The anti-lock control relay supplies power to the anti-lock brake controller. The control relay is only active when the brake controller is demanding to pulsate the brake fluid to the wheels. The anti-lock control relay breaks down over time and tends to fail.
How an anti-lock control relay works
The anti-lock control relay is just like any other relay in your vehicle. When power flows through the first circuit within the relay, it activates the electromagnet, generating a magnetic field that attracts a contact and activates the second circuit. When the power is switched off, a spring pulls the contact back up to its original position, switching the second circuit off again.
The input circuit is switched off and no current flows through it until the brakes are fully compressed and the computer senses that the wheel speed has dropped to zero miles an hour. When the circuit is completed, then power is sent to the brake controller until the demand for added braking efforts are no longer needed.
Anti-lock control relay signs of failure
The driver of the vehicle will experience a longer time to stop the vehicle. Plus, in hard stops, the tires will lock up and cause the vehicle to go into a skid. Also, the driver will not feel anything on the brake pedal as they perform a hard stop.
Engine light and ABS light indications
When the anti-lock control relay fails, the engine light may come on. However, most vehicles are equipped with a bendix system controller and the ABS light will come on when the brake controller is not energized during a hard stop. The ABS light will begin to flash, and then after the third time the brake controller has failed to energize, then the ABS light will remain on.
Part 1 of 8: Verify the condition of the anti-lock control relay
Step 1: Get the keys to the vehicle. Start the engine and take the vehicle for a test drive.
Step 2: During the test drive, attempt to step on the brakes hard. Try to feel the pedal pulsate. Remember if the controller does not engage, then the vehicle may go into a skid. Make sure that there is no inbound or incoming traffic.
Step 3: Check the dash for an engine or ABS light. If the light is on, then there may be an issue with the relay signal.
Part 2 of 8: Preparing to work on replacing the anti-lock control relay
Having all of the necessary tools and materials prior to starting the work will allow you to get the job done more efficiently.
- Allen wrench set
- Boxed end wrenches
- Cross tip screwdriver
- Electrical cleaner
- Flat tip screwdriver
- Needle nose pliers
- Ratchet w/metric and standard sockets
- Torques bit set
- Wheel chocks
Part 3 of 8: Preparing the vehicle
Step 1: Park your vehicle on a flat, hard surface. Make sure that the transmission is in park. If you have a manual transmission, make sure it is in either 1st gear or reverse gear.
Step 2: Place wheel chocks around the rear tires that will be remaining on the ground. Engage the parking brake to the lock the rear tires from moving.
Step 1: Install a nine volt battery saver into your cigarette lighter. This will keep your computer live and keep your setting current in the vehicle. If you do not have a nine volt battery saver, that is okay.
Step 2: Open the hood and disconnect the battery. Remove the negative post off of the battery terminal. This discharges the power to the neutral safety switch.
Part 4 of 8: Removing the anti-lock control relay
Step 1: Open the hood to the vehicle if it is not already open. Locate the fuse box in the engine compartment.
Step 2: Remove the cover to the fuse box. Find the anti-lock control relay and remove it. You may have to unscrew the an additional compartment if the relay is joined with multiple relays and fuses.
- Note: If you have an older vehicle with a brake controller in it with the Onboard Diagnostics First Addition, OBD, then the relay may be isolated from the rest of the fuses and relays. Look by the firewall and you will see the relay. Remove the relay by pressing in the tabs.
Part 5 of 8: Installing the anti-lock control relay
Step 1: Install the new anti-lock control relay into the fuse box. If you had to remove a cluster of fuses in an additional compartment, then you would need to install the relay and reinstall the compartment back into the fuse box.
If you removed the relay from an older vehicle with the Onboard Diagnostics First Addition, OBD, then install the relay by clicking it into place.
Step 2: Put the cover back onto the fuse box. If you had to remove any obstacles off the vehicle to get to the fuse box, be sure to put them back.
Part 6 of 8: Hooking the battery back up
Step 1: Open the vehicle’s hood. Reconnect the ground cable back onto the battery’s negative post.
Remove the nine volt battery saver from the cigarette lighter.
Step 2: Tighten the battery clamp up tight to ensure that the connection is good.
- Note: If you did not have a nine volt battery saver, you will have to reset all of the Settings in your vehicle, like your radio, electric seats, and electric mirrors.
Part 7 of 8: Testing the anti-lock control relay
Step 1: Put the key into the ignition tumbler. Start the engine up. Drive the vehicle around the block.
Step 2: During the test drive, attempt to step on the brakes hard. You should feel the pedal pulsate. Also look at the dashboard.
Step 3: After the test drive, check to see if the Check Engine Light or ABS light is off. If for some reason the light is still on, you can clear the light with a scanner or simply removing the battery cable for 30 seconds.
The the light will be out, but you would need to monitor the dash to see if the light illuminates back up after some time.
Part 8 of 8: If the problem continues
If your brakes feel out of the ordinary and the engine light or ABS light is on after replacing the anti-lock control relay, then there may be further diagnosis of the anti-lock control relay or a problem with the electrical system.
If the problem persists, then you should seek out help from one of our certified mechanic that can inspect the anti-lock control relay circuitry and diagnosis the problem.
This article originally appeared on YourMechanic.com as How to Replace an Anti-Lock Control Relay and was authored by Marvin Sunderland.