With a good wiring diagram and a good multimeter, a trained electrical professional can find the cause of almost any problem.
There are two basic types of multimeters, digital and analog. Analog multimeters have a needle and digital multimeters have an LCD or a LED display. WIth today's demand for accuracy in testing electrical systems, it makes more sense to have a digital multimeter but an analog multimeter still has its uses.
This article focuses on digital multimeters. A digital multimeter will have many functions built into it. As with any tool or piece of equipment, it is necessary to make certain you read and follow digital multimeter instructions and cautions. This will protect you and your electrical equipment.
All digital multimeters will test for voltage, current and resistance. These are the three functions needed when trying to diagnose a problem. When you purchase a digital multimeter, one of the most important things to look at is the meter's impedance, which is the meter's operating resistance. Most digital multimeters have very high impedance. Since the meter is part of the circuit being tested, its resistance will affect the current flow through that circuit.
Typical Amperage Test
If a digital multimeter has a very high impedance or resistance it will cause a slight increase in the circuit's current. This becomes a concern when you test electronic systems because the increased current draw can damage the components being tested or, at the very least, alter the readings or change a sensor signal. It's best to get a meter that has an impedance of at least 10 megaohms. That way the current draw is so low it becomes invisible.
Almost all digital multimeters have an "auto-range" features that will automatically select the proper range. Some digital multimeters will let you override this feature and let you manually select the range you want. Some DMMs do not have this option and must be set manually. Check the documentation that came with your digital multimeter and make sure you know and understand its different ranges.
Most digital multimeters that have an auto-range will have the setting either before or after the reading. Ohms are measured in multiples of ten and given the designation 'K' or 'M' with 'K' standing for 1,000 ohms and 'M' standing for 100,000,000 ohms. Amps would be displayed as mA, milliamps or 1/1000 of an amp or A for full amps. Volts will also be displayed as mV or volts. When you take a reading with a DMM that has auto-range, be sure you note at what range the meter is on. You could mistake 10 mA as 10 amps.
Typical Voltage Test
Most digital multimeters that have auto-range will show the reading with a decimal point. A reading of 1.2 amps will be 12 amps if you ignore the decimal point.
Digital multimeters do have a limit on how much current they can test. Usually this limit is printed at the point where the red lead plugs into the meter. If it says, "10 Amps Max" then there is a 10-amp fuse inside the meter that will blow if the current is above 10 amps. If you take out the 10-amp fuse and put in a 20-amp fuse, you will burn out the meter beyond repair. I would suggest buying a DMM that will handle at least 20 amps for automotive testing.
Many digital multimeters have an inductive pickup that clamps around the wire being tested. These ammeters measure amperage based on the magnetic field created by the current flowing through the wire. DMMs that have an inductive pickup usually will read higher current and have a higher limit. Since this type of meter doesn't become part of the circuit you do not need to disconnect any wires to get a reading.
Voltmeters are usually connected across a circuit. You can perform two types of tests with a voltmeter. If you connect it from the positive terminal of a component to ground, you will read the amount of voltage there is to operate the component. It will usually read 0 volts or full voltage. If you test a component that is supposed to have 12 volts, but there is 0 volts, there is an open in the circuit. This is where you will have to trace back until you locate the open.
Typical Resistance Test
Another useful function of the digital multimeter is the ohmmeter. An ohmmeter measures the electrical resistance of a circuit. If you have no resistance in a circuit, the ohmmeter will read 0. If you have an open in a circuit, it will read infinite.
An ohmmeter uses its own battery to conduct a resistance test. Therefore there must be no power in the circuit being tested or the ohmmeter will become damaged.
When a component is tested, the red lead is placed on the positive side and the black lead on the negative side. Current from the battery will flow through the component and the meter will determine the resistance by how much the voltage drops. If the component has an open the meter will flash "1.000" or "OL" to show an open or infinite resistance. A reading of 0 ohms indicates that there is no resistance in the component and it is shorted. If a component is supposed to have 1,000 ohms of resistance and a test shows it has 100 ohms of resistance, which indicates a short. If it reads infinite, then it is open.
Analog ohmmeters will need to be calibrated before they are used. There is an "ohms adjust" screw on the meter used to do the calibration. To calibrate the ohmmeter, you touch the red and black leads together and turn the adjusting screw until the needle is at 0. You should do this each time you use the ohmmeter and each time you change scales. DMMs do not need to be calibrated since they will self calibrate themselves. Holding the two leads together will confirm that they are, indeed, calibrated.
To check a wire in a harness you connect one lead at one end of the wire and the other lead to the other end of the wire. If the wire is good you will get a reading. If it is broken, you will get an infinite reading. This is useful in determining why a particular component is not getting power. Just be sure you isolate the wire from the circuit so your ohmmeter does not get damaged.
These are the three basic functions of all digital multimeters. Some digital multimeters will have many other features such as averaging where it will take a reading over a period of time and average it out. Some have a MIN/MAX feature that will hold the highest/lowest reading. Some will do specific diode tests, measure injector pulse times and even have thermometers.