Other Articles - September - 2021

Proper Bulb Choice Creates Adequate Electrical Load

Applying a proper load is critical to the success of certain electrical tests. Although some technicians routinely practice this technique, others are unaware of it. Consequently, they make diagnosis much more difficult than it needs to be.

Electrical troubleshooting is a vast topic that occupies chapter upon chapter of automotive textbooks. But simply reviewing loads and how they impact diagnosis may improve your troubleshooting skills. Here, I will focus on “loading” a frayed, lighter-gauge wire because techs have asked me about that particular scenario.

To broaden your perspective, I am including the results of experiments I performed on healthy as well as damaged wires. The outcomes may surprise you.


Okay, the basic series circuit consists of a battery, a positive side, a load device and a negative side. Most techs call the positive side the hot side; they call the negative side the ground side.

Current is electrical volume, measured in amperes (amps) on an ammeter. Ideally, the proper amount of current should flow from the battery, through the entire circuit and then back into the battery again. The current flowing through a circuit is often called the “load.” For example, a tech who measures 10 amps flowing through a circuit may state that the load on that circuit is 10 amps.

Resistance is anything that opposes the flow of current. Traditionally, resistance has been measured in ohms on an ohmmeter. There is a certain amount of normal resistance in wires, cables, connections, etc. But excessive resistance is THE most-common killer of healthy circuits. Typical examples of excessive resistance are frayed or severed wires and cables. Other causes include broken, loose, dirty or corroded connections. The load is an electrical device that does work. Common examples of loads are bulbs, horns, flashers, radios, relays, solenoids, injectors, ignition coils, motors and computers.

I just mentioned normal and excessive resistance. The load always creates the largest amount of normal resistance in a circuit. Substituting a load with less resistance than the OEM one increases current flow through the circuit. Substituting a load with greater resistance reduces current flow. During my last experiment, I changed the electrical load on the circuit by substituting a different light bulb.

Voltage is electrical pressure, measured in volts on a voltmeter. Now let’s bring resistance back into our discussion. Excessive resistance, the big killer of healthy circuits, does more than restrict current. It also causes an abrupt, abnormal change in operating voltage at the exact location of the problem. For example, imagine that some copper strands inside a wire are broken. A substantial change in operating voltage occurs at the site of those broken strands. Traditionally, techs have called this sudden change in operating voltage a “voltage drop.”

Last but not least, remember two vital details. First, excessive voltage drop and excessive resistance go hand in hand. So, you will find excessive resistance wherever you find excessive voltage drop and vice-versa.

Second, voltage drop is proportional to the current flowing through the wire, cable, connection, etc. That is why loading a suspect wire – perhaps one with broken strands inside it – is vital to revealing an abnormal voltage drop across that wire.


Suppose you suspect that a wire may be damaged based on the vehicle’s symptoms and your initial diagnosis. Imagine that this is a lighter- gauge wire buried inside a wiring harness that is difficult to reach. Needlessly replacing this wire could be very costly.

I tried to recreate this situation by load testing sensor wires and measuring the voltage drop across them. Then I intentionally damaged the wires. Next, I loaded the wires again and repeated the voltage drop checks.

Anyway, I retrieved sensor wires from some wrecked Toyotas. The wires, which were unharmed, were reference-voltage wires running from the PCM to various engine sensors. The outside diameter of each wire measured 0.060 inch (1.50 mm), including its insulation. Each wire contained 7 strands of copper. Figure 1 shows a spot where I carefully removed the insulation and spread out those strands.

Ultimately, I created two groups of sensor wires for the experiment. Each wire in the first group was 32 inches long; each wire in the second group measured 72 inches.


Some techs who use an ohmmeter may struggle with a diagnosis because they do not know a key limitation. Simply stated, an ohmmeter cannot and does not apply a substantial load to the wire, connection or component you are testing with it. Consider the following example, which was the first part of my experiments.

I chose one of the 32-inch sensor wires as well as a 72- inch wire. I carefully measured and noted the resistance reading across each length of wire. Each one measured zero ohms. As shown in figure 1, I had stripped some insulation and spread out the strands of both wires.

Next, I snipped one copper strand at a time (Figure 2). Then I repeated the resistance check after each snip. The important lesson here is that a professional – grade ohmmeter could not detect damaged wire strands. Its resistance reading did not change after each snip. Instead, each wire still measured zero ohms after I had cut 6 of the 7 strands inside it!


