Over the years, I have learned some very good electrical diagnostic routines that have made finding a variety of electrical problems both fun and very fulfilling. I have heard it said that when hunting for electrical problems, there is no right and no wrong way to do the job. I would question, and most likely argue, that point.
The process of electrical problem analysis for me started many years ago with some training, and has continued with much practice and thought about how this process can be made better. Of all the classes I have taken on electrical problem analysis, two classes stick in my mind as great classes. One was a two-hour class done by our local Interstate battery distributor and the white-haired old guy talked about the simplicity of using volt-drop testing when looking for unwanted resistance problems. That was a simple class, but the content of the class has stuck with me over the years. The other class was done by a great electrical trainer, Vince Fischelli. The class was about critical thinking and using the built-in loads on the vehicle (starter, headlights etc.) to load the circuits and actively test for proper circuit operation.
These two trainers captured my attention on electrical testing and from the things I learned from them, I was able to turn “the finding of electrical problems” into a game. So, how do you play the game; what are the rules?
Rules of the game
Many years ago, Mr. Ohm and Mr. Kirchhoff wrote down some really cool stuff about electricity. These laws deal with voltage, resistance and current flow and how they all play together. Once you gain an understanding of these rules (electrical principles), playing the game can be fun.
When I got into this business in 1992, I was told to throw away my conventional test light, since using it on the electrical systems of “today” would kill computer sensors and drivers. Here we are 25 years later, and I have a very good selection of test lights using a variety of bulbs. Do I use them for testing today? You bet I do, although I have learned there is a time and a place for every tool and if these test lights are used in the proper places, they can be great tools.
Admittedly, one of the tools that I have replaced test lights with is the Power Probe. This is a wonderful tool but it, too, can be used in the wrong places. If it is used improperly, computer drivers magically go away. Every tool has its place and knowing the place to use the tools can be a mark of a true craftsman.
Equip yourself
When it comes to finding electrical problems, the first piece of information you need is a wiring diagram. We have all heard wiring diagrams called “electrical road maps” and this seems like a good description, since not only does the diagram show you where the electricity flows, but also information like how the circuit is designed and what components are in the circuit are also included in the diagram. Without a wiring diagram, you have no way of knowing which wires power the component or which wires are ground or control wires. These few pieces of information are very crucial to the operation of any electrical component.
If you want to test the powers and grounds in a circuit, you can use a volt meter, a bulb test light, or an LED test light, but stop and consider what you are testing. It doesn’t do much good to use a volt meter to test the voltage on a circuit that is not loaded, since the voltage of a non-grounded circuit will always be system voltage, regardless of the resistance in the circuit. The circuit needs to have a load put on it for the test to be accurate. This will require you to either turn the circuit on, or simulate a load with a bulb test light, or even a variable load tool I built that uses 1157 bulbs. With this tool, I can regulate a load from 2.5 amps through 15 amps. It is sort of like a test light on steroids. With a circuit properly loaded, you can get an accurate voltage and volt drop measurements.
Another valuable piece of equipment needed to make testing of electrical circuits easy is a scan tool that will communicate with the modules on the vehicles and have the capability of bidirectional control of the components. Being able to turn components on and off without having the engine running or the vehicle in operation can be very valuable. Let’s say you are working on an A/C compressor clutch that will not lock up — being able to have the engine off and using the scan tool to activate the clutch while testing the power and ground can be a real time saver.
Last but not least on the list of time savers is having such a simple thing as a package of colored highlighters to identify the different parts of the circuit. Being able to identifying the different parts of the circuit with different colors can make it simple to just glance at a wiring diagram and see which part of the circuit is powered, grounded or switched, or even if there is a different voltage other than vehicle system voltage. All this information can be found in a wiring diagram, if you spend the time to print it out and study it.
Let’s play!
In the shop is a nice-looking 2001 Mazda 626 with an overheating problem. The vehicle is powered with a 2.5 V6 engine with automatic transmission. The odometer has recorded 205,000 miles and the vehicle is nice and clean. This is a vehicle I have serviced in my shop for a few years.
