Electrical System Faults

Jan. 25, 2021

With the ever increasing technology on today’s vehicles comes with ever increasing electrical demands.  You would think that electrical failures would be more prevalent than ever on today’s vehicles.  

It doesn’t seem that there are more component failures, but there are quite a few wiring issues that will at times cause a component failure.  Components such as batteries, alternators, starters and control modules can stop working because of a simple wiring issue.  

When looking for wiring issues you might come across a wire that has been pierced, cut or rubbed through.  Any time a wire has been compromised the electricity that runs through it will be diminished.  

As you can see in Figure 1, this wire has been rubbing along a transmission case to the point where the strands of wire were starting to break which increased the resistance in this particular circuit. This also was grounding out on the transmission case (see Figure 2), and causing the input speed sensor to start reading erratic. Once that happened,  a diagnostic trouble code was set.  

Other forms of wire harness failures could involve the intrusion of moisture or water.  

Figure 2: Wires rubbed on transmission case.

As you know, wiring harnesses are made up of copper strands running inside a plastic covering.  Any time a wire has been breached with excessive moisture you’ll have corrosion buildup on the copper.  If you recall television ads from years ago that described growing your own Chia pet, corrosion on copper looks very similar.  

Another failure area to check are your connectors.  The connector shown in Figure 3 goes to a DEF (diesel exhaust fluid) module.  This connector allowed water to get in between the connector and the component due to a failed seal that was supposed to be on the connector.  That seal, which resembles a rubber band, is located where the connector plugs into the DEF module.  

Figure 3: Corrosion on the connector of a dosing module.

Always perform a good visual inspection of your connectors, the wires and the way the harnesses are routed. Improper wire repairs will also cause a component to fail prematurely. Shown in Figure 4 is an example of a wire that has rubbed through, with an improper wire connection.  This wire was repaired using a butt splice connector that should have been shrink-wrapped on the ends. With the connector knots sealed this is a perfect imitation for corrosion to reside. 

Figure 4: An exposed wire and improper repair.

Remember, corrosion is resistance and if there is too much resistance in a circuit then that  can cause a component to fail prematurely. 

When working on automotive electrical systems it’s important to have the proper tools. First and foremost a good working multimeter, electrical system tester and a soldering gun, brushes and other various tools are necessary to perform a complete and efficient job. There are many different types from which to choose. There is not a one-size-fits-all. You need to choose which tools are best for the type of job you’re working on. 

Figure 5: A major electrical failure at the starter.

Repairing wiring harnesses can be a bit of a challenge at times.  One of the most difficult things to do when it comes to harness repair is to actually get to the harness itself. Today’s vehicles are tightly compacted.  Sometimes you have to move many components just to get to the area of repair.

Whether you’re repairing a harness or replacing the harness, many times it boils down to cost. Some harnesses are very expensive and possibly not worth replacing due to the value of the vehicle.  In that case repairing the harness might be the best option. 

When dealing with an older vehicle, you might not be able to find a replacement harness due to the fact that it has been discontinued.  If you’re lucky you can find a good used harness but you don’t want to replace a harness that’s not much better than the one you have. This is strictly a judgment call.

So what about the  components themselves? Many factors can contribute to a component failure. That can be improper mounting, wear and tear, or maybe even the wrong part. 

Let’s take a look at a starter motor.  This starter (see Figure 5) experienced a major electrical failure. This wasn’t due to an internal failure, but an external failure. This starter had a positive battery cable that had rubbed to a ground causing excessive heat. After prolonged heat buildup the solenoid that the battery cable was attached to started to melt. After a while it got to the point that the battery cable attachment lost its attachment to the starter. 

This starter failed due to the improper routing of the positive battery cable. This vehicle owner was very lucky that this vehicle did not catch on fire. 

Other types of starter failures can be internal. One of the biggest reasons a starter will fail is due to the fact that there’s not enough amperage to engage the starter to the flywheel and have enough power to turn the engine over. This can be caused by a weak battery or a battery that is not correct for the vehicle. 

Vehicle design engineers have determined what size battery is needed to cover all the electrical systems and accessories for a particular vehicle. It may seem obvious, but it’s critical to have the correct battery in the vehicle. An under-performing or low-cranking amp battery will not provide the components with the proper electrical requirements.  

Another way the starting motor can fail is due to excessive resistance in the battery cables. That could be from a poor connection or a corroded connection to an internally corroded battery cable. Any time you suspect a starter motor failure, be sure to perform a voltage drop test on both your positive and negative battery cables before you replace the starter. You could have corrosion buildup inside the cable that has migrated in between all the strands, a condition that may not be readily visible. Without performing a voltage drop test, the new starter motor will have a short life span. 

The voltage drop test is very simple. Set your meter on the DC volt scale. Attach one lead to the positive battery cable connection and the other lead to the other end of the positive battery cable. If your cable is in good condition your meter should read zero volts or very close to that. If you are getting anything different than that then you are measuring excessive resistance. 

Figure 6: A Cummins code description seen on the Insite diagnostic tool.

Perform the same test on the negative side.  Also make sure that both your cable ends are clean and tight. 

