Monday, July 22, 2024 Detailed Auto Topics
You May Also Like
AGCO Auto Quick Tip:

Try our new Category View for Detailed topics segregated by their topic.

Now it's even easier to find the information you need.

AGCO Auto Quick Tip:

You can also receive our Detailed Auto Topics, delivered to your computer, as soon as they are written, please subscribe to our RSS feed.

AGCO Automotive Detailed Topic Blog


Detailed Topics

Solving automotive problems often means diagnosing the electrical system. Electrical circuits are a much larger part of a modern vehicle. Knowing how to check the basic electrical components can make auto repair easier.

How to check fuses

Fuses protect the electrical circuits on vehicles. They design a fuse to burn or blow out, at an amperage below the point where damage occurs in the circuit. Engineers size fuses for about twice the normal current draw. For instance, if a device normally draws 10 amps, a fuse roughly 20 amps in size might protect it. The fuse burns out at 20 amps, which is below the current that might damage the circuit. They show fuse amperage by the color of the fuse, and a number marked on the exposed face.

the small metal contact on the fuse can be used for testing

Finding a blown fuse is not always easy. There may be several fuse boxes or blocks in a vehicle. Often, there is one or more under the dash and another under the hood or in the trunk. An owner’s manual is handy in locating the box in which the suspected fuse may exist.

small metal contacts on a fuse are for testing

An easy way to test fuses is with a volt-meter.  This check is done with the ignition on. With the black volt-meter lead grounded and a pointed probe on the red lead, the tiny piece of exposed metal is touched. Touch one side of the fuses and then the other. If the meter reads voltage on one side, but not the other, the fuse is bad. If neither side reads voltage, the circuit is not active. This means the key may need to be on, or another problem exists upstream of the fuse.  If both sides of the fuse read voltage, the fuse is good.


Pulling a fuse out of the box to check it has many disadvantages. Seeing some blown fuses is difficult and a bad fuse may seem good. Electrically testing the fuse will show if it is bad or good. Another problem with pulling fuses is, keep-alive power for the various modules on the vehicle may be lost. This results in poor idling, a loss of diagnostic information and sometimes accessories that do not work.

The terminals in a fuse box are fragile, and we may damage them by twisting and pulling on hard to remove fuses. This may cause a loose connection and a future problem. Using a voltmeter to check fuses will be far more certain, quicker and will avoid these problems.

How to check a standard relay

A relay is a magnetically operated switch. The purpose is to allow a low-amperage circuit to control a heavier device. A relay is often used with computers to control analog devices. The computer may supply power or a ground to a low-amperage relay that turns a high-amperage device on or off.

typical GM fuel pump wiring diagram for trucks

In the diagram above, the computer supplies current to the relay, which is grounded. Once energized, the relay conducts power to the fuel pump. This arrangement allows the computer to control the pump which is too heavy to control directly.

Testing a five terminal automotive relay

A typical automotive relay has four or five terminals. These terminals often have numbers, as shown above.  This circuit can be tested with an ohmmeter.  Conductivity is indicated by a reading of near zero resistance.  An open circuit would show OL or open-lead, which means infinite resistance.

The terminals numbered 85 and 86 power a magnet that closes the relay. Terminal 30 is the controlled-circuit input. Number 87 and 87a are the switched connections. On the five-terminal relay, 30 and 87a are connected, when no current is applied. This provides a closed switch until power is applied.

Testing a five terminal automotive relay for switching

When terminals 85 and 86 are energized, 30 and 87a are disconnected, and 87 is turned on. On a four-terminal relay, 87a is omitted, and 30 and 87 act as a single switch. These relays can be tested by checking continuity between 30 and 87a. Next power and ground are applied to 85 and 86 while checking number 30 for continuity to 87.

With fuel pumps, testing can be a bit tricky. The computer will at most power the relay for a few seconds when the key is switched on. Continuous current is applied after the engine starts.

In the fuel pump diagram, current at terminal 87, when the switch is turned on, indicates a good relay and fuses. No power at 87 would require checking for power at 30. If there is no current flow at 30, the fuse would be checked. If there is power at connection 30, power and ground would be checked at terminals 85 and 86.

The relay is used to control a great many things on automobiles. Power windows, headlight circuits and windshield wipers are a few. All relays will have a controlled circuit and a low-amperage on-off circuit that energizes the magnetic coil.  On vehicles that use several of the same relay, on different circuits, switching one for another is a quick way to verify a problem.

The fuse and the relay account for a large percentage of the problems the average enthusiast will encounter. Learning how to check these devices makes diagnosis far easier.

Post or Read Comments (0)

Please click the link above to leave your comments


Registered visitors are always invited to leave their comments and thoughts by using the form above. If you need to you can login here or register here.

You can also win a free AGCO coffee cup, by reporting any errors you find, with this form.