Measuring voltage:
With a multimeter, we can measure the voltage (volt) across electrical components such as the battery, wiring, switch, and lamp. In this context, it is also called a “voltmeter.” The multimeter is connected in parallel across the circuit and set as follows:
- Turn the dial to the V for volts (voltage);
- In this case, select DC voltage;
- Red lead in the V input;
- Black lead in the COM input.
The red lead is the positive lead and the black is the negative. At the ends of the leads are probes. Hold the red probe against the positive terminal of the battery and the black against the negative. This way we measure the voltage difference in the battery, which reads 1.5 volts on the display.
The battery voltage of 1.5 volts is directed to the positive terminal of the lamp through the positive lead when the switch is closed. Using the multimeter, we measure the voltage difference across the lamp: the lower point is positive and the casing is the ground. The probes should be held against the positive and ground to measure the difference across the lamp.
When the switch is opened, the circuit is interrupted, and no current flows through it, causing the lamp to turn off. The multimeter then reads 0 volts with this difference measurement. Since the switch is on the positive side of the lamp, the lamp is left without voltage. Later sections will delve into positive and ground-switched lamps and their associated measurements.
Measuring current:
With the multimeter, we can ascertain the amount of current flowing through a circuit. It’s critical to connect the multimeter in series to allow the current to pass through it. In this setup, it’s referred to as an “ammeter.” Set it as follows:
- Turn the dial to A for Amps;
- With this type of multimeter, each time the A setting is chosen, press the yellow button to switch from AC to DC;
- Insert the red lead into the 10A input;
- Insert the black lead into the COM input.
To place the multimeter in series, interrupt the circuit at some point. This can be done by removing the fuse or opening the switch. Connect the probes where the circuit has been interrupted. The following two images depict current measurement with an open switch. Measurements are taken in Amperes and milliamperes. Further explanation follows the images.
As we can see from the images, current can be measured in two settings.
- The first measurement is in the Ampere setting. In this setting, current up to a maximum of 10 Amperes can be measured;
- The second measurement is in the milliAmpere setting. In this setting, currents up to a maximum of 400 milliamperes can be measured, equivalent to 0.4 A.
If you are uncertain about the current flowing through a circuit, it’s wise to measure in the 10A setting first. If the current is less than 0.4 A, you can switch the probe to the mA input and set the dial to mA. Don’t forget to press the yellow button to switch from AC to DC. The measured value correlates, but is more precise in the mA setting.
- 0.15 A equals 150 mA;
- 147 mA equals 0.147 A (this setting is more precise).
Errors sometimes occur when measuring current. The most common mistakes are shown in the following two images.
If a measurement is carried out and the user (in this case, the illuminated lamp) is working correctly, but the multimeter reads 0 A, then the meter might still be set to AC, or the circuit isn’t interrupted. The current follows the path of least resistance, which is through the closed switch. Essentially, the multimeter is now connected parallel to the circuit. Nothing will be damaged. Once the switch is opened, the correct value appears on the display.
If the current exceeds the fuse’s capacity, the fuse will break to protect the multimeter’s electronics. In the mA setting, that’s 400 mA. You’ll notice this if the meter is connected correctly, yet the user isn’t functioning and the meter reads 0 mA or 0 A. In this case, it might be advisable to carry out the measurement in A, as this setting is protected up to 10 A and is less likely that the fuse is faulty or will fail.
Measuring resistance:
The third measurement we conduct with the multimeter is resistance measurement. This allows us to check electrical components for internal shorts or interruptions. In the images below, two measurements are shown determining the resistance of the lamp. The multimeter now functions as an “ohmmeter” and is set as follows:
- Turn the dial to the a9 (ohm) position for resistance measurement;
- The red lead is inserted into the a9 input, which is the same input used for voltage measurement;
- The black lead is again inserted into the COM input.
The resistance of the lamp is 1.85 ohms, indicating it is in good condition. Note: when the lamp is on, resistance changes due to temperature. We can’t measure resistance while it’s on, but right after turning off, the measured value will drop significantly.
