starter, battery testing, battery load test, how to test car battery, test battery voltage, no start diagnosis, starter chatters, starter clicks, starter does not click, continuity test, DVOM

Id: DfLyh43iihM

Length: 33min 15sec (1995 seconds)

Published: Sat Apr 04 2015