How to test car fuses and relays

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How to test car fuses and relays

A car relay is a particular kind of remote-control switch. It is magnetically operated and is used to control electrical circuits from a distant point. It controls electrical components like fuel pumps, air conditioner motors and radiator fans.

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But relays use flexible mechanical contacts that wear or burn, effectively blocking current to the circuits they service. Fortunately, a car relay is easy to test. Troubleshoot any particular relay in your car and find out whether you actually need to replace it. Find the particular relay you want to test. Depending on the circuit it controls, the relay may be located under the dashboard or inside the engine compartment in a junction block.

Turn the ignition switch to on to energize the particular circuit you are testing, if necessary. Connect the alligator clip from a test light to any good ground on your vehicle. Probe the wire coming out of the relay and going to the component with the tip of the test light. If the bulb in the test light glows, there is voltage, and your relay is working properly. Probe the wire or wires feeding voltage to the relay with the test light following the same procedure used on the previous step.

If the light glows, there is incoming voltage. Otherwise, the relay is not receiving voltage. Check the voltage source. Switch the ignition key off. Unplug the relay from its electrical connector, making sure not to break the locking tabs on the relay.

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Identify the relay's power and control terminals. Some relays show a circuit diagram on top of the case to identify these terminals. Check for continuity between the two power terminals using an ohmmeter.

There should be no continuity. If there is continuity, replace the relay. Connect a jumper wire between the battery positive terminal and one of the control circuit terminals on the relay. Connect the other control terminal to ground with another jumper wire. If you do not hear a click as you make the second connection, reverse the connections. If you still do not hear a click, replace the relay. Connect the jumper wires as you did on the previous step.Home Articles Relay Test How to Test a Relay Easy step by step procedure to test an automotive relay, this information pertains to all relay controlled circuits.

Step 1 - A relay is used to control switch a high amperage electrical circuit with a low amperage one, for example a radiator fan can pull up to 25 amps when in use, which would burn the computer circuit that controls it, a relay is used to bridge this circuit to prevent electrical damage.

If this information is not available, check the owners manual or Google Images. Relay Identification Step 4 - Once the relay has been identified, gently grasp or touch the relay in question, have a helper turn the ignition key to the on position, then crank the engine over, the relay should click in one of the ignition switch positions.

If so the trigger circuit of the relay electrical system is working, if not continue to next step. Note: If the relay clicks and the circuit is still not working there is a good chance the contacts inside the relay have shorted.

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Feel Relay Action Step 5 - Next, remove the relay for inspection, grasp the relay and pull outward while slightly wiggling the relay housing, note the orientation of the relay, it must be installed the correct direction. Removing Electrical Relay Step 6 - Once the relay has been removed, inspect the relay terminals for signs of extreme heat or corrosion.

how to test car fuses and relays

Inspecting Relay Terminals Step 7 - The relay is mounted in four electrical terminals housed in plastic and when overheated due to overload or resistance caused by a poor connection can distort and melt. Inspect Electrical Terminals Step 8 - Use as small metal scribe or tool to scrape clean any corrosion to ensure a good connection once the new relay is installed. Clean Electrical Terminals Step 9 - Most relay's describe the internal circuit by an illustration on the side of the relay.

Relay Terminal Identification Step 10 - Each terminal is identified at the relay base.

how to test car fuses and relays

Relay Terminal Identification Base Step 11 - To test the trigger or primary side of the relay set up a test light by connecting a scribe to a test light clamp, being illustrated in the picture below while testing the tail light socket. Scribe in Test Light Clamp Step 12 - Once the test light is set up connect each end to terminals 86 and Now start the vehicle and operate the accessory switch, the test light should illuminate, if not the switch or circuit ground has shorted.

Note: if the circuit is computer controlled a delay could be programmed into the operation of the accessory, additionally if a cooling fan is being tested the engine must reach operating temperature before the computer will trigger the circuit.

