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How to interpret automotive wiring diagrams

Monday, October 2, 2017 - 07:00
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We use wiring diagrams in many of our diagnostics, but if we are not careful, they can sometimes lead us to make decisions that are not accurate, which can lead to wasted diagnostic time, unnecessary parts costs for the replacing parts that are not defective, and sometimes even missing a simple repair.

One area where I have noticed a wide skills gap when helping other technicians diagnose a problem is in the use of wiring diagrams — not reading them, but more importantly interpreting them. While there have been several very informative articles and training classes on the subject, the one that has had the greatest impact on improving my circuit diagnosis was a technique invented by Jorge Menchu of AESwave called Color Coding. His technique uses various colors to represent what types of signals to expect at certain points in a circuit and help narrow down where the problem is by seeing what is and isn’t working as it is designed to. I point this out since the colors I used to highlight the circuits in this article are based off of this technique, and I also use this information to design/change my diagnostic plan. A color coding kit (AES# 02-WDCC) is available from

But alas, even when using circuit wiring diagrams and having proper techniques, there are times when the provided information does not show the whole picture, which can cause inaccurate diagnostic summaries and wasted replacement of components.

When it isn’t the bulb

How often when a vehicle comes in with a complaint of a bulb not working do we or the customer automatically just install a new one? In 95 percent of the vehicles that have this concern, bulb replacement fixes it, so for the most part it could be a valid first step. However, if it doesn’t work, it can turn out to be a problem vehicle especially if the wiring diagrams get a little complicated. This is what happened on a 2008 GMC Acadia SLT that had 82,439 miles with the complaint of an inoperative RF turn signal. The technician who was originally assigned the repair order started with a replacement bulb, but found that this repair was not going be that easy. Apparently, the bulb has already been replaced by either the customer or another shop so their next step was to determine if there was correct voltage and ground supplied to the bulb; a quick check with a Digital Multimeter (DMM) showed no voltage. Looking at the wiring diagram for the exterior lighting, they determined that the Body Control Module (BCM) was at fault because in their thought process that is what supplies the voltage to the turn signal and since the Right Rear Turn Signal was working; it must be getting the request from the Multifunction Switch. The tech checked the powers and grounds to the BCM and they were OK, so a new BCM was installed and set up. Obviously if I’m writing about this vehicle, that didn’t fix the concern. 

Figure 1 - The only code that appeared in the BCM was a B2615 for Courtesy Lamp Control, but since the circuit description did not have anything to do with the exterior lighting I kept my focus on the turn signal concern.

Like most diagnostics, if I’m not sure how a system is designed to work I do some research before testing. This is also the point at which I print out a wiring diagram and highlight what a correctly working circuit should look like. I find the same circuit also includes the turn signal on the RF side-view mirror, and I noticed that it is not working either; however, the Right Rear Turn Signal is on a completely different circuit and is working as designed. I also gathered from the wiring diagram that the BCM (Connector 4 Pin 5 DK BLU/WHT wire) is what controls the circuit once the input from the turn signal switch is received (Connector 1 Pin 16 DK BLU/WHT wire). Since the BCM controls the turn signal circuit, it’s a good idea to check for codes and when I did I found a B2516 Passenger Compartment Dimming 2 Circuit (Figure 1). A quick look up of the code with a description of the circuit shows that this is related to the courtesy lighting circuit, which I notice is not working. This does not seem to have any effect on the exterior lighting circuit so I decided to stay focused on the turn signal problem and keep that info in the back of my mind. Now I remove the RF turn signal bulb socket to start my voltage tests. I can see from the wiring diagram that the ground for the RF turn signal — in this case G102 — is a constant; it is the first signal I check with my LOADpro voltmeter Leads to test the circuit under a load. Next we move on to the supplied voltage side of the circuit. Since the BCM is easily accessible by the driver-side kick panel, I perform my testing there.

Known good – known bad?

