After 1 Year of rebuilding my 87' FC Turbo II it was time for a test drive. I noticed that the ABS-Light on the dash lit up after just a few meters of driving. When braking hard, the wheels lock up. So it's clear that the ABS does not work.
As the ABS-Light only turns on when driving the car, I assumed that there would be a issue with the speed sensors. I measured all the sensors and the ABS unit as described in the workshop manual. Everything looks good so far, exept for one speed sensor.

The 3 speed sensors, Front Left, and both Rear sensors works fine. I've measured the signal while a friend was turning the wheel. All signals have about the same amplitude. But the Front Right Sensor have a much higher amplitued as the 3 others but still a perfect sine-wave signal.
I can't figure out, why one sensor works and generates a perfectly sine wave signal, same as the other sensors, but with a much bigger amplitude (about 8x - 10x more).
Since the sensor does not provide the expected voltage, it should be broken, as described in the manual.

But it seems weird to me. I expect a broken sensor to not provied a sine-wave signal or some other weird signals but not just a bigger amplitude.
Does someone had the same issues or know more about the sensors than I do?

I've added some pictures from the workshop manual and of the speed sensor with the gear.

 

have you looked at all the teeth on the problem wheel? its possible that there is junk in there or its just broken or something

 

I think i need to disassemble the front wheel hub and take a closer look.

 

Did you get to the bottom of this? I'm looking at the same. Like your scope display, I'm getting about 800mV-1v p-p when turning the wheel but the FSM suggests 50mV - 60mV.
o we know if this is open circuit measured or with everything connected up? Seems to be way out. This is measuring what i thought was a good sensor so it must be correct.

I cant find a replacement in the UK for my faulty front sensor so I trying a MK2 MX5 sensor, seems to work ok its just getting the clearance to the toothed ring that the issue.

D

 

I solved it by replacing the whole sensor. I dont know exactly what was wrong with the old one but with the new everything works great. I also had some contact issues with the calbe/plug.
An option can be to adjust the distance between the sensor and the gear with a feeler gauge. This might help for adjusting the voltage.