Unfortunately yours is a pre-1992 240 ABS system that doesn't use the OBD system to store diagnostic codes that would help pinpoint the reason the ABS warning light is on.

You did of course clear the original ABS fault(s) by disconnecting the battery?

Check the front ABS sensor tips for any signs of damage (chipped edges). Also check the front sensor cables and connectors for signs of damage (open up, clean and reseat the connectors). The reluctor teeth need to be mildly clean and the gap to the sensor tip has to be approximately correct. I imagine you're well past all that by now.

The ABS warning light is not associated with hydraulic system issues, but rather ABS sensor and control module issues, so basically electrical. Apart from the physically obvious, there are voltage tests you can do to see if things like grounds, control module connections and that relay are okay. The relay is certainly a suspect area once you verify external damage. I see mention elsewhere of a fuse at or in the control module (up behind the glovebox in your year, later year 1992-on under the front passenger seat) that gives an ABS fault when it blows, but that was discussing a '92 240.

Refer to the ABS section of the 700/900 FAQ here for tips and procedures https://www.brickboard.com/FAQ/700-900/BrakesABS.htm. Your 1990 240 system should be very similar (if not identical) to the early non-OBD 700/900 ABS systems described in the FAQ.

An early 1990s brake service manual and full proper wiring diagrams would be useful (preferably 240, even 700 would do), but may be difficult to find online. A quick look for a suitable bootleg manual didn't turn up anything. Apparently Bentley has not too bad a section on ABS.

Hope some of the above at least helps you get to the next level on this. Many of us have had minimal issues with our ABS systems beyond keeping things clean and inspecting for damage.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now