So the needle would hit 3500 rpm in high gear and bounce a bunch and go to zero temporarily. With the air or heater on, the tach would not register over zero until I would hit 1800 to 2000, then suddenly spring to life. Could not for the life of me figure out ---WTF?

Then I replaced the mono valve heater control. All seems to be well again. No odd tach readings or needle bounces at all. Strange. Care to postulate a theory--- comments welcome

Regards

Run-em
1983 S D-aka-SPARKY THE DIESEL

Check tach amp (on drivers fender...black thing) and make sure its connected well.

Mine used to do this too...it eventually died...and I replaced it with a junkyard unit.

Do a search for "cigarette butt".....this little trick worked to "fix" my tach.

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1983 300SD "Helga"