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Use the test light grounded to check for power, and then switch the test light lead to the power side of the battery to check for circuit ground. Testing Circuit 86 and 85 Step 13 - Next, use a piece of wire automotive wire 20 to 16 gauge and strip both ends exposing the copper wire. Turn the ignition to the "ON" position and jump terminals 87 and 30 the relay is now jumped and the accessory should activate, if so the relay has failed. If no power is observed at either 87 or 30, the fusible link or maxi fuse has shorted.

Example: Testing the radiator cooling fan relay, the cooling fan should be operating. Testing Circuit 87 and 30 Step 14 - When replacing a relay be sure to match up the terminal location from the old relay to to the new unit. New Relay Step 15 - When installing the new relay be sure the orientation is correct or the relay will not work. Reinstall Relay Cover Additional Testing. When a particular two wire accessory is not operating, use a grounded test light to check for power at the wiring harness either wire.

If there is no power, the electrical system must be tested starting with the fuse then the relay. If power is present use the test light connected to battery power to check the ground circuit. Some relay's can differ from this configuration but follow the same principle. To confirm the wiring configuration consult a wiring diagram from Google Images or a service manual. Step 1 - To test for intermitted relay failures which are common, remove the relay in question, take a small wire strand, about two inches long and insert it into the relay connector terminal 87 or Step 2 - Next, reinstall the relay while keeping the wire strand inserted and clear of any other terminals or metal ground.

Step 3 - With the wire strand secured in the relay terminal, attach the wire to a small automotive bulb and socket and ground.

How to Test a Car Relay

Note: A side marker bulb and socket work great because of its small size, make the wires long enough so the bulb can be seen while driving. Step 4 - Temporarily mount the small bulb in a visible area to be seen while driving, masking taped to the hood or dash works well. Step 5 - This test can be used to test all aspects of the wiring circuitry by moving the bulb ground to power.

The bulb will now illuminate when the relay is use, and will go out to signal a failure. A relay is switch that utilizes an electrical trigger signal to activate.

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Once activated the relay connects an electrical supply to a particular accessory. These accessories can range from the main computer PCM powertrian control moduleradiator fan, fuel pump, door locks etc.During these challenging times, we guarantee we will work tirelessly to support you.

how to test car fuses and relays

We will continue to give you accurate and timely information throughout the crisis, and we will deliver on our mission — to help everyone in the world learn how to do anything — no matter what. Thank you to our community and to all of our readers who are working to aid others in this time of crisis, and to all of those who are making personal sacrifices for the good of their communities. We will get through this together. Updated: April 9, Reader-Approved References.

Relays are discrete devices as opposed to integrated circuits that are used to allow a low power logic signal to control a much higher power circuit. The relay isolates the high power circuit, helping to protect the lower power circuit by providing a small electromagnetic coil for the logic circuit to control.

You can learn how to test both coil and solid-state relays. All the NC contacts should read 0 ohms, or 0 resistance, while the NO contacts should read infinite resistance. Then, connect the relay to a power source, listen for a click, and check all of the poles again.

This time, the NC contacts should have infinite resistance, and the NO contacts should have none. Keep reading to learn how to test a solid state relay! Did this summary help you? Yes No. Log in Facebook Loading Google Loading Civic Loading No account yet?

how to test car fuses and relays

Create an account. We use cookies to make wikiHow great. By using our site, you agree to our cookie policy. As the COVID situation develops, our hearts ache as we think about all the people around the world that are affected by the pandemic Read morebut we are also encouraged by the stories of our readers finding help through our site.A fuse is a low-resistance resistor device that protects a circuit from becoming overloaded.

It is a short piece of wire that is designed to melt and break apart when exposed to an excess of electrical current. A fuse is connected in series to the circuit it protects. A blown fuse usually causes an electrical short or an overloaded circuit. The most common fuse to burn out in a car is the 12v power outlet, also known as the cigarette lighter.

This is often caused by leaving a cell phone charger in it for a long time, or because of the occasional stray penny that falls into an exposed power outlet. A fuse box is located in the car and houses the fuses. Some cars have multiple fuse boxes with many different fuses. If something electrical in your vehicle suddenly stops working, start by checking the fuse box and get a certified mechanic to take a look and diagnose any electrical problems.