A lamp ages as it burns for more hours. The tungsten filament becomes thinner and evaporates against the inside of the glass. This can be seen as the lamp darkens. A darkened lamp will fail soon. In the second measurement, this occurs: the tungsten filament is broken and the lamp is no longer working. The circuit is thus interrupted. Because the connection is broken, the resistance has become “infinite.” In this case, the multimeter displays OL. Some multimeters may then show “1.”
With the ohmmeter, we can perform the following measurements:
- The internal resistance of electrical and non-electrical components;
- Diagnosing interruptions in an electrical circuit, such as in circuit boards or wiring;
- Finding electrical connections using the continuity mode;
- Locating a ground connection;
- Check whether the test leads are in good condition.
The last measurement is crucial in diagnostic work. If a test lead is in poor condition, it affects every voltage or current measurement with the multimeter or oscilloscope (the latter can only measure voltage).
If a test cable has been pinched or has been frequently twisted and pulled through intensive use, the connection may be lost when held at a certain angle. This can be checked easily by holding the ends of the test leads together: the resistance should be about 0.1 ohm. Is the resistance significantly higher, or OL? Then the test leads are no longer usable.
Another example of resistance measurement is measuring the glow plug found in a diesel engine.
- A good glow plug has a resistance of about 6 ohms.
- If the glow plug is interrupted, the resistance is infinitely high.
- In case of an internal short (where the coil and casing make internal contact), we theoretically measure a resistance of 0 Ω and actually measure a resistance of 0.1 Ω due to the “always present” resistance in the test cables, as described in the previous section on checking the test cables.
See the page on glow plugs for more information about their operation and measurement techniques.
V4 measurement:
This website describes the voltage levels, signal transmission, and measurement techniques of various types of sensors, actuators, ECUs, and networks. You can find these on the respective pages of the temperature sensor, passive, active, and intelligent sensors, relays, and CAN-bus. On these pages, measurements specifically focus on the topic.
For troubleshooting, we often use the voltmeter and sometimes the current clamp. Rarely, if ever, are current and resistance measurements conducted during diagnostics:
- Since current measurement requires interrupting the circuit (undesirable), and current levels don’t provide sufficient information about potential losses as the current strength is consistent throughout the circuit. Also, the ammeter is limited to 10A, but using a current clamp that isn’t restricted to a set current strength may sometimes be desirable.
- Resistance measurement is only recommended for determining a connection or interruption. In all other cases, we’re measuring an “unloaded” resistance, making the resistance value unreliable.
The above implies we almost always use the voltmeter in our diagnostic work. For complex diagnostics, we use an oscilloscope, which is also a (graphical) voltmeter. With the voltmeter, we measure voltage differences and losses in a loaded situation, so when the user is active. This makes the measurement result the most practical.
To aid in voltage measurements with the voltmeter, mastering the V4 measurement is helpful. Through four voltage difference measurements, the approximate cause of a poorly or non-functioning user can be identified. This section explains how to perform the V4 measurement, expected measurement values, and identifying a fault.
The V4 measurement involves using a single voltmeter for a difference measurement at four specific points. These four measurements are known as V1, V2, V3, and V4.
Note: for a PWM / duty-cycle controlled user, the V4 measurement isn’t feasible; an oscilloscope must be used in such cases!
V1:
The V1 measurement is the initial test performed. Here, we measure the battery voltage. All subsequent voltage measurements are compared to this value. Before measuring, the user must be turned on, as heavy users could cause the battery voltage to drop by a few tenths of a volt without indicating a fault. Set the multimeter correctly (see the section on measuring voltage) and place the probes on the battery’s positive and ground terminals.
Should the engine need to be started during the V4 measurement, the battery voltage due to the alternator’s charging voltage will be higher. Perform the measurement again in this case.
V2:
Next, measure the voltage difference across the user. Naturally, the user must be turned on. With a lamp, this is straightforward: use a switch to activate the lamp. Sometimes activating the user is slightly more complex, such as with the electric fuel pump in the tank. In that case, start an actuator test through a diagnostic device or let the engine idle.
