Working on my mom's pickup. 92 D350 was 518 auto, now Getrag conversion. Speedo worked fine for a few years after the conversion. The last few years it has worked intermittently. So far I have checked the following.
1) Mechanical stuff coming out of transmission. All is in good working order- VSS, ratio adaptor/reducer, drive gear on tailshaft is tight, driven gear is in the correct place with good teeth.
2) VSS verified good in my pickup truck.
3) i have checked wiring to the PCM. All is in good shape.
4) wiring behind gauge cluster is good.
5) I swapped gauge cluster into my pickup a year or so ago for a test drive. Worked flawlessly.
6) leads me to believe somewhere in the PCM the signal may be getting lost. About 2008-09, in the course of a few months, she lost all pcm controlled functions- AC, voltage regulation, OD engagement (hence, the 5spd swap), cruise control. But the speedo worked for awhile after that. I'm stumped. Ideas?