Tachcometer not working.
 

I cannot find something in the cars wiring that is most likely a fuse. This is a 1986 300SDL that had to be boosted to start at work, noy once but twice. I don't have a manual so I am hoping someone with a far greater knowledge than I can help. It would be greatly appreciated.

Just about to turn 300K on the old girl, and she seems to be quirky this week. Besides I gave away her best friend to a son (300D) and park her by a 65 Buick convert, maybe it's hurting her feeling.

Check the fuse in the overvoltage relay! Is the ABS light on as well??

On my Range Rovers, tach is run by signal from the alternator. Not sure on the Benz. A bad alternator could be the cause of your starting failure.

Diesels can't use a "normal" tach pick up, so they have a little different system. I understand that on the W126 diesels, there is a plug on the driver's side of the engine bay that is part of the tach system, and that plug can wear out over time, and the connections become loose. This means the tach stops working even though the other instruments are all fine.

Tach
 

No ABS light, where the heck do I find the overvoltage relay?? Checked in the fuse box and the one listing the tach was good. I am for sure not a mechanic so I do appreciate your help.

I will recheck the alternator output, but it could be a contact there. It was not an easy start, the battery had shorted.

Since it worked up to the point of the battery dieing I don't think it is the plug in, but something to check as well.

Lost Tach
 

it appears the ABS system might not be working, the light is not on, but no longer comes on when you turn the key on as it used to. Where would this over voltage relay be located??