Most cars have more than one fuse box - some vehicles may even have three or four. Car manufacturers tend to install fuse boxes in different locations depending on the make of the car. It is best to refer to your owner's manual to locate the fuse box you need, and also to determine which fuse controls each circuit. Step 1: Remove the fuse. With the car completely off, locate the fuse in question and remove it by grabbing it firmly with the fuse puller stored in the fuse box, or with a pair of needle nose pliers.

Step 2: Inspect the fuse. Hold the fuse up to the light and check the metal wire for signs of damage or a break. If you see either of these, you will have to replace the fuse. If you do not have a fuse diagram to locate a specific fuse, you can test each fuse individually with a test light.

Step 2: Inspect the fuse with the test light. Attach the clip for the test light to any bare metal, and use the probe of the test light to touch each end of a fuse. If the fuse is good, the test light will light up on both sides of the fuse.

If the fuse is bad, only one side will illuminate the test light. Once the damaged fuse is detected, be sure to replace it with a fuse of the same type and amp rating. Identifying and replacing a damaged fuse on your own can save you time and money. However, if the same fuse is blowing repeatedly or if certain electrical components are not workingit is advisable to enlist a certified mechanic to inspect the electrical system to identify the reason the fuse keeps blowing and replace the fuse box or fuse for you.

The most popular service booked by readers of this article is Electric Problems Inspection. Our certified mobile mechanics make house calls in over 2, U. Fast, free online quotes for your car repair. Schedule Electric Problems Inspection. Service Area. Average rating from customers who received a Electric Problems Inspection.Remove all rings, watches, and other jewelry when working on electrical system.

A digital multimeter DMM is one of the most useful test instruments in any automotive electrician's tool kit. Every DMM is slightly different but there are some general guidelines you can follow for its use.

An old analog voltmeter will work too but be careful of polarity when using one of them or you can peg the needle and do permanent damage to the meter movement. Locate the fuse block on your vehicle with the help of a work light or flashlight The main fuse block is usually located underneath the dash on the drivers' side of a vehicle. There will also be a power fuse block located under the hood near the vehicle's firewall. The blocks contain two types of fuses, the older "glass cartridge style" fuses and the newer "blade style" fuses.

The blocks may also contain re-settable circuit breakers.

How to Use a Voltmeter to Test a Fuse Block

Clip the negative, black test lead to a good ground using the alligator clip attachment. Remember the car's metal frame is one side of its electrical circuit. If you have a meter that must be set manually, set the meter's "Function" switch to "DC Volts" and set the "Range" switch to a scale that will allow you to read volts.

On most manually ranging meters, that will be the volt range. Locate the small holes on each end of the blade-type fuses; those holes give you access to the blades inside the fuse. Insert the pointed tip of the red test lead first in one hole and then in the other. If you read volts to ground on one end but not the other, the fuse is blown and needs to be replaced.

With glass fuses place the red test lead tip on the silver-colored end caps to check them. Double-check your ground connection if all the fuses read "0" volts to ground.

If there is no power to one fuse but all the others are functional, the problem lies with the fuse block itself. Based in Colorado Springs, Colo. Walch spent 40 years working in the electrical trades and holds an Associate of Applied Science in applied electrical engineering technology from Alvin Junior College. Hunker may earn compensation through affiliate links in this story.

Step 1 Locate the fuse block on your vehicle with the help of a work light or flashlight The main fuse block is usually located underneath the dash on the drivers' side of a vehicle. Step 2 Clip the negative, black test lead to a good ground using the alligator clip attachment. Step 3 Locate the small holes on each end of the blade-type fuses; those holes give you access to the blades inside the fuse.

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How to check a fuse in a car

Jerry Walch. Show Comments.During these challenging times, we guarantee we will work tirelessly to support you. We will continue to give you accurate and timely information throughout the crisis, and we will deliver on our mission — to help everyone in the world learn how to do anything — no matter what.

Thank you to our community and to all of our readers who are working to aid others in this time of crisis, and to all of those who are making personal sacrifices for the good of their communities. We will get through this together. A blown fuse can be annoying, but it actually helps prevent electrical damage and fire. Fortunately, fuses are inexpensive and easy to replace.

