Dan, it's pretty simple on the TH400's It's a single prong sender at the tranny, a single wire from there to the switch at the throttle linkage and just the switch.

I suspect that they may have loosened the wire at the tranny? Bumped it, or? As that is the only area they worked with right?

I'd check the connection there at the tranny on the drivers side rear, and then trace the wire top make sure it's not binding with the linkage and or shorting anywhere etc...

Brett - 67 Drop Top 455!