The vehicle came to the shop with an overheating complaint. When interviewing the person who drives the vehicle, I found the temperature gauge would climb when the vehicle was climbing steep grades. More questioning found the problem to also be after the vehicle was driven for an extended period of time.
Checking the basics, I found the cooling system low on coolant. Any time I see this condition, my thoughts turn to combustion chamber or head gasket leaks or a plugged up radiator. Before I can condemn the engine, I first need to check for any external coolant leaks. If there are external leaks, they need to be fixed and then the engine tested for combustion leaks.
Pressure testing the cooling system revealed a heater hose with a pinhole leak that was loosing coolant. This repair was simple, with just a piece of heater hose. The other coolant hoses were inspected with no problems found.
Being thorough with testing problems like this is very important, since there are several things that can cause an overheat problem. The low coolant is a sure bet to cause an overheating problem, but is there more? Is the leaking hose a symptom of another problem, or is it the cause of the problem?
With the cooling system holding pressure and full of coolant, the engine was ran until it reached operating temperature. The cooling system was tested for combustion gasses with none found. The next place to test is to verify proper cooling fan operation. This is easily done by running the engine until it reaches the proper temperature to turn the cooling fans on. To know this information, a scan tool must be used to verify engine coolant temperature and the cooling fan operation.
Before I get ahead of myself, I need some information on how the cooling fans on this vehicle work. The quickest place to find this information is with a wiring diagram. The wiring diagram shows two electric fans on the vehicle. One is labeled cooling fan and the other is the condenser fan. If needed, more information on the fan operation could be found in the service information, although, at this time the wiring diagram information told me all I wanted to know. The wiring diagram shows both the cooling fan and the condenser fan are operated by separate relays, which are controlled by the PCM.
This is great information. Since the PCM is controlling the fan relays, there should be some scan data PIDs and possibly some bidirectional controls for these relays. By using the scan tool data, I found the right side fan (condenser fan) would come on when the ECT (Engine Coolant Temperature) reached 245-248º. When the A/C is turned on, only the condenser fan runs. By watching the scan tool, I can see the relays for both the cooling fan and condenser fan are being commanded on.
Since the scan tool tells me both the condenser fan and the radiator cooling fan relays are being commanded on by the PCM, I need to know one more thing before I do any testing. I need to know where the relays are located. Service information says the relays are behind the battery and in front of the fuse box. Looking at the vehicle there are three relays in that location and they all look the same. These relays are plain mechanical/electric relays. Any time these relays are turned on or off, they make a “click” noise, and will also make a small vibration. All that is needed to determine which relay is which is to use the scan tool and cycle the relays. By feeling the relays with my hand while they are activated, it is an easy, quick and accurate way to find which relay is the cooling fan relay.
With the relay located and the relay removed from its socket I find the relay is a four pin relay. By using the wiring diagram, I find two of the terminals should be powered when the ignition key is turned on. Testing the terminals in the relay socket, I found power at these two terminals. All that is left to test is to verify the ability of the PCM to ground the relay, and the ability of the circuit to flow current to the cooling fan motor. By using the scan tool to command the cooling fan relay on, I found a path to ground for the relay coil, but there is no path to ground on the terminal that powers the cooling fan motor.
The problem is now narrowed down to either an open circuit in the cooling fan motor, or an open in the power supply circuit between the cooling fan motor and the relay terminal. The problem was found at the harness plug that connected to the cooling fan motor. The positive wire terminal had overheated and had to be replaced.
By using a scan tool, a wiring diagram and a logical diagnostic process, this problem was quickly and easily found.
With the electrical problem fixed and the cooling fans operating as designed, the vehicle was test driven up a mountain pass with a 6 percent grade. The engine performed well, and the cooling system cooled the engine properly.
When problems like this come in, it is too easy to jump to conclusions at the first problem found, which in this case the cooling system was low on coolant. Be through with the diagnostic process and test the complete system and let the system test its self, by making it work as it was designed to work.
