Tach problems

[John T]

I have a 1995 Mack RD690s. Wondering if anyone would have an idea of why the tach gauge would work when it is cold but once the truck warms up the tach gauge changes readings. When the truck is cold it idles at 800 rpms and revs to 2200 rpms as soon as it warms up the gauge shows idling at 400 rpms and revs to 800 rpms. It does not seem like the engine idles different when the change occurs. Any ideas would be great. Thanks in advance 

 

[mrgumby]

Have you changed the sensor   Or is it cable drive.  Seen the sensor cause a tack to do strange things. Had one not move much couple hundred rpm then stop wiorking then start again and one spun the tach all the way around 

[John T]

I have tried the sensor, same out come not sure the proper gap between the flywheel and the sensor I just matched it up to the old sensor and installed. Any idea what the distance is supposed to be?  

[Rob]

The countdown circuitry is failing internal to the tachometer itself. Probably a simple electrolytic capacitor but not cost effective to repair really. You install the "Hall Effect" transmitter threading into the housing till it lightly touches a flywheel tooth. Then back the transmitter out 3/4 of a turn and lock it into place.