F800GS ABS sensor problem

[andyredz]

My abs light came on and stayed on last week. A mate used his 911 and diagnosis was front sensor. Replaced at very ouch cost. Light remains on. Diagnosis from 911 still saying fault on front sensor. If anyone has any ideas please help! Bike is on 08, 22k. Thank you.

 

[Nick V]

Hi Andy!

The tricky part of that fault code is that when the GS-911 says 'The front ABS speed sensor is faulty', it's only telling you that it's detected a report of an open circuit (or intermittent open circuit) somewhere between the speed sensor and the engine ECU or ZFE-high.

If replacing the sensor had no effect, the good news is that you've eliminated it as a possible cause of the issue. The bad news is that you've more likely than not got a break in the relevant portion of the wiring harness. I'd recommend plugging the GS-911 back in again and noting how many times the fault code has been logged. If it's only once or twice, chances are there's a constant break, and that's (relatively) easy to find. If it's many times, you may have an intermittent break, which is much harder to isolate.

If it were me, I'd start by disconnecting the battery, then disconnect the multi-plugs leading to the engine ECU (under the seat) and ZFE-high (in front of the engine airbox) and checking for any plug terminals that aren't making proper contact, or have been pushed back into the plug.

If all is in order, I would refer to the Haynes manual for the ABS wiring diagram, and use an electronic multimeter set to the 'Continuity' setting to check for an open circuit between the relevant sensor plug terminals (there are two) and the relevant engine ECU or ZFE-high terminals.
The good news is that the ABS speed sensor circuits are not resistance-dependent. In other words, if there is a break in either of the relevant lines, you can simply repair it by re-soldering and re-shrink-tubing the broken wires.

Best of luck!

 

[andyredz]

Hi Nick

Thank you for such a comprehensive reply. I do have a Haynes manual, and will borrow a multimeter for a look see if checking the plug terminals doesn't yield results. It may be a week or two before I can do it, but will post results...

All best! Andy

 

[Nick V]

Hi again Andy!

Had a squizz at the Haynes circuit diagram last night. (The relevant wiring diagram is on page 8.30 of the Haynes manual.)

Your procedure will be as follows:

1. Disconnect the battery negative terminal.
2. Disconnect the front wheel speed sensor from the wiring harness.
3. Disconnect the multi-pin plug from the ABS control unit. (Luckily, if I'm reading correctly, you won't need to dismantle the bike's front end to get to the ZFE-high - I believe the 'ABS control unit' is a separate, tiny ECU that lives on the ABS regulator itself.)
4. Check for continuity between:

• Wiring harness speed-sensor plug terminal 1 (Yellow wire with brown tracer) and terminal 5 on the ABS control unit plug.
• Wiring harness speed-sensor plug terminal 2 (Yellow wire with red tracer) and terminal 16 on the ABS control unit plug.

All terminals on all BMW multi-pin plugs are numbered - you just need a fairly powerful light to see the numbers.

Let us know how it goes!