1979 300SD Tachometer Problems
 

How exactly does the Tach work on a W116 300SD 1979? Mine shows the correct RPMs at idle but when you throttle up the needle drops to 0...then goes back to the correct revs when you go back to idle. I don't know if the amplifier is bad or what...any ideas?

99.9% chance your tach amp is bad. It is the little black cap looking thing under the hood by left front fender. Unscrew that. Then pry the two parts apart and stick a cigarette butt or something that won't catch on fire and put everything back together. If it fixes it, then you know your amp is bad. Head on over to eBay to buy another one.

They fail every so often. I think I'm on my 3rd one with my 116 now.

The digital tach amp upgrade worked well for me, it has been some time though and I don't remember the username of the guy selling them