Every vehicle has many electrical components, which all need to function properly together as a unit in order to function properly.
These electrical components are responsible for every system in the vehicle from turning on the headlights to getting the air conditioning working. If something is malfunctioning within the electrical components, you are likely dealing with a faulty relay and you want to know how to test a relay.
Since there are numerous relays in the vehicle, you want to know these straightforward simple steps.
Relays work like a switch, allowing one circuit to control another one even if the amperage levels differ. Once you follow these straightforward simple steps you will be able to determine the problem and replace it quickly even as an amateur mechanic.
Important Relay Safety
Testing automotive relays is simple, but there are a few precautions to take for safety. If you don’t know what you are doing, it could lead to creating huge problems or injury.
First, relays should never be replaced with parts from other cars or something lying around. Using an incorrect relay will cause it to surge power and short itself, damaging the car’s electronics.
Secondly, you should handle your relay with care. If you drop it, it may ruin the wires and they could melt or burn, or injure the person testing it. This is also the reason you don’t want to modify a relay.
Because the relays deal with electricity, do not use wherever there are flammables gases. Work in a safe location in the garage or workshop when testing automotive relays.
Finally, always go to your service manual before working with any relay. You might feel like you understand the basics, but it is important to have everything replaced to factory specifications.
Materials Needed to Test a Relay
You don’t need many tools to test an automotive relay, and you’re likely to have most of these supplies at home in a garage right now.
- Test light
- Ohmmeter, or digital multimeter
- Vehicle service manual
- Replacement relay
- Jumper wires
You will get the best result if you organize supplies before you start. You will want to have everything at hand’s reach so that you do not have to get away from what you are working on!
How to Test an Automotive Relay
#1. Locate the Relay.
Before testing the relay, you’ll need to locate it. Some relays are below the dash, while others might be in the engine bay.
The location will rely on whatever the relay controls. If you aren’t sure where it is, look in the vehicle service manual wiring diagram.
#2. Swap the Relay with a known working one.
The simplest way to test the relay is to swap it with a working relay. This can, however, ruin the good relay and/or create additional expenses.
If you decide to swap relays, please clean the connectors and inspect the pin while you have it removed.
#3. Use a Multimeter.
With the multimeter, you can test the relay by swapping leads to each input/output pin while the unit is off; Monometer should range 50 to 120 ohms on most relays.
If the meter reads Open or Out of Range, that usually indicates a defective coil winding. If that happens, you need to buy and install a replacement relay.
If the reading is ok connecting leads between the switch pin and input. An open relay will show OL or Open.
#4. Add Power.
After you check the relay wiring, it’s time to power up the relay with a 12-volt battery and you should hear a click in the relay coil closing the connection. In the case of diode in the relays, polarity is important, but not for 4-pin relays.
Connect your test light across the ground and the switch terminal. The test light will read the current and light up properly if all is working right – if not the light will stay off. The test light should go off when you remove the jumper.
#5. Test Switch.
If your tests showed everything is working correctly, you can now move on to the switch. A bad contact point can cause voltage to drop or no voltage at all. Remove the test light and put the multimeter in DC.
Then connect the leads on the switch pins and this reading would be the same voltage as your battery voltage normally 12V.
Next check the resistance on the switch with the jumper disconnected, energize the relay and move the multimeter to ohms so you can read the resistance of the switch. The energized relay that is open should read nearly 0 ohms; the closed relay will read OL or Open.
If after all this you can’t verify the relay is functioning properly, you might have to ask for help from a professional electrician.
What is an Automotive Relay?
Every vehicle today has relays, which are also used in aviation and marine engines.
Essentially, a relay allows a circuit with low amperage to control a circuit with high amperage. A relay works similarly to a switch in that it enables a system to control an external system with higher amperage.
As an example, if you’re trying to run your car headlights based on a switch in your car without a relay, you very likely would go above the amperage capacity; this could cause things to melt, or in the worst case start a fire in your automobile.
Instead though, there is a relay that would control the operating system. A relay can also control feature systems, like the headlight function with the windshield wipers, or when you turn on the stereo and extend your radio antennae.