yes yes yet another tachometer post. sorry.

anyways the vehicle in question is an 85 w123 300d California model. so its a 3 prong/crank trigger tach specific only to that year and possibly 1984.
so far ive bought a new engine computer, and gauge cluster, also grabbed a new pickup from a w124 om603 at the junkyard. still the thing does not work.
i realize the easiest method would be an oscilloscope and to check the signal directly, but im not an electrical engineer nor do i know what exactly to look for aaand i aint got one of them electrical gizmo boxes (yet ). sooo before i do that... what part am i forgetting to throw at this problem. the new computer fixed the temp gauge but the tach still does not work.


back story behind the tach failure, i fried it by hooking up a battery wrong in the dark with the engine running. alternator is fine the old computer and tach were not. this car has an om 603/5speed swapped in place of the original 617/slushbox. when i first completed the swap the tach functioned normally. only after me being stupid did it fail. so if that gives any insight to my problem please let me know lol.

As far as the "computer" goes, are you talking about the unit under the plastic kick panel on the right side? I have seen the pins on the connector corrode. This will also prevent the A/C to work, as the KLIMA needs a tach signal to engage the compressor. However, reading your back story, you may have something far more serious than just some corroded pins. Either way, be sure to use a KNOWN GOOD tach amplifier (computer). I am not sure if James Dean (user here) still repairs these units anymore, but give him a shout.

EDIT: I am curious how you put a 603 trans behind a 617?

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All Diesel Fleet
1985 R107 300SLD TURBODIESEL
2005 E320 CDI (daily)

LOTS of parts for sale!


EGR block kit
http://www.peachparts.com/shopforum/diesel-discussion/355250-sale-egr-delete-block-off-plate-kit.html

1985 CA emissions 617 owners- You Need This!
Sanden style A/C Compressor Mounting Kit for your 616/ 617 For Sale + Install Inst.

Sanden Instalation Guide (post 11):
http://www.peachparts.com/shopforum/diesel-discussion/367883-sanden-retrofit-installation-guide.html

yes im talking about the egr ecu thing. 003 545 5132 pass kick panel.
i already replaced the computer with a ebay unit same deal for the gauge cluster. im pretty sure the computer works cause my temp gauge started working again once i replaced it. i guess the next logical question is how do i check it? scope and signal? orr is there a more shade tree method? im guessing i just need to pony up and buy a scope...

anyways besides the computer, tach it self, and pick up wire, is there any part of the system im missing? also is there a consensus as to what happens or what is fried when you do what i did? (hook up a battery wrong while running)

to answer your question about the trans, i kept it attached to a 603. did some other stuff too. the trans days are already numbered.

ill look for that user and dm him. thank you.

well i resolved the tach issue by finding a picture in a thread on here lol. there was an overload relay behind the glove box on the passenger side "a pillar" that had a burnt fuse. final piece of the puzzle. also this was the final gauge issue i needed to rectify. even the clock works now lol. thanks to this bulletin board

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85 300d 0m603, 717, he341, 50mm external W/G with open dump, intake manifold as well as intercooled. need to figure out a different trans before i throw more fuel at it. the 717 trans and 8" ceramic clutch are already pissed with stock elements.