The Trainer #32: How To Read An Automotive Block Wiring Diagram

The Trainer #32: How To Read An Automotive Block Wiring Diagram


if you’re going to be successful at troubleshooting electrical problems you need of course to be able to read our diagram you need to understand the electrical fundamentals and you need to know how to use your tools we have material online to cover the latter two today we’ll focus strictly on reading a schematic so let’s get right off the start let’s just say we’re going to be dealing with an AC compressor clutch that isn’t working that’s the customer complaint AC is not blowing cold we’ve narrowed it down to a pressure pledge that is not coming on and we need to be able to troubleshoot that circuit so the first thing I want to do is I want to come over here and pick up my service information system in this case we’re going to use Mitchell’s protoman platform the subject vehicle will still uses a 3.1 Chevy lumina just for the fun of it and we’ve come down to service information and the wiring diagrams now a block diagram is a representation of only the past and circuits related to a particular system if you take a look at an OE diagram you’ll see everything laid out in one and it can be a little more difficult to trace a problem so most service information systems are going to offer block diagrams like the ones we’re going to take a look at but no matter which style you use you first have to become familiar with the language of the diagram like reading a paper roadmap what do the symbols mean what do the colors mean and we’re going to use that by checking out know Mitchell’s information you can see we have wiring diagram color deviations what the symbols mean we can get to know exactly what that is so when we go back to the actual diagram we’re not lost where the ground locations are where the diagram component locations are these are all information that we can use to help us understand this the schematic itself now the other thing we need to know is when we’re dealing with a circuit of course for a circuit to work properly we have to have a complete path from whatever component is doing the worst and that work in this case the AC compressor and the source of electric potential which we generally use the battery for so we need a complete loop but if we’re going to take a quick look at a new AC diagram and see if I can move this down in size so you can see the whole thing there you go ah you’ll notice that it’s laid out typically with the power slide at the top of the diagram and then the ground side down here the base again just representing the components part of that circuit but if you look closer at the top you see that it doesn’t end at the battery here in this case is entering telling us where does then the fuse block and which fuses are involved fighting run hot and run hot at all times I’ll cover that in a moment but if I want to know how it gets to the battery that I need to use a diagram called a power distribution diagram and I just want to show that to you real quick so you know where we’re at and here’s an example of a power dip our excuse me power distribution diagram let me bring that up to where we can blow it up and see it a little bit easier just to give you an example you can see what is this showing is where the fuses or components are located could be the ignition switch or straight to the fuse depending on how it makes its way back to the battery but this is what I want to be able to know getting all the way back to the battery and in some cases if I’m troubleshooting something say like a parasitic drain I want to know what fuse feeds what I can use this diagram to find that out so that’s what you need the power distribution diagram for for other Diagnostics not really going to affect us on the topic today just once you familiar that you do need this in order to find your weight all the way back to the battery on the same side of that coin is the ground side and here’s an example of a ground distribution diagram swing that up so you can see that better and again it’s telling me how I’m getting back to the battery on the ground side and remembering the information the ground locations but I can use that to look up where for example G 108 is or G 109 is if I have a ground sight issue again I need to follow eventually that path all the way back to the battery the power and ground distribution diagrams help me do exactly that get all the way back to the battery okay so that’s so much for the basic resources let’s take a closer look at the actual diagram itself we’re going to mess with and this is the AC circuit that contains the AC compressor clutch and I need to identify the components that make up a complete circuit I guess let me make prayer Phi’s that I know the AC compressor in this car isn’t working okay that’s the component that does the work in the circuit is the only reason we have the circuit in the first place isn’t it that would be in the equivalent of a tail light that’s the light that’s the load in the circuit in that particular circuit now I have to get power to that load typically from the battery now yes there those are going to tell you that when the car is running the alternator is the source when eventually it’s all going to get tied back to the battery in the ground through the battery so that’s what I’m going to use as my source in addition to the components that were identifying the circuit are the circuit protection devices now the circuit protection device is always wired into the hot side of the circuit don’t need protection on the ground side do it so if the wiring gets to a ground point before that load before that working component you know what happens the wiring goes up in smoke so we have fuses circuit breakers fusible links these are the circuit protection devices that I’m going to identify in the diagram okay next what turns it on and off a lot of circuits are controlled by relays so we’ll probably want to focus on on that as we go along today the relay does serve two functions one of course is as a control device or a switch now if I have all these basic elements I have a circuit so if I on my diagram so let’s bring that back up and let’s start by identifying the load I think we can do that zoom right in get a little more I want you be able to see it really good okay and here’s the AC compressor