My point was that the vss isn't just for the speedo and cruise. OD does need a reading from it. An erratic reading can ause the od to drop out of engagement. Which is why I also asked if the speedo and cruise worked correctly.

I asked about engine running since the op didn't specify a diesel, gassers with a flaky tps will run oddly.

I agree with the rest of your information, and yes even the 518 can be converted to lockup. But that's a compete rebuild option, not a minor repair.

And one last input, besides the cold eninge switch, there is a trans temp switch that will kick the od off if the temp gets too high. IIRC the switch kicks off at like 250 (way hot)

Still, as with you I bet the tps is funny, a sweep test will confirm it.