When using a scope to diagnose a problem, it is a good idea to have a known good signal to compare a possibly defective signal to, so I also monitored the Left Front Turn Signal input and output (Connector 1 Pin 16 LT BLU/WHT and Connector 5 Pin 4 LT BLU/WHT respectively), since I know this side is working as designed. As you can see (Figure 2), both inputs are working correctly but only the LF turn signal output is being generated by the BCM; nothing is happening on the RF turn signal output circuit. I also turn on the hazard flashers as another input source to the BCM and have the same result with an inoperative RF turn signal output. Next I use a Power Probe to apply battery voltage to the RF turn signal circuit at the BCM harness with the connector unplugged and the directional bulb at the RF corner illuminates. This tells me that the circuit is intact and can handle the load when applied. Now I am starting to see why the previous tech suspected the BCM.

Figure 2 - This is the scope capture from the BCM input and output controls of the turn signals.  Notice that the input is received, but there is no output for the RF turn signal.

While looking at the wiring diagram for the exterior lighting circuit, I notice a few pins that are B+ supplies to the BCM, and one of the fuses is even labeled as the Right Turn Signal. Something important to remember when testing voltage supplies and grounds to a module is to look at the actual module wiring diagram. While the exterior lighting wiring diagram shows some power supplies, it does not show the whole picture of the module itself (Figures 3, 4).  I start with the grounds first; it looks like Pins 1 and 5 (Connector 3, both BLK/WHT) and Pin 9 (Connector 4 BLK) are the grounds, and all three test fine. Next I move on to verifying the voltage supply pins. I find that there are four pins, all RED/WHT wires numbers 1-4 that are supposed to have B+, but find that Pin 2 does not; it is an open circuit. Guess where the voltage supply comes from?  Remember the code in the BCM for the courtesy circuit? The fuse that supplies B+ to this pin was open. After replacing the fuse, the RF turn signal worked.

Figure 3 - (Diagram courtesy of Mitchell Pro Demand) The wiring diagram for the Exterior Lighting shows only 3 B+ inputs for the BCM, which all tested fine.
Figure 4 - (Diagram courtesy of Mitchell Pro Demand) The wiring diagram for the BCM itself shows another B+ input at Pin 2, note that this does not show on the Exterior Lighting wiring diagram and was not tested by the original tech that diagnosed the vehicle.

I asked the other tech again to verify that he tested all the B+ supply circuits; he said yes and showed me the exterior lighting diagram and found it only lists Pins 1, 3 and 4 as B+; however, Pin 2 is not shown on the exterior lighting circuit. That is why it is important to use the actual module wiring diagram to check for B+ and ground. I do not understand why this power supply would affect only the RF turn signal, especially since there appeared to be a dedicated fuse for the right-side turn signal, but it does go to show that we must not get tunnel vision when performing something as simple as a lighting circuit diagnostic, as there may be a bigger picture.

Not quite done

So the vehicle is fixed right? Well, sort of. The RF turn signal is working (Figure 5), but the directional on right side-view mirror is still inoperative. As stated before, the wiring diagram shows both the RF turn signal and Passenger Outside mirror are on the same circuit, in fact the mirror is spliced to the same wire from the BCM before going through the Underhood Fuse Block so it eliminates that part of the wiring automatically. Well, it looks like the best place to test is at the connector for the mirror itself, so we can see if the voltage signal is present and test the ground. After removing the door panel the problem was pretty easy to see: the mirror that was on the vehicle was incorrect for the application, the connector pins for the mirror side of the harness did not align to the pins in the original door harness, and there was a second mirror harness connector on the door that did not have anything plugged into it.

Figure 5 - The scope capture of the repaired circuit showing all inputs and outputs are working as designed.

Someone had just attached a side-view mirror that looked correct (on the outside) from a GM vehicle with different options. In hindsight I could have saved myself the trouble of removing the door panel by trying to move the mirror glass with the controls as none of the functions of the mirror worked. Repairing the circuit for the RF turn signal restored the double-time flashing of the right turn signal indicator on the instrument cluster; the inoperative side-view mirror directional did not affect the rate of the flasher. I did not find out what caused the courtesy fuse to blow, but I don’t know what happened to the original side-view mirror on the vehicle, either.

