Accurate electrical diagnosis is one of the most difficult challenges to be found when working on today’s automobiles. Considering the large number of systems that use electricity in order to operate, being able to quickly diagnose electrical problems can be a real asset. Whether you are attempting to diagnose an engine control issue, an automatic ride height system, or a power window issue, the electrical systems all utilize the same principles.

Often, diagnosing electrical problems is more mental work than it is turning a wrench. Ninety percent of solving an electrical problem will be done without ever opening your toolbox. The last ten percent of the solution is involved in testing and repairing the circuit.
When it comes to computer controlled systems there is sometimes even more mystery to the way electricity works. Some technicians have been known to resort to replacing a computer, not because they know it is bad, but because they are not sure how it works and they see nothing else wrong.

When replacing the computer doesn’t work, they then try a different part, only to eventually discover that it was faulty wiring that was causing the computer to provide improper output.

By understanding what a computer needs in order to function properly and then ensuring that it has everything it needs to do its job, a technician can successfully determine whether the computer needs to be replaced.

Following a few simple, and easy to remember steps, will mean a confident diagnosis and a “fixed right, first time” repair for your customer.


Now that we understand what electricity is, why would electrons want to move?
Electrons need some sort of “push” to leave their atom and get moving. This push is called voltage. It is helpful to think of voltage as electrical pressure.

The unit of measurement for voltage is the volt, generally designated as ‘V’. The letter E is used in formulas to denote voltage when discussing the relationship of voltage to current and resistance. This comes from that fact that voltage is considered electro-motive force.
There are two types of voltage: alternating current and direct current.

Direct current voltage (DC voltage) is voltage that is applied in one direction all of the time. Many automotive systems use direct current to get their jobs done. Automotive batteries supply direct current to the electrical circuits.

Alternating current voltage (AC voltage) is voltage that is applied in one direction at one given time, and the opposite direction at another given time. The electricity found in the electrical outlet of your house wiring is an example of alternating current voltage.

There are some automotive systems that will use, or produce AC voltage instead of DC voltage. Examples of these might be various speed and position sensors, electric motors, or alternators.

Measuring Current

Voltage is responsible for putting pressure on electrons and making them leave their atoms. The term for how many electrons are moving is current. You can think of current, or amperage, as the volume of electrons that are moving.

The unit of measurement for current is the ampere, or amps for short. An ampere is a known quantity of electrons passing a single point at a given time. The actual quantity of electrons in an amp is equal to 6.24 x 1018 electrons per second (or one coulomb of electrons per second). The letter I denotes current in formulas.

Like voltage, current comes in two forms: DC current and AC current. DC current is electrons moving in one direction only, while AC current is electrons moving in one direction at a given time, and then the opposite direction at another given time.
Most electrical systems on automobiles utilize direct current.


The third term that is associated with the movement of electrons is resistance. Resistance is opposition to the flow of electrons. Resistance gets in the way of electrons moving and can build up electrical “pressure” at the point of resistance so that less voltage is available to move other electrons.

The unit of measurement for resistance is the ohm. The ohm is show by the Greek letter Ω. In formulas, the letter R is used to denote resistance.

To measure voltage with your meter, first place the black meter lead in the “com” jack. Secondly, place the red meter lead in the “V” jack.

Now you must set the meter to read the voltage that you intend on measuring. To do this you must first decide whether you are measuring AC or DC voltage. Most automotive systems are operating on DC voltage. However, if you are measuring sensor output voltage at the sensor itself, or at an ECU input, then you may need to set the meter to read AC voltage. Knowing how the sensor works will be critical to choosing which voltage setting to use.

Next, set the meter to “Auto Range”. If your meter does not have an “Auto Range” function, then you will have to decide approximately how much voltage you expect to encounter. Set the meter to read the highest voltage you think that you are likely to encounter. You can always change the meter range later if your meter reading seems to not be accurate enough.

With the meter now properly set up and the meter leads plugged in, it’s time to measure some voltage. There are two ways of measuring voltage: voltage available and voltage drop.

Voltage Available

Voltage available is voltage that is present at any given point in the circuit. You can measure voltage available by placing the red meter lead at some point in the circuit (switch contact, fuse, connector, etc.) and the black meter lead on a known good ground. This test will tell you how much voltage, or electrical pressure, is present to do some work at a given point in the circuit.

