1994 F-350 IDI Turbo/E40D Tachometer issue
 

Hi gang,
Istarted my truck today, and noticed the tach still showed 0 rpms. I goosed the throttle and it started working, but as soon as rpms dropped to idle the tach dropped to 0 again.
Driving, the tach reads correctly as long as the engine speed is above idle, but if you let off the throttle to stop at a light, as soon as the rpms drop below about 900 rpms, the tach needle drops to 0. As soon as you give it throttle, it starts working normally again.

Any ideas?

Truck is running fine, but this may/may not be related.
Lately, when shifting from Park into a gear, sometimes I have to rev the engine up a little to get it to shift into gear...otherwise it sits like it's in neutral. Does not happen all the time, and it happens equally when hot or cold. Once it engages, there's no other problem.. it then drives and shifts fine. I checked the fluid level, it's dirty but not burnt... and full. I figured it likely is overdue for a trans filter and fluid change, so I hadn't worried too much about it. Now I'm worried that there may be more wrong than I thought.
Thanks!

Easy things first, pull the tach sensor and clean the business end and reinstall. It's a magnetic pickup so it picks up debris and crap over the years, sometimes all it needs is a cleaning.

ya its $75 ford only part.