Common electrical failures in a vehicle’s charging system can vary depending on the vehicle type and load requirements for a specific vehicle. An alternator’s job is to help keep the battery state of charge up to where it needs to be for proper operation and to assist the various electronic components of the vehicle to operate correctly. Reverting back to the battery, if the battery can’t maintain a proper charge it will place an excessive strain on the alternator, making it work harder than it was designed to do. 

That alternator will fail faster than its average life expectancy. Another premature failure would be attributed to heat. If the engine temperature is higher than what it should be, especially under the hood of vehicles with today’s cramped  engine compartments, the heat generation will slowly degrade the internal components of the alternator. 

Another issue I have seen is the front bearing of the alternator (which is usually sealed and packed with grease) will start to degrade the internal lubrication of the bearing and eventually will damage the bearing, shaft and even the pulley. An alternator can actually fail prematurely due to another component not operating as designed. That can be a component like a power steering pump that is either misaligned or not mounted properly. This could cause a vibration that over time could damage the alternator’s internal components as well as the front or rear bearings of the unit.

Considering the ever-increasing number of electrical components of today’s vehicles, it’s critical that the battery, starter and charging systems need to work properly. Let’s take a look at an electrical system that is starting to fail and is creating a drivability issue. 

Figure 7: A Cummins code description. Per the code, a cylinder imbalance was cited.

An example is a 2005 Kenworth W900 with an 8.3L Cummins engine. Remember, it doesn’t matter what type of vehicle you’re working on... electrical system failures can happen on any vehicle. The driver stated that between 40-65 mph, the truck would exhibit an intermittent fish bite or chuggle at steady speeds. 

As you well know, trying to nail down an intermittent problem can always be a challenge at times. Since I don’t have a commercial driver’s license, I rode with the driver while I monitored the scan tool to see if I could catch the culprit. Since this vehicle has a Cummins engine, I hooked up the Cummins Insite which is the factory scan tool, and monitored for any stored codes that would alert me to the drivability problem that the driver was having. 

Once I retrieved the codes, I saw a diagnostic trouble code that was describing the type of concern this vehicle was experiencing. The code was a 0951 which is a power imbalance between cylinders detected by the ECM (see Figures 6 and 7). 

Figure 8: When the Cummins cylinder balance test was performed, no imbalance issue was shown.

During the test drive, I monitored the scan data and I didn’t see anything that looked amiss. When we were able to pull over to a safe spot I decided to run a cylinder power balance test to see if it could be a mechanical problem. The test passed without any issues (see Figure 8). The next step was to look at the snapshot data that was retrieved when the code set and I noted in the charging system PID (on-board diagnostics parameter IDs) that the voltage was lower than it should be (see Figure 9). 

This had me thinking more in the lines of an electrical system issue as opposed to a mechanical issue. I decided to take a look at the electrical system and I started with the batteries. I performed an electrical load test on the truck’s two batteries which are run in series. I isolated both batteries and tested each one separately. 

Each battery has a rating of 900 CCA. One battery tested out at 710 CCA and the other one tested at 580 CCA. Looking up the history on this vehicle it looks like the batteries haven’t been replaced in six years. I recommended that the batteries be replaced as a maintenance practice before anymore testing can be done. I had the driver take the truck on another road test as I wanted to see if replacing the batteries made a change. 

The driver stated the truck didn’t act up at all. I went back in and re-scanned the truck and there weren’t any codes stored. What was happening was the control module was losing the proper operating voltage that it needs to sufficiently keep the truck operating as it was designed. 

Remember, vehicles with control modules are pretty picky on obtaining the correct voltage to operate the systems that they control. This vehicle has been operating like it should without any issues reported by the driver.

Figure 9: Critical data PID on Cummins Insite. 11.81 was showing when the misfire was occurring, which is what turned on the light. The 12.38 showed when the code first set.

That was just one scenario where an electrical system that’s not operating like it should can cause another system to show a fault. Catching electrical system and component failures can be easy to do with the proper training and the understanding of how the system operates. 

In a previous issue I wrote an article on electrical system troubleshooting and I stated numerous times that any electrical diagnostic troubleshooting starts with the battery. The battery is the heart of the system. If the heart is bad, the rest of the system will fail. 

How did I know to check the code that was set on that Kenworth for a voltage issue? It doesn’t matter what vehicle you’re working on... if you find a diagnostic trouble code set, you should always look at the set criteria, meaning what does it take to set that code.... and then analyze the data that set when that snapshot was taken. Scan tools may not be as fast as a lab scope, but they can provide a substantial amount of useful data if you look at them closely. Knowing and understanding the vehicle system or systems that you’re working on and how it operates will go a long way in aiding your diagnosis.

About the Author

Edwin Hazzard

Edwin Hazzard owns South East Mobile Tech in Charleston, S.C., which is a mobile diagnostic and programming service providing technical service to many automotive and body repair shops. He has 38 years’ experience in the automotive industry. He currently is an automotive trainer, a board member of TST (Technician Service Training), a member of the MDG (Mobile Diagnostic Group), a member of the Professional Tool and Equipment advisory board for Pten magazine, a committee member of Nastaf, and is a beta tester for multiple tool makers.