The voltage available test is good for determining if you have voltage present, but it does not do much in the way of diagnosing what is wrong with the circuit.

For example: say you’ve placed the red lead on the B+ wire leading into a power window switch and the black lead on a known good ground. You observe a meter reading of 9.4 volts, but the window does not go down when the switch is operated.

All that you’ve really determined is that some voltage is getting to the switch, but you haven’t measured anything that can lead you to what is wrong with the circuit.

Voltage available is a good test to use when you want to see whether voltage is present at a point in the circuit or not. Think of the voltage available test as a “go, no go” type of test.
Do not use the voltage available test to determine whether a circuit is functioning properly or not. There may be voltage present at a point in the circuit, but that does not mean that the circuit has what it needs to work.

The second voltage test, and by far the most useful, is called a voltage drop test.
We say that voltage is “dropped” when some form of load uses all, or a portion of, the voltage available.

Loads come in the form of light bulbs, motors, solenoids, heating elements, etc. Other loads may be found in the form of corrosion, poor switch contacts, or even an open circuit. When a load of this type “uses up” or drops voltage, then it may not leave enough voltage available to the rest of the circuit for the circuit to work properly. A voltage drop test will help you to isolate where voltage is being dropped, or used up.

Unlike resistance measurements where the circuit is not turned on, a voltage drop test is performed with the circuit working (i.e. the headlights are on, the window is being moved up and down, etc.).

To perform a voltage drop test you must place the meter leads at two points within the circuit. Remember, during the voltage available test we place the leads at one point in the circuit and the other at a known good ground. In the voltage drop test, both meter leads will be placed in the circuit.

With the meter leads placed in the circuit, you are now measuring how much electrical pressure, or voltage, is present between the meter leads. If you measure any voltage at all on the meter display, then there must be some sort of load between the meter leads.
Your next job is to determine whether the load that is represented on the meter display is a load that is supposed to be there, or whether it is a load that may be harmful to the circuit. Remembering a few simple rules will allow you to determine if the voltage drop you are measuring is “normal” or not.

  • A meter reading of less than 0.1 volts represents a wire, or switch that is operating normally.
  • A meter reading of full source voltage (i.e. 12.6 v or more) represents all of the available voltage being dropped. This could be due to an open wire, or it may be a “normal” load that happens to be the only load in the circuit.

Current Measurement

The third measurement that can be useful in electrical diagnosis is the measurement of current, or of how many electrons are flowing. Remember, current is measured in amperes, or amps for short. Like the voltage drop test, an advantage of measuring current is that the circuit will be performing its function, making this a “real world” test.

There are three drawbacks to measuring current. The first drawback is that current specifications for even common circuits can be hard to come by. This leaves the technician to find a specification on their own, usually by comparing the problem vehicle and problem circuit to a normally operating circuit on a comparable vehicle. This takes time and can lead to inadvertent problems on the known good vehicle.

The second drawback to testing current during diagnosis is in the way the meter must be installed in the circuit. Since current measurements are actually measuring how many electrons are flowing in the circuit, all of the electrons in the circuit must pass through the meter. This means that the meter must become part of the circuit.

An ammeter must be installed in series with the circuit, at a location where the maximum current within the circuit will flow through the meter. This involves opening the circuit up and connecting the meter leads in series with the circuit.

There are a number of good locations for opening up a circuit up to install the meter, with the most popular, and often the easiest, being at a circuit fuse. This is done by removing the fuse and installing the meter leads on either side of the fuse connectors.

The third drawback to using current as a diagnostic tool is that you may not know how much current is flowing before you install your meter. The current flowing is often more than what your meter can handle. Although a quality meter is fused, often these fuses will blow when trying to measure current because the circuit being tested uses higher current than what your meter is designed for.

To get around these last two drawbacks, technicians will use an “amp clamp” or an inductive ammeter. This device uses the electromagnetic field that forms when electricity flows through a conductor in order to determine how much current is flowing in a circuit.
Amp clamps have two significant advantages: firstly the circuit does not have to be opened up to install the ammeter, and secondly the amount of current flowing in the circuit cannot damage the meter. The disadvantage is that you must purchase an amp clamp that measures the range of current you expect to find. It is not possible to purchase a single clamp that will measure all current ranges.