1986 SDL Tach reads fast
 

The tachometer on my "new to me" 1986 300SDL is reading consistently about 60% fast. It indicates 1100 at idle and 5000 at 75 MPH. It is not jerky or intermittent so I am ruling out a loose connection. Is there an adjustable dashpot (or something similar) somewhere in the system? Where does the tach get its signal from? I looked through the FSM but found nothing.

Usually the tach problem is assocciated with the sending unit mounted on the drivers side inner fender, in front of the GP relay. It is round, with a plastic retaining strap, this unscrews from the plug. You will note a cable leading to the front of the engine at the flywheel. I am unaware of any adjustment.

'85+ do not have a Tach Amplifier.

I appreciate your response. The round Tach Amplifier does not exist on mine. I am under the understanding that in 86 they switched to a different system.

Anyone know how the tach is signaled on an 86 or newer?

1985 and newer do not have the Tach Amp.



It's the EGR computer down in the passenger kickpanel that controls it as I recall.