So the car came in with a complaint of engine overheating in stop-and-go driving. It didn't take you long to determine the electric radiator fan wasn't turning on. A simple trick that will cause the Engine Computer to turn on the fan is to unplug the coolant temperature sensor. When you do that, the fan relay clicks but the fan doesn't run.
What do you know so far? The computer, coolant temperature sensor, and low current wiring are working. What are the possible causes of the problem? Not many. The fan motor, relay contacts, and high-current wiring are all that is in the circuit. Should be an easy diagnosis.
The logical first step would be to unplug the connector for the fan motor and check for battery voltage when the relay turns on. That turns out positive. Voltage is present. Before jumping on the fan motor as the suspect, don't forget to check the ground circuit. In this story it proves to be fine also. That only leaves the fan motor.
Okay, so the new fan motor is installed. All that's needed is a quick check of its operation and you can give the car back to the customer. But wait a minute. The engine is getting hot but the fan hasn't turned on. Time to double check your work.
With no obvious problem found, you connect a scanner and command the fan relay to turn on. The relay clicks as it did before but the fan doesn't run. Time to start all over with your diagnosis.
This is exactly how the story played out for a very experienced transmission mechanic working on a mid-1980s Dodge Aries. He had a very good understanding of electrical theory but relatively little experience working on electrical problems. The only thing he did not test previously was the fan motor so he used a pair of jumper wires connected directly to the battery, . . . and the fan ran just fine! So the motor is good, the 12 volt feed is good, and the ground is good, . . . and the motor doesn't run. After struggling with it for most of the day, the new guy in the shop pointed out he was using a very expensive high-quality digital voltmeter to take the voltage and resistance readings. "Try using a cheap test light instead". The mechanic's argument was, "why would that matter?"
In fact, once he did perform the same tests with a test light, he found there really wasn't 12 volts to the motor. The question now was why would the voltmeter detect battery voltage but the test light didn't? The answer lies in how the two pieces of test equipment work. The voltmeter has a very high impedance, (resistance), so it does not complete the circuit and let current flow. This would be comparable to a pressure gauge on the compressed air line in your shop. No air flows through the gauge for it to do its job. The air line could be bent, kinked, obstructed, squished, plugged, or a valve could be partially closed. Any of those things would restrict air flow (volume) but as long as any tiny amount of air could get through, full pressure would eventually be seen at the location of the gauge. Has the light bulb over your head clicked on yet?
You have pressure at the gauge but when you connect the hose to an air tool, it doesn't work. All you hear is a tiny hiss. So, you have pressure and a good working tool but it won't run. This scenario correlates perfectly with the fan motor. Everything tests good but the circuit doesn't work. The problem with the air tool is the air line is restricted, . . . not blocked completely, just restricted to the point not enough air can get through. The same thing can happen in an electrical circuit, and in fact, that's what threw the mechanic off track.
The problem was caused not by an open circuit, but by excessively high resistance in the circuit. With an open circuit, which is a much more common defect, both the test light and the voltmeter would have indicated 0 volts at the fan motor's connector. Very high resistance would prevent sufficient current flow to light up the test light, and of course it would prevent sufficient current flow to run the motor. But unlike the motor and test light, the voltmeter doesn't rely on current flowing in the circuit to measure the voltage, just like the pressure gauge doesn't rely on air flow to measure pressure in the line. With no current flow through the voltmeter or any part of the circuit for that matter, there will be no voltage drop across the undesirable resistance. It doesn't matter what the value of that resistance is, there will be no voltage drop. Since we started with full battery voltage and dropped 0 volts in the circuit, we end up with full battery voltage at the fan motor's connector. That's what the voltmeter showed.
This is one instance where the cheap test light provides a more accurate test result than the expensive voltmeter. By its very nature, it requires current from the circuit to flow through it to turn on the light bulb. That bulb completes the circuit which allows current to flow, but because the undesirable resistance is so high, most of the battery voltage is dropped across it leaving very little across the test light, . . . or across the motor.
The logical place to expect to find excessive undesirable resistance would be between the relay contacts. The scanner was turning that relay on and off as part of the test procedure it was programmed to perform so the mechanic went right to it with his test light. He didn't need to figure out which wire was which function or even look at the service manual because he found one terminal where the test light would turn on and off each time the relay clicked on and off. Since the light didn't respond the same way at the other end of the wire, the resistance had to be in that piece of wire.
At this point another detail should be included. This circuit is fused on newer cars with a large plug-in fuse, but up through the late 1980s it was common to use fuse wires called fuse links. It was rare for one to burn open so easily-replaceable fuses were not considered necessary. A fuse link wire is a regular wire that is smaller in diameter than the rest of the wires in that circuit. That makes the fuse link the weak link in the chain. What is different is the wire's insulation. It is designed to not burn or melt when the wire burns open.
In a regular fuse, when the element burns open, an open circuit exists, but in a fuse link wire a carbon track is left behind. That carbon is deposited on the inside of the insulation and it is a conductor. The same carbon tracking can occur inside a distributor cap. Even though it is very high in resistance, it can conduct the current to ground that was supposed to go to a spark plug. In the fan problem, that carbon track allowed battery voltage to appear at the fan motor's plug. Even though its resistance was very high, the voltmeter didn't allow any current to try to flow so there was no voltage dropped across that resistance. That left all of the battery voltage to appear at the plug. When the test light was used, current tried to flow in the circuit. That caused most of the battery voltage to be dropped across the carbon in the burned-open fuse link and almost no voltage left to be dropped across the test light.
Further testing of the old fan motor proved it was tight and barely turning. That means it wasn't producing much back EMF so current flow went up so high, the fuse link burned open. See the story about back EMF in "Starter Systems, Theory of Operation". The car needed that new fan motor. All that was needed to complete the repair was to solder in a new fuse link wire.