clutch coil now that’s the load in our diagram and I know that I have to have a complete path from the load through the battery and back again correct so from the positive side to the load through the load then the ground back to the battery so that is the circuit path I need to identify and as I’m making my way from the load back to the battery on either side of that circuit because this is where I’m going to start at the load I’m going to identify all those other elements as I go along what is the circuit protection devices what are the control devices you know what tasks do I have to be worried about and what can I tell from where in the flow that that path is located it doesn’t matter where I start I have two wires coming off the club off the compressor clutch I just want to find which one goes back to the battery so we’re going to take this one right here just for example we’ll start here and let me blow that up little more so we can see it really good again one of these is going to go to ground one of these is going to go to power I need to identify which one is which and what is what’s in the path along the way so let’s just pick one we’ll take the one on the left we can see that this is labeled MCA can you tell me what that YZ what that means no color assigned is what that stands for and that just means that it could be any color usually it’s black that’s what you see in a Sherlock harness that plugs into the compressor itself so we’re going to follow that down we see that it ties in here to the AC compressor clutch diode okay with a one-way flow okay what’s the diode for well the dial is there to protect the computer we mean P well when I have this energized this is a coil isn’t it this is a magnetic field and when I turn that a/c compressor clutch off that field collapses and it can send back a lot of voltage back along the wiring and the diode is there to suppress that so it doesn’t get to those tender little drivers in the control module so I’m not going to go that way and I worry about that way let’s go this way and we see very quickly they wind up at ground point 110 at the lower-left from the engine so now I can go back to that ground distribution diagram and find out how 110 gets back to the battery keep in mind the body the car is one big ground and just because you verified for example the taillight is grounded to its ground point in the trunk doesn’t mean that it’s grounded all the way through to the battery and could explain why the taillight or any other component for that matter is not working properly always had to get all the way back to the battery so we’ve done it on this side now we’ve now identified the ground side of this circuit so then we know this wire is going to ground this one must be going to battery let’s see if we can follow the path to it can we come off on a no color assign wire back to the diode color changes to a dark green wire that’s what this stands for in the system and comes in at pin eight on the ACE compressor relay located in the under hood electrical center now what’s a relay that’s a control device isn’t it and that’s exactly where we’re coming in here from the diagram you can see this is a switch that’s the control side something has to turn that relay on though doesn’t it it’s electronic switch so we’re going to worry about that later right now I just want to see where is the power supply the path to the battery from the compressor clutch coil itself now these components on the diagrams are shown in their normal state well having the AC on is not normal state is it we have to turn that on so this is showing what it looks like in the off position if I turn it on the coils energize in the relay pull the switch down to this contact here that’s going to be where we go to continue our path back to the battery so now we follow it along we come out on connecting pin f6 colors now change to a brn looking that up in your wiring breviary she’ll see that stands for brown and this is even nice because if you can see it these are actually color coded on the schematic so that’s that helps so okay let’s track that brown wire follow that along and now we come to a symbol representing a splice in other words all of these wires are connected at this point in the wiring harness now you can follow all of them if you want but remember the aim the goal here is to get back to the battery and as I mentioned earlier tower tends to be heading towards the top of the page so I’m going to take the vertical line and follow that again I’m trying to get back to the battery okay now here we go now we’re in the fuse block the under hood electrical Center number one engine AIE mis fuse 15 amp fuse the fuses the circuit protection device isn’t it and here you’re going to see hot and run now what does that mean that means that the ignition key has to be in the run position to complete the connection between the battery and this fuse I can refer back to the power distribution diagram locate the underhood electrical center number one and the e/m is fuse in the power distribution diagram to follow it all the way back to the battery do I need to worry about that right now maybe maybe not hot at all times mean the key doesn’t have to be in any position but that fuse has got power going to it continuously and of course you may see hot start hot and start and run different combinations indicating when power is fed to that fuse but for our purposes I’m back to the battery on the power cycle circuit at this point so now I can do some some idea if this is hot and run that means I have to turn the key on and when I do I should be able to measure 12 volts at the brown fuse so that means if I come all the way back down to the AC compressor clutch coil and the relay where that brown fused terminates or that brown wire terminates I should be able to measure with my voltmeter twelve volts here if I start the engine and I turn the AC on because I have to have a running in order for the compressor to turn don’t I then I should measure that same 12 volts here if this switch closes but if I want to check the entire path I should be able to come all the way back up to the compressor and measure it directly at the compressor now this is part of voltage drop testing and we have several videos on voltage drop testing that I encourage you to check out you’ll see some links right here on