ABS light on constantly - not OVP

[snakehips916]

1993 190E 2.3 4cylinder auto USA model.

Happy New Year everyone.

For the second time in 2 years my ABS dash light is on all the time.
The 1st time (about 18 months ago) it was accompanied by poor starting.

I rectified this with a secondhand OVP relay (part # 201 540 3745) - this had 2 fuses compared with original's 1 fuse (part # 201 540 3245).

Car has been running fine until 1 month ago; I was driving along and ABS light came on and has stayed on ever since. Poor starting followed - predictably! I checked OVP relay to find one of the fuses blown.

I Replaced fuse - ABS light still on and poor starting. Changed OVP relay for a spare I had (part# 201 540 3245. Starting better-than-ever but ABS light still on. Changed OVP relay for brand new item (part# 201 540 3245).
Result; starting not quite as good and ABS light still on.

Any ideas? I read somewhere the ABS sensors may need cleaning?! My useless Haynes manual doesnt tell you about ABS so can anyone help me?
Also, can anyone tell me where I can download a decent factory workshop manual or equivalent.
Many thanks
Nick