I've had my SERVICE ABS light on for awhile now. Couldn't read codes through grounding H at the ALDL, because some stored codes disable that function. I reckon I had such a code.

So, after a lot of research, I realized that the only way to access those codes and to erase them, I would need a Tech-1 scanner. However, I did not want to spend that much money, as I already had an OTC Monitor 4000e.

With the 4000e, all I needed was the ABS adapter cable (238464). This cable had three wires that plug into ALDL as follows--black>A-- red>H-- blue>M. I was able to read current and history codes, as well as erase codes. Furthermore, I am able to see in MPH and range, in real time, for each wheel speed sensor.

It turns out, that I had a code 63, which is the modulator valve relay. The relay tested good, on the bench. Then, it was off to the FSM flow charts. This is where I ran into another road block.

The first item in the flow charts calls for a pinout box (J35592), to check circuits. So, I found one online for about $50. It turns out, I also needed the supplemental electrical FSM too; luckily, I had that. I found that I had an open ckt 2.; fixed a corroded wire that was inside a connector...FINALLY problem solved.

So, I couldn't have fixed the ABS without both, the FSM and the supplemental electrical manual, OTC Monitor 4000e, ABS adapter cable, and the pinout box. Oh ya!! and a lot of research and patience.

In conclusion, the 90-91 ABS system is unique to those years. For other years, you'll have to check your FSM and do a little research. I hope this can help someone else, in the future. Here are some pics of the tools I described here.
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I wish I'd bought the ones over in parts before they sold. I put it off and now I don't have the FSMs

 

Thanks

 

Bump for pics added to original post