@Slowbrick thanks for the links, they explain much.
@Philia_Bear is technically correct; strictly speaking, there is no
direct connection from the diff speed sensor to the ECM...
but there is a connection from the (electronic) speedo to pin 34 of the ECM, and the speed signal to the speedo comes from the Hall effect sensor in the diff.
Digging a wee bit further, I found
this post by malloy1 on Turbobricks, describing his 4-pulse-per-revolution setup, which he's attached to his overdrive, to make the ECM think it's getting a signal from the speedo.
So it appears pin 34 needs a square wave, approximating 4 hz per revolution of tailshaft, to do its speed sensing thang. The latter model diff sensor thing either does 48 or 96 pulses per turn depending on year model and if ABS, etc,, so I figure that speedo must have a divide-by-12 or divide-by-24 circuit in there somewhere,
If only there were waveforms.... oh, wait, here's some!
http://cleanflametrap.com/speedo.html
The day has been saved again by cleanflametrap.
And by
@Forg, who asked the right questions in
this thread, posted aaaaalll the way back when our soon-to-be-S80 was but a twinkle in a Torslanda's eye.
