Tach affecting trans shifting?

[The Warden]

Hello!

The tachometer on my 300SD works when it feels like it (read: very rarely). I'm pretty sure the tach amp needs to be replaced. However, I've noticed something strange. When the tach's working, my shift points change. When the tach's non-functional, the tranny shifts at about the RPM's I'd expect for an OM617, but when the tach is working, the shift points go up almost so high that I have to completely let off the throttle for the tranny to shift from 3rd into 4th when getting onto the freeway.

I thought these trannies were only manipulated mechanically and via vacuum (except for the kickdown switch). Is there another electrical connection somewhere?

Any thoughts would be greatly appreciated...thanks!

[The Warden]

Finally have a chance to reply...work's been insane...

according to my parts database, the 300sd still uses a mechanical linkage to the transmission for throttle pressure, which determines the shift points. The vacuum modulator, which controls shift firmness, has alot of whizzbang attachments going to it - obviously because a diesel engine produces NO vacuum on its own.

Actually, on that note, does your database say anything about where the vacuum modulator is? I know that on the 123 and 126, they put it on top of the valve cover, but all that's up there on the 116 is the mechanical linkage (which I recently completely re-greased...the difference is night-and-day!!).

Id be interested to know how the tach works on a diesel anyway! unless theres an actual home position sensor on the crankshaft or something. . . now ill have to do my research so i can sleep at night

I'm pretty sure that there's a crank position sensor. I've never looked at how the tach's set up on an M-B, but that's how Ford did it. I think Dodge did something similar with the first-generation Cummins diesel as well, and that's how all aftermarket diesel tachs I've seen handle it (except a few which use a sensor on the alternator to sense the field, but I personally don't trust those as much).

The simplest diesel tach setup I've seen is the one that Isspro has. You glue a magnet to the crank pulley, and put the sending unit on a supplied bracket in the immediate vicinity. The sending unit is activated by the magnet, counting how many times it goes around to get the RPM's.

There's also the old mechanical-drive tachs, seen mostly on truck and marine diesels...works sort of like a speedo...