The ABS isn't working on my 1991 940 turbo wagon. Model plate on rear tailgate claims 940SE.

The ABS light triggers at about 40kph (25mph). ABS works fine below that, tested on wet roads.

I had a bad connection on the rear sensor several months ago and fixed it. The speedo needle quivers a bit sometimes, not sure if speedo issues. I changed the original due to the speedo not working (it was not because of the rear sensor).

Resistance reading for each front sensor static is 1.1K ohms.

The front sensor and toothed discs are newish and very clean.

I get the proper resistance at the ABS controller connector for all sensors so wiring is fine.

The surge relay activates when powered but info in manual is wrong.

My Volvo brake manual hasn't been a great help as it's given some wrong info. and doesn't mention how to troubleshoot for ABS ight at 6mph.

I only have DTC connections for ECU issues, nothing for ABS.

I tried disconnecting both front ABS sensors and measuring the mV after driving 25+ mph. BB FAQ said 50-700mV is typical. On the drivers side I got 0.1mV and zero on the passenger one.

After this testing the ABS error lights at 6mph with all sensors connected.

I noticed a new addition to the FAQ since I printed it. Haven't tried this from (Diagnostic Unit Malfunction: Surge Protector Failure.
[Tip from IO333]

Any help appreciated.
cheers