Good morning. I agree, the thought of modifying the tone ring without having the diff open is a little daunting. I'd be tempted to pull an earlier axle and start over rather than do that (of course spoken by the guy who isn't doing the work-lol).
As to why; I rebuild the clusters quite frequently and I've always wanted a means to bench test the ABS units like I do the regular ones. To that end, and knowing there is an issue with the schematic, I conned a service manager into loaning me the 92-93 wiring supplement a couple of years ago to no avail. With the rain stopped, I have the Bentley here with me now and I'm refreshing my memory of what was a waste of time back then. The supplement is just a supplement and had the same schematic as the Bentley. No corrections. My first issue is the grid lines of the schematic on top of the wires, terrible. Does the wire on the right leaving 229, the pickup, go to the speedo or not? That would be five wires, so probably not. Who knows? What is the loop created by connections A1 and F1? It's right there at the speedo, but doesn't seem to connect. What is the line leaving the right side of the speedo marked 31/6 that just terminates above the F1 loop? Is that where the other side of the pickup connects? If so, why does it go down instead of up on the page? And why does it just end in a little dot with no color code or indicated connection point? Very annoying.

The only thing that makes sense is that the mystery 31/6 wire is the fourth wire in the angle plug to the speedo head. The schematic does show four, where the earlier speedos only have three leads in the plug. This appears to be power in (from my test last week) and I'll bet it's from the ABS controller, but I admit I threw up my hands when I took a look at the harness on the controller.

We all agree that what needs to happen here (in a perfect world) is to modify the freq signal that the counter is counting. But it only drives the odo???!!! That's incredible. Uh-oh, where does the speedo drive take it's signal from if that's the case? Even if we did something in the line to the cluster from the pickup it wouldn't affect the speedo reading?

DS