[diesel_mercedes] Reasons for a 300SD tach that doesn't go over 1900 RPM?

 

One of the discoveries I made while running Son's 300SD with him yesterday is that the tach doesn't go over 1900 RPM.  It seems accurate below that point, but when it gets to that point, it just stops moving as if the needle had just hit a peg.  It does this whether I'm revving the engine with the transmission in park or if I'm out on the highway--there's no way that engine is turning 1900 RPM at 70 MPH.


I can't remember if the tach circuit is the same for the 300SD as it is for the D.  Any ideas what could be causing this?  I haven't yet pulled the binnacle, which I'll need to do to replace all the burned out dash bulbs, so I haven't actually inspected the tach mechanism yet for any kind of physical obstruction.


Interestingly, this tach works consistently and stays on.  The tach amplifier seems to be working properly....


--Conrad J.

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Posted by: conrad.jacoby@gmail.com
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