Have a 88' F250 with an automatic that I dropped a 6.9L in. The tach just bounces around 700-800rpm when I accelerate. Any clues on where to start?

 

Start by doing the least costly, least time consuming thing and work your way up.

Start by removing the sender from the IP gear housing and clean the tip with your favorite solvent. Sometimes they get dirty and throw off the readings. Cost-$0 Time- >10 mins.

A new sender is about $50 from the dealer.

 

A dead tach sender should/would send the automatic trans into a "failure management mode" (limp mode) with harsh engagements and firm shift feel and an abnormal shift schedule. It has no effect on a manual trans only the Tach readings.

Check the wires where they exit the big nut on the oil filler housing , known to short out due to bare wire.....

Replace the tach sender on the oil fill housing a big 1" nut with 2 wires. Ford only part Engine RPM sensor E5TZ-17B384-A about $50. You can remove and clean it but usually changed later too.

You can test it too, hold it in the air away from ferrous metal, using an ohm meter lead on each wire you need DC resistance between 2000-3000 ohms.

 

good thread, i have this problem and was also told to check the main ground for the harness.

 

Tach sender is a Hall effect device so no real power just pulses.....there are several connections that could be part of a problem.

 

My transmission has been shifting pretty roughly since I dropped in the 6.9L. I will try to clean it first and go from there

 

was driving last nite and the od lite started flashing when i would decelerate. i started freaking til i realized the tach would drop to zero after it got to 1000 rpm. i could decelerate to a stop and go again, but it never shifted hard because the tach would work after getting above 1000. wiggled the wires coming out of the sensor (looked ok) and wiggled the connector. problem went away (for now)

 

Cleaned off the sensor really well with ether and it still won't work, I will try to ring it out with the DC meter to see if i get anything