In the next experiment, I recreated a common, 3-wire sensor setup and checked voltage drop across its reference voltage wire – in good condition as well as damaged. I had an analog Toyota VPS (Vapor Pressure Sensor), which contained a reference voltage terminal, ground terminal and output signal terminal. Using some of my sensor wire and a booster (“jump”) box, I fabricated a VPS setup on the workbench.

On the vehicle, the PCM applies 5.00 volts to the “reference” line. But during my experiment, a USB port on the booster box provided the 5.00-volt reference voltage (Figure 3). I routed one of my 32-inch sensor wires from the USB port to the reference-voltage terminal of the VPS. My measurements showed that 0.008 amp (8 milliamps) was flowing through this reference line.

Like the previous experiment, I removed insulation from the test wire and spread out its 7 copper strands. I measured the voltage drop from the USB port to the reference terminal of the VPS. I cut one strand at time and repeated the voltage drop test after each cut. With all 7 strands, the voltage drop from the USB port to the VPS measured 0 volts. Surprise – this voltage drop still measured 0 volts after I had cut 6 of the 7 copper strands!


Finally, I performed a traditional load test and measured the voltage drop across a sensor wire. Then I repeated the voltage drop check after cutting copper strands within the wire. (I call this a “traditional” load test because electrical specialists and some vehicle manufacturers have recommended it for years. They disconnected the ends of the suspect wire from the circuit. Then they connected them to a bulb and a 12.00- volt source.)

First of all, I used the regular booster box terminals to supply 12.00 volts to my test circuit. Second, a 32- inch long sensor wire served as the hot side of my test circuit for one group of voltage drop measurements. A 72- inch sensor wire was the hot side in another set of measurements. I already had removed a section of insulation, exposing each wire’s copper strands.

Third, I checked each wire’s voltage drop with all 7 strands. Then I repeated the voltage drop after cutting 2 strands, then 4 strands and finally, 6 strands. Fourth, an automotive bulb (with the appropriate bulb socket) was the load during these tests. A sensor wire powered one bulb socket terminal; the other socket terminal was grounded to the booster box. I happened to have 3 different – but common – bulbs handy, so I used them.

The first one was an 1157 taillight bulb that carried 2.43 amps. The second one was a 9006 headlight that carried 3.75 amps. The third was a 9004 headlight that carried 7.35 amps. So, the electrical load on each test wire would be 2.43 amps, then 3.75 amps and finally, 7.35 amps.

Here are just a few examples of the results. Using the 1157 bulb, normal voltage drop for the 32-inch long wire was 0.08 volt. Normal voltage drop for the 72-inch wire was 0.16 volt. Using the 1157 bulb, voltage drop with 6 strands cut from the 32-inch long wire measured 0.17 volt. With the 1157 bulb, cutting 6 strands from the 72-inch wire increased voltage drop from 0.16 to 0.26 volt.

Using the 9006 headlight, normal voltage drop across the 32-inch long wire was 0.11 volt. Normal voltage drop for the 72-inch long wire was 0.25 volt. Using the 9006 headlight, voltage drop across the 32-inch wire increased from 0.11 to 0.25 volt with 6 strands cut. After cutting 6 strands of copper, the voltage drop across the 72-inch wire increased from 0.25 volt to 0.42 volt.

Last but not least, the 9004 headlight caused a normal voltage drop of 0.11 volt on the 32-inch long wire and 0.51 volt on the 72-inch wire. With the 9004 headlight, cutting 6 strands caused a voltage drop of 0.66 volt on the 32-inch long wire and 0.92 volt on the 72-inch wire.


Clearly, applying a heavier electrical load produced definitive, measurable voltage drop values. Clearly, the greater the electrical load, the greater the voltage drop across a healthy wire as well as a damaged wire. And, the longer wire had a greater voltage drop than the shorter wire did.

Perhaps the biggest surprise of this last experiment was that I could not see any obvious change in bulb brightness during each test. Some techs may argue about a perceived change in bulb brightness. But it’s tough to argue with actual voltage drop measurements.

Earlier, I emphasized that this brief article is no substitute for more-extensive electrical training. But I hope it gave you some perspective on the limitations of an ohmmeter as well as the impact of electrical load on voltage drop measurements.

Last but not least, there are various tools and testers with which you can load a suspect wire. I am not knocking these products. But I am suggesting that in a pinch, a proper bulb and bulb socket may suffice as a load tester.