Replace Your ABS Relay or Abs Controller

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Getting Ready to Replace Your ABS Relay

Replace your ABS control unit.
Your replacement ABS relay or control unit. photo by Matt Wright, 2008

If you're haunted by your ABS light and you've narrowed the problem down to the brain that controls your ABS, the ABS relay (or ABS controller), it's time to replace it.​ You've already checked the fuse. The dealer will charge big bucks to do this repair, but you can save a lot of money if you replace your own ABS relay. When your ABS is malfunctioning or off, your car's safety is compromised. If your vehicle is equipped with any type of traction control or stability management system, there is a good chance this is disabled, as well.

Difficulty Level: Novice

What You'll Need

  • New ABS control unit
  • Screwdrivers

The following how-to covers ABS relay replacement on a C-Class Mercedes, but it's similar in most vehicles. Your unit may be inside the car rather than under the hood, and it might be bigger. Check it out ahead of time to be prepared.

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Accessing the ABS Controller or Relay

The ABS ECU Controller Relay Cover
Remove the cover to the ABS unit. photo by Matt Wright, 2008

Before you begin: Whenever you're working with your car's electrical system, especially when dealing with sensitive electronic components like an ABS control unit, be sure to disconnect the negative battery terminal to be sure you don't cause any damage.

The brains behind your ABS and traction control systems are protected by a plastic shield to ward off moisture, ​mice, and other mayhem. The protective box will either be under the hood or in the passenger compartment. Sometimes it will even be exposed but behind an access panel under the dashboard.

The cover to the ABS relay or control unit will be held on with screws or just clipped into place. Carefully remove the cover to expose the fuses and other stuff inside.

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Disconnect the ABS Relay Wiring Harness

Take note of the ABS wiring arrangement.
Carefully remove the wiring from the ABS unit. photo by Matt Wright, 2008

With the cover off, you'll be able to locate the ABS block you need to remove. Your unit may be by itself, which makes things easy because you just replace it. If your vehicle is set up like this one, the ABS relay (circled above) is grouped with other electrical components in a sealed compartment. If it's not obvious, you can locate the ABS relay by looking at the new part you just purchased and comparing it to what's in there.

Before you start yanking wires, take a good look at the way it's set up, taking note of anything that might be important, for instance, if there is one big bundle of wires that is on top of a smaller bundle this might be the way you have to put them back if you want to get the cover back on. If you have a digital camera, a snapshot of how things looked before you disconnected it all can be very helpful. You'd be surprised how something that seems so simple when it's together can get really confusing later.

Carefully remove all wiring from the ABS unit. A couple of small screwdrivers can help you to push in those little release tabs or help you carefully finesse the wiring plugs out.

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Remove the Old Faulty ABS Control Unit or Relay

Remove the faulty ABS ECU controller relay unit.
Slide the old ABS unit up and out. photo by Matt Wright, 2008

With the wiring removed and pushed out of the way, you'll need to remove the faulty ABS controller. It may be held in by screws, or it might be secured by a slide-in type of holder like the unit pictured above. Simply slide it up and out.

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Installing the New ABS Relay and Finishing Up

ABS relay wiring.
Carefully deal with the ABS wiring. photo by Matt Wright, 2008

With the old relay out, you just need to slide the new ABS unit into place the same way the old one came out. Be sure all wiring is out of the way before you press it into place, so you don't smash or crimp any of them. Now install all of the harness plugs the way they came out. It's almost impossible to get it wrong because the plugs are designed to fit only in the proper hole. Many are also color coded.

Be sure to properly reinstall the protective cover to keep moisture away from the sensitive electronics. Reconnect your battery, and you're good to go!

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