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ABS Power Toggle Switch

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    ABS Power Toggle Switch

    I would like to wire in a toggle switch to turn the ABS system on and off. My chassis a 91 318iS so keep in mind it does not have the ABS relay under the dash like the earlier cars, it's part of the ABS pump itself. The ABS pump and ABS computer power run through the C101, pin 20.

    Ideally how I would like it to work:
    • With the ABS enabled, the cluster light is off and the toggle switch is switched on and the LED light is illuminated
    • With the ABS disabled, the ABS light on the cluster would turn on, the toggle switch would be switched off and the LED would turn off

    My original plan was to tap into the red/yellow power wire that leads into the ABS computer under the dash. Another option would be to cut into the ground wires going to the ABS computer, although my rudimentary electrical knowledge leads me to believe that if I switch the ground I won't be able to utilize the LED on the toggle switch.

    On my car the red/yellow power wire comes off Pin 20 of the C101 and it splits to power both the ABS pump and the ABS computer. To keep all the associated wiring inside the cabin, I planned to tap the red/yellow right by the ABS computer instead of in the engine bay.

    Would one be preferable to the other? Would killing the power input to the ABS computer kill the ABS system but also allow the cluster light to turn on? Would it be better if I tapped the red/yellow wire further back closer to the C101?

    Let me know if I am on the right track with this. Searching didn't come up with much, most people just said "pull the relay" to disable ABS. For reference, here is a link to the ETM for this chassis: the ABS computer and relay are on pages 26 and 27 of the PDF: click here

    For anyone curious about this, I did some testing yesterday. With the ABS computer unplugged, the ABS light comes on in the cluster. Next, I snipped the red/yellow power wire in the ABS computer connector and the desired result: ABS light turns on in the cluster, and if you reconnect it, it turns off. So I am going to switch the red/yellow wire, will give me the desired outcome of a toggle switch to turn ABS on and off on the fly.


      I like the idea. Thanks for posting the research
      '89 325i OBD2 S52 BUILD THREAD


        So I discovered something about this arrangement over the weekend. When you turn the key to ACC and the power wire to the ABS computer is disconnected, the ABS pump doesn't finish the self-check/prime and keeps going. It seems if you have the power connected and then you cut it off after the key is already on, it performs the test fine and doesn't stay on after you cut the power.

        I cannot tell if this causes any issue when driving the car as I haven't driven it yet. I also cannot tell if the ABS pump continues trying to prime once you actually start the engine. It didn't seem like it, but my engine is so loud it was hard to tell.

        If you completely disconnect the connector/harness to the ABS computer, the ABS pump never tries to prime at all. So there is probably a better way to cut the power than the way I went with.

        I'll keep an eye on this and report back. Hopefully someone with more knowledge of how the ABS system works can chime in as well.


          Still having trouble with this. Now that I have grounded the toggle switch, you can always hear the ABS pump priming when the toggle is switched off.

          It appears that by only splicing the wire close the ABS control computer, power is still going to the ABS relay and turning on the pump even when the computer is off.

          My new line of thought: switching the red/yellow power wire before it separates to the pump and computer (closer to, or before the C101). That would prevent power from going to the pump relay AND the computer when the switch is off. I believe it would still turn on the cluster light as well when disabled.


            I've not examined the ETK for your car to know it this will work, but it seems logical.
            The car makes it possible, but the driver makes it happen.
            Jim Levie, Huntsville, AL


              On someone seemed to have had luck installing a switch via the power wire @ pin 20 of the C101, before the the wire splits to both the pump and the computer. I'll report back after giving that one a try.


                To clear myself of ignorance, what do you plan to use this for and what benefit(s) are you expecting from it?


                  Originally posted by captain awesome View Post
                  To clear myself of ignorance, what do you plan to use this for and what benefit(s) are you expecting from it?
                  I plan to run ABS at all times, but there have been several instances in my ownership of this car that I have needed/wanted to disable ABS for troubleshooting purposes. While wiring my switch panel, it just seemed like a quick and easy thing that could be helpful again in the future.

                  Example 1: I accidentally installed my front wheel speed sensors on the wrong sides of the car. Applying the brakes made the ABS system go crazy, since the readings from the sensors were off.

                  Example 2: On the track I was trying to track down a shudder during high speed braking that felt just like the ABS was activating prematurely. It turned out to be worn CABs causing the shimmy, but turning off the ABS for a few laps helped me rule that out quickly.


                    Sounds logical to me. It would also be nice to switch off and prove or disprove it's ability to help on the track. Obviously that probably all depends on driver, but it's something you could test at the flip of a switch against your current data. I'm guessing it's going to help more than hurt though.


                      Yes, I think I'll probably be faster with ABS as well!

                      Happy to report that moving the wire splice closer to the C101 fixed my issue! There are no ABS pump running/priming issues with the toggle switch now. I simply tapped into the red/yellow wire BEFORE it splits to the ABS pump and ABS computer separately.

                      This is on a late model car where the ABS power wire (red/yellow) runs through the C101 on Pin 20.

                      On earlier cars, you can tap that wire @ the relay that is under the dash on the driver's side. The power doesn't run through the C101 on earlier cars.

                      Hope this helps someone!


                        The ABD failed on my 190E 2.3-16 racecar. I didn't really care, since I don't rely on ABS anyway. But the car became undriveable on track as the rear brakes kept locking up.

                        Turns out the brake proportioning is controlled by the ABS system. When it was disabled, the brake bias shifted so far to the rear that you basically lost the ability to brake. Even the lightest brake pressure would lock up the rears. Flat-spotted some very expensive tires. Had no fun.

                        Turns out the same issue exists with the E36 chassis. Not sure if it's also an issue with the E30, but it's definitely something to consider before you disable ABS.
                        1987 Mercedes 190E 2.3-16: Vintage Racer
                        2010 BMW (E90) 335xi sedan: Grocery Getter


                          That's not the way E30 ABS functions, but thank you for the heads up.