- The voltage across the user should roughly equal the battery voltage, with a maximum difference of half a volt. If this is the case, there is no voltage loss in the positive or ground, and the V4 measurement is complete;
- If the V2 measurement voltage is more than half a volt lower than V1, there is voltage loss. In such cases, measure voltages at V3 and V4.
V3:
This measurement determines the voltage loss on the positive side, between the battery positive and the lamp’s positive terminal.
- The loss should not exceed 0.4 volts;
- Below 0.4 volts is acceptable;
- More than 0.4 volts indicates a point of resistance on the positive side.
V4:
Lastly, perform the loss measurement between the lamp’s ground and the battery’s ground. The criteria are the same as for the V3 measurement: a maximum loss of 0.4 volts indicates otherwise a point of resistance.
Check:
The battery voltage distributes across the circuit. All partial voltages (V2, V3, and V4) equal the battery voltage (V1). This is evident in the example measurements:
- V1 = 12.0 v
- V2 = 11.7 v
- V3 = 0.2 v
- V4 = 0.1 V
This allows us to use the following formula:
If the calculation deviates significantly, a measurement error was made. One must verify which value is illogical. For instance, it’s impossible for a lamp to function at 12 volts when the battery voltage is 13 volts with a 12-volt voltage loss.
Below are five potential faults identifiable with a V4 measurement. To conserve space and maintain clarity, images of the “actual” voltmeters have been replaced with circles containing numbers.
Fault 1 – lamp lights dimly:
The lamp illuminates less brightly than others in the vehicle. Understandably, as it only operates at 7 volts instead of 13 volts. The V3 measurement shows a 6-volt loss on the positive side. There’s a point of resistance between the battery’s positive and the lamp’s positive where 6 volts is consumed. This voltage loss detracts from the user’s operating voltage.
Possible causes:
- A damaged wire before the fuse, between the fuse and the ECU, or between the ECU and the lamp;
- A poor connection for the fuse in the fuse holder;
- A poor wire connection or connectors at one of the black dots in the diagram;
- A failure in the ECU.
To pinpoint the point of resistance, move the negative lead from the V3 measurement to the bottom of the ECU. If 6 volts still measure here, the voltage was not lost in this wire, and the issue is higher up. However, if 0 volts measure above the wire, this wire is damaged and needs replacement.
Fault 2 – lamp lights dimly:
Again, we face a lamp that lights dimmer than the others. The measurements show a 6-volt voltage loss at the V4 measurement. Here too, 6 volts are necessary to overcome a point of resistance on the ground.
Possible causes:
- A damaged wire between the lamp and a ground point;
- Corrosion at the contact points of the cable eyelet and ground.
If the point of resistance is in the wire, mounting a new wire between the lamp and a ground point suffices. If the wire is sound, unscrewing, sanding, and cleaning the ground connection before remounting the wire and retesting may help.
Fault 3 – lamp lights dimly:
All lamps light dimly. When measured, the battery voltage (V1) is too low. Voltage loss measurements (V3 and V4) are fine. Charging (and perhaps testing) the battery suffices to resolve the issue.
Fault 4 – lamp does not light:
The lamp doesn’t light. However, the voltage across the lamp is 13 volts, and no loss is detected.
Possible causes:
- The lamp is faulty: a broken filament interrupts the circuit. The 13-volt voltage and ground still reach the lamp, thus a “good” voltage difference is read at V2;
- Poor connector connection as the metal connectors have lost their clamping force. Frequently disconnecting or connecting the lamp plug may create space between the metal pin and lamp connection.
A defective lamp can usually be visually assessed: the filament is visibly broken. If desired, measure the lamp’s resistance with an ohmmeter. An infinitely high resistance indicates an interruption.
Fault 5 – lamp does not light:
Once again, we encounter a lamp that does not light. The expected voltage difference at V2 is now observed at V3, indicating that the top of the fuse has a good positive and the bottom has a good ground. Given the measurement, the fuse appears to act as a user consuming the 13 volts, but this is incorrect.
The cause of this fault is a defective fuse. Just like the previous fault with the broken filament causing a broken circuit, the fuse interrupts the circuit here.
