Okay so I have an 89 lifted 4 with 33/12.50-15s and I do understand I will need a 30 tooth speedo drive gear to get it on line speed wise.
What is bugging the hell out of me is my tach is dropping below 0 ( almost 750 rpm I am guessing) after I start it up. Sit at 0 rpm when not running,
Soooo anyone have this little pain in the butt problem? Could use the answer before I take the dash and go aftermarket.
Which with only 135k on it seems like a losing deal with everything else working as should.

 

It's likely to be a bad tach, or maybe even simply a bad solder joint on the back of the tach. There's also been an issue with some (not sure of the years) XJs that have a connection issue at the back of the cluster. You might just be able to clean that connector and make it work again.

 

agreed with agreen here, however your estimate of RPMs match what mine says electrically. per NickInTime designs: – Engine Idle (~600-900) Proper Idle should be around 750 RPM
REM Gauge Readouts Summary – NickInTimeDesign

 

Thanks, pulling it out Monday have to finish my Harley build first.
It has been down for a month and I am going nutz not riding it.
Then it's the dash while the motor is at a machine shop to work
the block and heads of the 4.5 stroker build on her.
Again my thanks for the info.

 

Yes Sir Cyber I'm an ASE Master tech for over 25 years and the
majority of the time as a tune tech so 750 +/- was an easy # to come up with.
Thank for your input. Was hoping for an easy fix and a resolder or cleaning works.