1995 Peterbilt 379 with DDEC III Speedometer issues

I've been round and round and back again with this one, I missed an important and kinda obvious fact.

Problem was the speedometer needle was behaving erratically, slow/fast, all the way around the clock at times.

My truck was installed with an 'Opti-Idle' system. Wiring diagram SK25038 was the closest I found that was of assistance.

What I missed was that there are 2 speed sensors at the rear of the transmission, where the driveshaft enters - one at the 7 o'clock position, and one at the 12 o'clock position. 7 o'clock feeds the ECM (wires 556/557, in positions E2 and E3 on the 30 pin ECM plug in) . . .

The only added clue I had, was that the scan gauge reading the J1587 data connector in the cab , would read road speed correctly, and the cruise control was functional.

What I erroneously thought, was that ONE speed sensor had a junction in the harness that split a single speed sensor into both the ECM and speedometer head. Dummy Dum Dum driver - read the schematic again and look closer at the situation.

12 o'clock is a twisted pair that feeds directly to the speedometer head - (wires 197/239).

Both sensors are two wire sensors.

The 12 o'clock sensor evaded my eyes, being up on top, dirty/greasy, and blending in with other hoses/wires. Prior to this, I had sent the speedometer head to JJT Speedometer (Jared) in Colorado, TWICE, and swapped in @wore out 's speedo head to completely rule out the head. I'd replaced the ECM speed sensor, and was about set to take the ECM connected pair of wires out of the harness before I found the second sensor.

Again - this is somewhat specific to the Detroit S60 DDEC III w/opti-idle configuration, but I thought I'd leave this here for anybody down the rabbit hole on this one.

If you Google search this and look at other posts on TTR, you will find some helpful posts with schematics downloads posted.

Good luck.