this video that will take you there to help you make these tests a little easier so very quick diagnosis if I have a key in run and I have the engine running a C’s commanded on and I have no power coming in I need to backtrack it to see where that power was lost if I do have power then the next step is to make sure the ground sight is okay using a voltage drop testing technique and that’s either going to tell me I have a ground issue or problem with the clutch coil itself but to continue our discussion on diagrams let’s just say that we don’t have power here and we trace it to no power at the EA pin we suspect our relay is not turning on yes I can jumpered these two to see if it comes on it may be a bad relay but it also may be that the relay itself is not being turned on so now this portion of the relay the cell monoi portion really that now becomes a load in the second circuit doesn’t it still trying to figure out why the compressor clutch coil isn’t working no power getting to it no power coming out of the relay command it on so we’ve got to figure out why the relays not working well here’s the brown wire if we trace that back to the load we’ll end up at that splice we saw earlier okay that’s telling us that if this this side coming in to this either relay is good on f6 there’s power here there should be power at f8 so we can verify that first and if that’s the power side to the relay control the load rather then the dark green but what’s this dark green /wh t that means it’s a dark green wire with a white stripe along it it’s called a tracer I know you’ve seen those but under hood you’ll see different color wires and habeler stripe running down the side that’s the tracer so now instead of a dark ring like I had on he ate I’m looking at a dark green with a white tracer coming on 10e six that’s got to be the ground side so let’s see where that goes we’re going to follow that down still trying to get to the ground side the battery to complete the path to that secondary load and doing a little confusing very careful here we go dark green and white okay depending on the VIN we’ll tell you what connector and what pin and that connector it is but the dark green and white is the relay control and what the PCM and for the AC system that’s not unusual the AC has the say-so of whether the compressor comes on or not it’s it’s an emissions impacting device isn’t it and it gives us an idea of what information that PCM needs in order to turn the AC on pressure switch AC request and we can track these other sensor inputs to see you know what needs to be there or you can check your service information system theory of operation for the AC so that you understand what the computer has to see in order to turn that driver on and ground the controls of the solenoid side of the relay am i really worried about that right now not at this point if I can verify that the ground is indeed being made and that’s not an issue so let’s talk about troubleshooting that side the circuit very quickly we’re going to come back up to the relay power coming in should be there hot and run we already established that on the brown wire when I turn the AC on I should be able to do a voltage drop check on 10 e6 to see if the AC is indeed being pulled the ground if not then I need to take a look at the inputs I can use my scan tool for that to see if the inputs the PCM requires in order to turn the AC on or being met that could be something like we all know low refrigerant for example could cause that but if this is complete and this goes to ground and it still doesn’t work that I have a problem here in the relay on mechanical side the switch side don’t now another test that we can make is current what if I put a current clamp on the brown wire I turn the AC on if I measure current flow of Oh half an amp or so and I know current is passing through the solenoid and the circuits working just fine if I put it on f6 on the brown wire then I should get three to five amps because of the compressor clutch coil if I say that then I know the circuits just fine electrically I have a mechanical issue with the coil itself could be excessive air gap or mechanical defect so I hope at this point you can see how reading a schematic is not that difficult it’s a skill that you can master fairly easily and the information that you can obtain along the way so very quickly recap the first step in reading a block diagram is to identify the load or the working component that you’re trying to troubleshoot in our case it was the AC compressor clutch coil and then to follow the wiring coming from that load until it gets back to the battery both on the power side and on the ground side because any low has to have a complete path in order for it to work now along the way we weren’t identified the other elements necessary for electrical circuit that was the circuit protection devices the fuse and the control devices in this case the relay and once we have that determined we also spend a little bit of time talking about how the relay and electrical relay is really an electronic switch that can be both be a load here on this side or a control and if it’s not working we have a secondary circuit that we need to diagnose and we follow the same steps here’s the load then we’re going to follow the path back to the positive side of the battery and then to the ground side in this case through the PCM because we added PCM involved we knew that the relay was controlled by a control module that needed to see certain inputs before it would make that decision to complete the switch and the path to ground well I hope that was some help to you in reading block wiring diagrams the one issue that may throw you is when you’re dealing with electrical components say a MAF sensor for example that has more than two wires going to it still the same things still apply you first want to identify the power and ground side of that component you have that load so take one wire at a time until you get back to the battery either on the power side of the battery or the ground side of the battery then you can follow the remaining wires to see what they do are the essential input to a control module or something else thanks so much for taking this time to join me on this edition of the trader I’m Pete Meyer I’ll see you next month


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