erratic tach

[Slammin Sam]

The tach on my 89 B2 occasionally shows higher rpms than the engine is turning. I first noticed it occasionally when the cruise was engaged. Lately it has happened without cruise on. All the other gauges are behaving as expected. What should I look for?

 

[RonD]

1989 Tach signal comes from TFI module or the "-" side of coil.

Look here: http://www.therangerstation.com/tech_library/Tachwiring.shtml

Looks like 1990 and up mostly used a Tan wire for tach signal, but 1989 2.9l shows Green wire.

I would check coil and TFI module on distributor for that wire and make sure it is not frayed.

Cruise uses the Speed sensor so no connection to Tach system except for what all systems in the truck have in common........Grounds, so could have a loose ground in the cable.
Has there been in water intrusion in to the cab in the past?

Also check the Ground strap the runs from the back of the Drivers side head to the firewall, make sure it is clean and tight, thats the main ground for the cab electrics

 

[Slammin Sam]

Thanks for the input Ron. Before I had a chance to check the ground, the drivers seat recliner took a dump, so the B2 is parked until I have time to try to fix it. Factory manual says "no replaceable parts" and to buy the whole unit to repair. Its the split bench and not worth repairing. I guess I'll be finding those buckets sooner than I'd planned.