Understanding Your Car's Relays
Many car owners are baffled by intermittent electrical gremlins, from a car alarm that keeps going off without reason to central locking not working correctly. Often, the culprit isn't a complex wiring loom issue but a simple, inexpensive component: the relay. A relay acts like an electrically operated switch. It uses a low-current circuit to control a high-current circuit, allowing a small signal to switch a much larger load, such as your starter motor, fuel pump, or headlights. When a relay fails, the circuit it controls simply stops working. Knowing how to test a car relay is an invaluable skill for any DIY mechanic.
Relays are typically found in your car's fuse box, often integrated with fuses, or in separate relay boxes under the bonnet or dashboard. They are usually square or rectangular and made of plastic, with several metal pins protruding from the base. These pins have specific functions, corresponding to the coil (energising the electromagnet) and the switch contacts (making or breaking the circuit). Identifying the correct relay for the circuit you suspect is faulty is the first step in the testing process.
The primary reason a relay might fail is due to wear and tear on its internal components, particularly the contacts which can become pitted or corroded over time. Overloading the circuit the relay controls or voltage spikes can also damage it. Understanding these common failure points helps demystify why a specific relay might be causing your car alarm going off constantly or other electrical anomalies.
Identifying the suspected relay is the first hurdle.
Prerequisites and Tools to Test a Car Relay
Before you can effectively test a car relay, you need to gather a few essential items. The most crucial piece of equipment is a multimeter. This versatile tool allows you to measure voltage, resistance, and continuity – all vital for diagnosing a relay's health. Ensure your multimeter is set to the correct setting for the test you're performing, typically resistance (ohms, Ω) or continuity (often indicated by a buzzer symbol).
You'll also need a way to identify the specific relay you intend to test. Your vehicle's owner's manual is the primary resource here, providing a fuse box diagram and often detailing the function of each relay. If you don't have the manual, you can often find diagrams online for your specific make and model, such as a Corsa D fuse box diagram or Ford Fiesta fuse box diagram. Note that different car manufacturers use different relay types and pin configurations, so a generic diagram might not suffice for precise testing.
A basic set of tools, including screwdrivers or trim removal tools, may be necessary to access the relay box safely. It's also good practice to have a small notebook and pen handy to record your readings. Finally, understanding the different car fuse types (blade, mini, ATO, etc.) can be helpful, although for relay testing, it's the relay itself that's the focus. Remember to always work on your vehicle in a well-lit, safe area, and disconnect the battery's negative terminal before starting any electrical work to prevent short circuits or injury.
- A multimeter is essential for measuring electrical properties.
- Your vehicle's manual or an online diagram identifies relays.
- Basic tools and a safe workspace are required.
- Disconnecting the battery prevents electrical hazards.
Step-by-Step: How to Test a Car Relay
Testing a car relay typically involves checking its electrical continuity and operation when energised. There are two main methods: testing the relay in the vehicle or testing it out of the vehicle. Testing it out of the vehicle is generally more straightforward and definitive.
Method 1: Testing the Relay Out of the Vehicle
This method allows you to isolate the relay and test its internal switch and coil independently.
- Identify the Relay's Pins: Relays have a standard pin layout. Usually, there are four pins: two for the coil (often numbered 85 and 86) and two for the switch contacts (often numbered 30 and 87). Some relays have a fifth pin (87a), which is part of a changeover switch. Consult the diagram on the relay itself or in your manual to confirm which pin is which.
- Check Coil Continuity: Set your multimeter to the resistance (Ω) setting. Place one probe on pin 85 and the other on pin 86. You should get a resistance reading, typically between 50 and 200 ohms, indicating the coil is intact. If you get an open circuit (infinite resistance) or a very low reading, the coil is likely faulty.
- Test the Switch Contacts: With the multimeter still on resistance, place one probe on pin 30 and the other on pin 87. You should get a reading of zero ohms (or a very low resistance, indicating continuity) if the relay is un-energised. This is the 'normally closed' state for many standard automotive relays. If you get a high resistance or open circuit, the contacts might be dirty or burnt.
- Energise the Relay: To test the switch when energised, you need to provide a small voltage (usually 12V) to the coil pins (85 and 86). You can do this using a separate 12V power source (like a small battery pack or a jump lead from your car battery, being careful with connections). When you apply power to the coil, you should hear a faint 'click' as the relay activates.
- Re-check Switch Continuity: While the coil is energised, re-test the resistance between pins 30 and 87. It should now read zero ohms (or very low resistance), indicating the switch has closed. If it remains open circuit, the internal switch mechanism has failed. Remember to disconnect the 12V power source from the coil when you're done testing.
Method 2: Testing the Relay In the Vehicle (Indirect Method)
This method is less definitive but can help confirm if a relay is working by observing the circuit it controls.
Firstly, locate the relay in question using your car's fuse box diagram. If the problem is something like central locking not working or a specific light not illuminating, the relay for that circuit is a prime suspect. You can sometimes swap a suspected faulty relay with an identical one from a non-critical circuit (like a horn relay if your horn works) to see if the problem moves. If the problem follows the relay, it's faulty.
Always ensure the swapped relays are of the exact same type and rating.
Alternatively, you can use your multimeter set to voltage to check for power at the terminals the relay plugs into. With the ignition on and the circuit that the relay controls supposed to be active, check the correct pins in the socket. If power is present at the input terminal (e.g., pin 30) but not at the output terminal (e.g., pin 87) when the relay should be energised, the relay is likely the issue. If no power is reaching the relay's input terminal, the problem lies further up the circuit, perhaps with a fuse or the control switch itself.
Verification and Troubleshooting
After performing the relay tests, the verification stage is crucial. If you tested the relay out of the vehicle and it failed any of the resistance checks (high resistance in the closed state, open circuit in the coil, or open circuit in the un-energised switch contacts), then you have almost certainly identified a faulty component. The usual signs for a DIYer would be a failed resistance reading or no click when energised.
If you swapped relays and the fault disappeared or moved with the relay, this is strong verification that the relay is indeed the problem. If the problem was related to a specific circuit, like a car alarm keeps going off constantly, and after testing, you confirm the relevant relay is faulty, replacing it should resolve the issue. Always replace a faulty relay with one of the exact same specifications – check the part number and pin configuration.
When troubleshooting persistent electrical issues, remember that a relay is just one component. If testing shows the relay is functioning correctly, or if replacing it doesn't solve the problem, you'll need to look further. Check the associated fuses for that circuit; a blown fuse is a common precursor to relay failure and can prevent a relay from receiving the power it needs to operate. Examine the fuse box diagram for your vehicle to ensure you're looking at the correct fuse and relay.
Other potential causes for malfunctions include faulty wiring, loose connections, or problems with the control module or switch that commands the relay. For instance, if your central locking system isn't working despite a good relay, investigate the door lock actuators, wiring harnesses, and the central locking control unit itself. Sometimes, a component like an aftermarket alarm system might interfere with standard vehicle functions, leading to unusual behaviour like the car alarm going off constantly, which might point to the alarm's wiring or its own control module rather than a standard car relay.
A functioning relay is confirmed by proper continuity and activation.
If you suspect a relay is faulty but can't get a definitive reading, carefully inspect its socket for corrosion or damage.