Adding a tach to a 300TD
 

I want to add a tachometer to my 300TD - up until now it just had the big clock in the instrument cluster.

I've gotten a tach and tach amp at a salvage yard, now all that remains is how to hook it all up... I hope it's possible!

The plug for the tach amp on the driver's side fender wall is there; there is a gray wire leading somewhere into the engine that has three connections to it, and on the other half there are two more (in positions #2 and #6) that lead from the socket toward the firewall.

Stripping back the insulation on those two wires I've discovered they're power and ground wires (ground to #2 and power to #6). Nothing else leaves the tach amp socket for the firewall, but the tach amp cylinder itself has a prong at the #3 position, so presumably that's where the tach signal comes from (:confused: I'm guessing here).

At the back of the tach gauge itself are two prongs... I have the socket for this but of course I have no idea what to wire to it. Presumably the wire from the #3 position at the tach amp socket would go to one, what about the other?

Am I totally off? Does anyone have any idea? I wish I had a wiring diagram or something for the tach! Has anyone else attempted this before?

Did it!
 

Thanks to something posted by dieseldiehard in another thread (http://www.peachparts.com/shopforum/showthread.php?threadid=79238) I managed to get my tach functioning.

For the record, it turns out the green/black wire from the tach plug at the instrument cluster goes to the #3 prong on the tach amp, and the red/black one just goes to switched power. Everything works like a charm!

Not to rain on your parade, but how do you know it's accurate?

Sixto
95 S420
87 300SDL

I don't.

That said, it can't be too wildly off; it shows my idle speed to be about where it should and responds as expected to revving the engine... My car is in an undrivable state at the moment so I can't test more than that right now.

California emissions model tach
 

I wonder if it would be possible to rewire my '85 CA car's tach to work like the federal model? I can't figure out the problem I have since I can't find a wiring diagram specific to the CA model. I'd would like more than anything right now to have a working tach. The tach itself is fine but there is something that has gone bad within the wiring itself.

From my research in trying to get mine installed I found a lot of messages on the MBZ.ORG diesel mailing list from people who had done that. You might want to check it (use Google's advanced search option to search diesel.mbzarchives.org). It seems you can just bypass the EGR computer completely and send the wires straight to the tach.

Thanks for the advice! I somehow knew that the EGR comp has something to do with it.

Does not the FSM cover all the fancy pantsy emmesions crap on the 85 cali model? I thought I remembered flipping past it a bunch of times..?

But by all means, bypass away!

All I have is the CD manual and it seems to only cover the fed versions. Any chance ,as a favor, you could scan and e-mail me those pages?
~DJ