Another bulb in question

The next vehicle that was given to me was a 2008 Dodge Avenger with 112,976 miles and a 2.4L engine for a complaint of an inoperative right front low beam headlamp. A little background about this vehicle before it ended up in my bay: The customer has already tried replacing the bulb themself, however when the vehicle arrived there was no bulb to be found at the RF headlamp connector, in fact there was a new connector already spliced in with butt connectors (Figure 6). The technician who got to look at the vehicle first also knew the customer had tried replacing the bulb, so they installed a voltmeter across the bulb connector and turned the headlamps on - 12V! They assumed that maybe the customer had purchased a defective bulb, but it was not found in the vehicle, and we didn’t have another in stock to test. Since the bulbs are very easy to replace on this vehicle, he swapped the left front low beam bulb to the right side instead of ordering a new one, and he knew the left headlamp worked fine. Same problem: the bulb did not illuminate on the right side. He swapped it back to the left side and it again worked perfectly.

Figure 6 - The customer has already attempted to perform their own wiring repairs to the vehicle along with replacing the bulb, which was not in the vehicle when we received it for diagnostics.  Not sure why they used so many butt connectors but we had to make sure his attempted repairs were not negatively affecting the circuit.

I can understand the confusion and frustration of the technician since he verified he had voltage and ground at the connector with the headlamp switched on, so why was the bulb not working? He pulled a wiring diagram for the headlamp circuit and saw that the RF low beam headlamp is a fairly simple circuit that has a constant ground and voltage is supplied by the Totally Integrated Power Module (TIPM). So he asked me for a second opinion before recommending a new module.

When looking at the wiring diagram, I like to start with the ground side of the circuit and highlight it. I noticed that ground is constant as he stated, but it is also shared by the right front high beam bulb and the right front fog lamp bulb, both of which are working normally, so it doesn’t look like we have a problem with high resistance on the ground portion of the circuit. Another item I noticed is the customer replaced the connector to the RF low beam headlamp, with multiple butt connectors. Fortunately they were not affecting the operation of the circuit.

Figure 7 - A halogen headlamp is substituted for the missing bulb by wiring it into the connector.  This is also a great way to load test a system.
Figure 8 - A capture from the graphing multimeter shows that when the bulb is connected the supplied voltage drops to 0V, but when it is disconnected again the voltage returns. This is why the technician found battery voltage on their DMM when testing the circuit, it was not under load.

Next I move on to the voltage supply side of the system. Again as the technician stated, voltage is supplied to the RF low beam headlamp from the TIPM. So to verify my understanding of the circuit I used a graphing multimeter and back probe Pins 1 and 2 of the RF low beam connector and wired in a headlamp bulb (Figure 7) that I also use to load test the circuit. When the headlamps switch was turned on my GMM showed no voltage; unplugging my wired-in headlamp from connector I had battery voltage again. Reconnecting the headlamp to the circuit I experienced the voltage dropping back to 0V (Figure 8) .   

It appears to be a defective driver in the Totally Integrated Power Module (TIPM), but let’s not jump the gun until we verify the voltage and ground supplied to it first; we’ve already experienced that in our last case study. The TIPM is easy to access and had several connectors attached to the underside of the module. Using the actual TIPM wiring diagram and not the wiring diagram for the headlamp circuit, we find there is a larger B+ wire directly from the battery, which supplies voltage to the module and multiple grounds to check, again testing them under load since simple checking voltage it not going to reveal a problem as we just witnessed with the headlamp circuit. All the voltage and ground circuits to the TIPM are fine, but just to be on the safe side, I simulate the work of the TIPM and supply voltage to Connector 5 Pin 3 WHT/TAN wire to verify the integrity of the rest of the circuit to the headlamp and my wired-in headlamp bulb illuminates brightly, proving the problem is with the RF low beam driver inside of the TIPM.

The customer approved the replacement of the TIPM but did not want to pay us to replace the headlamp bulb; they had the bulb at home and would install it themselves. With the new TIPM installed I still wanted to verify my repair so I attached my tester headlamp in place of the missing headlamp bulb and it worked great, at least by doing this I can be confident when the customer installs their new bulb it will work.

As I stated in the beginning, the repairs themselves were simple but using the correct wiring diagrams and techniques to understand how the circuits that operated them are designed to work is the key. Not doing so makes it easy to get misled if a solid understanding of the wiring diagrams is not in place.

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