Before you go chucking the whole unit into the garbage, you need to get access to a high level scanner, be it a dealer Tech 2 or clone. I'm sure a Snap-on, Matco, or equivalent would work too.
You need to know if it's even capable of communicating and for our year, it's not CAN bus. That's why you need a different tool to read ABS codes, it's a completely separate communication protocol on a different set of wires located on the OBD2 DLC under instrument panel.
It could be that the problem lies OUTSIDE of the ABS. During POST(Power On Self Test), the unit is checking it's own memory, communication with the ECM and the presence of Wheel Speed Sensors. If any of these fail, the ABS light illuminates.
Here is something else that does cause the ABS light to come on: a stuck motor. The motor can be removed, two rivets need drilled, and the motor can be disassembled even further so you can clean out the buildup of dust caused by the motor brushes. After having started the engine and you begin to move more than 3-4mph, the ABS will cycle it's electric motor momentarily. If the motor doesn't cycle....yep, ABS light. This I know from personal experience. One day I didn't hear my ABS cycle and there was the light. The fix was a good motor cleaning and I haven't had a problem since. This was about a decade ago.
WARNING AND DISCLAIMER: removal and installation of the electric motor can be a bit tricky. Patience and small tools are mandatory to help push the pistons(of the hydro pumps)back in place to install the motor. Removing the upper control unit(super easy) while you remove or install the motor is mandatory. Don't say I didn't warn you.
I THINK I remember reading somewhere that if the BRAKE light is burnt out in the instrument cluster or if no bulb is sensed, the ABS light is turned on.
The point is that unless someone applied reverse polarity or if the voltage spiked well above 16volts(this is an estimate, not a actual spec), then it's not very likely that the control unit is damaged. I'm not saying yours isn't, but it's not very likely given what I have uncovered.
The lower electro-hydraulic portion, on the other hand, is near indestructible.....as long as rust from the lines is flushed away during regular bleeds. Rust particles can definitely affect the ability of the TWO hydraulic pumps to effectively do their job.
My dissection and analysis of my ABS unit is just a side project. Eventually, I'd like to learn how to communicate with it on a very basic level; not unlike a dealer tool, more powerful actually. I'd like the ability to change basic parameters and even study the algorithms they used to calculate when the unit activates. I just think it's impressive how much advanced technology for the mid 90's is packed into that unit.
My hope is that getting that deep into the firmware would illuminate the exact differences that are programmed for each GVWR and vehicle configuration. My truck is now 25 years old, how has someone not hacked into one of these things already? I guess it's more profitable to hack PCMs. :-/
Final thought and suggestion: do you have any other brake problems? Any problem that could cause the proportioning valve, where all the brake lines connect to the ABS, to activate it's switch?
