ABS Low Voltage Fix

By: AdvWisdom
Title: ABS Low Voltage Fix
Sourced From: advwisdom.com/a/abs-low-voltage-fix/
Published Date: Sat, 20 Jun 2020 00:04:40 +0000

ABS-II Low Voltage Fault Modification

By Mick McKinnon with words from Brian Curry, IBMWR President(s)

Please read the Disclaimer before attempting any work described in this document.

First Release 10/26/04
Final Release 10/27/04
Maintenance Release 11/19/04
Formatted for PDF by Tom Childers, 02/21/05
Original Formatting by NickJC

Please note there has been some phenomenal work done on the ABS I and II systems by Brian Curry. I suggest you see the fault code indices and how to get a reading BEFORE resetting your ABS system or proceeding with this document.

Legal Disclaimer

Warning: This modification is NOT recommended by any official BMW authority. Modifying your ABS function is serious stuff and should NOT be attempted if you are not comfortable with cutting into your wiring harness.  If you do not follow the instructions you can destroy your ABS Control Unit.  You are fully responsible for any modification you make to your motorcycle. It worked for me, I have no knowledge or assurance it will work for you.



Ever since it�s introduction in 1994, BMW�s ABS-II has been reported as prone to low voltage faults during engine start-up.  The ABS-II Control Unit begins monitoring the battery voltage as soon as the ignition is turned on and the ABS II control module starts the initialization and checkout sequence.  If the battery voltage drops below a preset threshold, ABS-II will fault with a low voltage indication.  I believe this low voltage threshold is set too high in most cases. 

When the engine is started and the starter motor is engaged, there is a very high current draw on the battery while the starter is operating.  The high current draw lowers the battery voltage.  If your battery is more than a few months old, and not maintained on a trickle charger or Battery Tender�, or if it is cold outside, your electrical system can fall below the low voltage fault threshold.  This fault can almost always be cleared by stopping and restarting the now warmer engine after a brief run period to recharge the battery.  This can be aggravating.

I looked at several solutions to this aggravating ABS fault.  One was a time-delay relay to start the ABS-II system initialization after the initial engine start.  These relays are available, but rather expensive.  Another solution would be a manual switch inline with the ABS supply voltage. However this would mean finding a place to install the switch and ensuring it could not be accidentally left turned off.  Neither solution was as attractive as the one this document describes. 


Overview & Theory

The theory is to automatically supply the ABS unit with a steady, stabilized voltage for the initialization and test procedure after engine start.  Since the engine starter motor inrush current sometimes causes the battery voltage to dip below the ABS preset low voltage fault level, having it perform the initialization and test procedure after that condition should eliminate the spurious fault. However, a long term low voltage condition will still cause a low voltage fault indication.

Adding a relay powered from the Alternator D+ output provides an effective automatic way to start the ABS system after the starting voltage/current variances.

The ABS unit gets its main power feed directly from battery via the ABS relay in the modulator assembly.  The voltage, supplied via the new relay, is used for ABS logic and control functions.  It comes directly from the ABS TEST relay in the relay box under the seat.

Once the system voltage has stabilized, engine running and alternator supplying voltage, the ABS initialization and test proceeds as it would normally.  This just delays the TEST until after the starter motor has drawn its big chunk of current with low battery voltage to start the engine

I personally never found the time to actually test this theory.  But thanks to Asko Paakkinen in Finland (who took the time to install and test this on his R1100RT), we have a solution.  Asko sent me a note describing how he modified his bike.  I added a few details and am documenting the modification for anyone who wants to experiment.  I performed this modification on my 1996 R1100RT and tested it with satisfactory results.

NOTE: SEE DISCLAIMER ABOVE, YOU are responsible for any modification you make to your motorcycle.

The modification looks like this on the R1100 wiring diagram.


Here is a drawing of the relay box under the seat, where the new relay will be installed. This will be helpful as you read the instructions on how and where you can install the new relay.

Modification Instructions

Review all the information below BEFORE cutting into the system(s) and proceed at your own risk.

  1. Remove the fairing, disconnect the battery and remove the fuel tank.

  2. Disconnect the ABS connector and remove the rubber boot from the lower end of the connector. The ABS connector looks like this:
      • Remove the three phillips head screws from the connector top and slide the connector body out of the housing, as shown in the next two pictures:

  3. Locate the pin #15 connector (solid green insulated wire) at the lower (cable end) of the connector. The pins are labeled on the ABS Control Unit, as shown below:

  1. Remove the fuse/relay box cover and remove the ABS warning relay (turquoise relay in the relay box right front portion). Verify, using an ohm meter, that the large blade connector at the front of this relay socket connects to pin #15 of the ABS Control Unit connector in Step 2. (0 ohms)

      • Connect 12� of at least 22awg hookup wire to the solid green insulated wire (0.5mm) below ABS warning socket base (this wire is attached to the large blade connector in the relay socket) and route it to the empty space just behind the yellow starter relay at the left side of the relay box. (TIP: I used 22awg Teflon insulated wire from http://www.ba-electronics.com/nte-wire.htm#wt)

  2. Cut the solid green wire mentioned in step #2 above about 2� from the connector body.
  4. Tape or use heat shrink tubing to insulate the solid green insulated cut wire end in the cable bundle.
  5. TIP: I drilled a 5/16� hole in the relay box left side and installed a rubber pass-through grommet to keep the relay box sealed, as shown below:


  1. Use more hook up wire to run a ground wire to the same empty space in the relay box. Any good ground can be used (TIP: I used a convenient empty 6mm tapped hole on the left side of my transmission housing just behind the starter:


  1. Run two lengths of at least 22awg hookup wire from the relay box to the ABS connector.  Route these wires along the existing cable harness and cable tie them to it.  (TIP: I used blue wire for the alternator D+ connection and red wire for the automatically controlled 12V to the ABS connector pin #15).

  1. Attach the new wire (red in my case) from the empty space in the relay box to the solid green insulated wire stub from connector body pin #15.

  1. Remove the D+ connector (1/4� slip-on connector) from the alternator and remove the housing cover. Attach the new blue wire to the connector and re-install the connector on the alternator. (TIP: I carefully opened the strain relief on this slip-on connector and soldered the new wire to the connector).

  1. Next install slip-on connectors on the 4 new wires just added to the relay box.  (TIP: I used insulated 90� slip-on connectors:


  1. I used a Bosch headlamp relay P/N 0 332 019 150 with integral mounting tab.  Mount by drilling a small hole in the front vertical surface of the relay box empty space just to the rear of the starter relay.

    1. Proper relay connections are critical to proper circuit operation! Connect the new green insulated wire from the ABS warning relay to new relay (K1) pin #30.


    • Connect the new ground wire to new relay pin #85 and the new blue wire from the D+ connection to new relay pin #86.


  • Connect the new wire, going to ABS connector pin #15 to one of the new relay #87 pins.


  • Mount the new relay using the hole drilled in step 9.



Finalizing and Operational Testing of your Modification

    1. Reinstall the fuel tank and reconnect the battery.  Look for smoke.


    1. When the ignition is switched on only the upper ABS lamp on the dash should be solidly illuminated.


    1. Start the engine. As soon as the Gen/Alt lamp goes out, both ABS warning lamps should flash in unison.


  1. If not, go back and recheck all your work to ensure ALL the connections are made correctly.


Everything works?  Good!  Reassemble the bike enough for a test ride.  Go find some dirty pavement and test the ABS function.  If all is well, you�re done and you will not have the aggravating low voltage ABS fault on startup.