i have a 1985 lb9 that somtimes at idle the tach will jump up to 4000 rpm but the engine is actually idling. this also sometimes happens while crusing between 1500-2000 rpm, tach jumps up to 4000 rpm but the engine is still just crusing... what should i look at for problems first?

 

SD,

There is a tachometer buffer circuit module that conditiones the tach signal to a useable analog signal for the dash instrument.

Check the wiring at the distributor, the connectors for the instrument panel backplane, and the tach module. It should have a white wire from teh module to the inst. panel.

------------------
Later,
Vader
------------------
"Let the bodies hit the floor!"
Adobe Acrobat Reader

 

where do i find the tach module? is it located somewhere on the firewall or under the dash?

 

It's fastened to the back of the tach itself, inside the cluster.

A bad cluster ground will also cause strange behavior... if more than one strange thing happens at the same time, it would be time to start looking at things like that.

------------------
"So many Mustangs, so little time..."
ICON Motorsports

 

cool, i'll check it out...