Look inside the fuse in question for black charred marks or a broken filament wire. Safety Warning: Never use a fuse with a higher amperage than the one that blew. Installing the wrong fuse can cause electrical damage or a fire. Tip: If your car is disabled due to a blown fuse, you could swap out a matching fuse that controls a non-essential device. For instance, if the fuse that controls your ignition blew, you could temporarily use the one for your radio, as long as the 2 fuses are identical.

Just remember to turn off the car before removing a fuse. To check fuses in your home fuse box, first find the fuse box, which may be located in your garage, basement, utility room, or attic. Once you find the fuse box, shut off the power at the main switch. To avoid overloading the replacement fuse, also unplug any appliances that turned off when the fuse blew.

After the power is off, locate the fuse that corresponds to the room where the power went out and twist it out of its socket.

Check if the fuse is broken by looking inside the glass for black char marks or a broken filament wire. To learn how to check fuses in a car, read on! Did this summary help you? Yes No. Log in Facebook Loading Google Loading Civic Loading No account yet? Create an account. We use cookies to make wikiHow great. By using our site, you agree to our cookie policy.

How To Test a Relay The Easy Way

As the COVID situation develops, our hearts ache as we think about all the people around the world that are affected by the pandemic Read morebut we are also encouraged by the stories of our readers finding help through our site.If a component that is fed with electricity through a relay See How car electrical systems work terminal of the battery to the feed terminal on the component, thereby bypassing the relay and supply wiring.

If the component still does not work, it is faulty; if it works, then the supply is faulty and the fault will be in the relay or the connections to it. Trace the supply wire back to find the relay — this is a small metal or plastic box which usually has four spade terminals and is located near the battery.

Check that a supply wire has not become detached from a terminal. Check each terminal for corrosion, especially the thin wire from one terminal which goes to earth on the car body — probably fastened under a screw or bolt near by.

A second thick cable goes from the relay to the component. A thin wire runs from the control switch on the steering column or dashboardwhile a second thin wire goes to an earthing point. Use a circuit tester to check whether current is reaching the relay. Clip one wire to earth on an unpainted part of the car and probe the feed terminal on the relay.

If the tester lights, there is power arriving at the relay. If it does not light, check the connection at the battery. If the tester lights, turn on the switch inside the car which controls the component and use the tester again to check for power on the thin wire leading from the switch to the relay.

If there is no power, use the lamp to check the input and output terminals on the switch. This will tell you if current is reaching the switch from the battery, and if the switch is passing the current when switched on.

If there is power at the relay, use the tester on the relay earth terminal the second thin wire. No current flowing to earth means that the relay unit is faulty and must be replaced.

If the relay is earthing properly, leave the control switch on and use the tester on the relay terminal which feeds the component. If there is no power, the fault is in the relay again — probably the contacts are burned or stuck in the open position. Burned contacts can also fuse together, so that they stick in the closed position, so the component does not switch off.

In either case, replace the relay. Remove the suspect relay and replace it with another of the same type. If the component works, the original relay is faulty.

If the component still does not work, check the terminals in the base of the relay connector block with the circuit tester probe. For the tester bulb to light there must be a good contact at the test points. That is the reason for the sharp probe, and for the sharp teeth on the clip. The probe is useful for poking under the plastic covers of spade terminals and snap connectors without the need to disconnect them.

Sometimes it is convenient to use the probe to prick through the insulation of a wire if other access is difficult. Apart from the circuit tester, another useful aid is a test lead — a 10 ft 3m length of wire with a crocodile clip at each end. This allows you to make direct connections from the battery to components which are some distance away, for example the rear lights, rear-mounted electric fuel pump and fuel-tank sender units. On most cars the negative battery terminal is earthed.

From the positive terminal another heavy cable goes to the starter solenoid switch, which feeds current to the starter along a third heavy cable. A wire leads from the live side of the solenoid not through the switch itself to the ignition switch. Another wire leads from the live side of the solenoid to the ammeter if fitted on the instrument panel. Thus, the ammeter is always live, and always shows whether any power is being discharged.


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