Tach variation

[milomilo]

I have a 1989 Ranger, 2.9, and the tach reading will change often, at all speeds. It varies from 2500 to 3200 while driving at highway speed of 60 as an example. Sometimes it will do this when cold or hot. It lasts from 2 to 30 minutes and then goes back to normal. Last oil change I added a quart of Rislone, thinking it would clean off any sludge from any sensors, but no luck so far.

Where is the sensor for the tach so I am remove it and see if it needs cleaning or replacing?

 

[IMenriched]

Prolly a bad connection somewhere...but you prolly already suspect that.

not quite exactly info your searching for...but maybe it will help.
http://www.therangerstation.com/tech_library/Tachwiring.html

 

[Rangermilitia]

tach sensor

on most 2.9 the tach sensor is in the distributor, the hall effect sensor. try replacing that. recon its a green wire off of the distributor tfi module. you can also hit it on the driver side of the engine bay. green/yellow i think. just start the truck and start unplugging. the one that makes it die harnesses the right wire... well thats how i found it

 

[holyford86]

if the truck runs fine and there are no codes pointing to ignition problems then the problem is elsewhere, replacing parts and hoping a problem will go away is not a way to diagnose the problem. There are no sensors inside the engine for the tach signal, but if there were they would probably be hall effect and unaffected by the nonmetallic sludge. The green wire at the coil is the tach signal wire, check the connection there and if the connection is good try attaching a tachometer there and see if your rpm problem exists on a different tach. Need some more info here, is this the factory tach or is it something that you installed?