1984 300SD tachometer troubleshooting

[dshaver001]

Hello folks!

This is my first post here, and I want to say thank you to the members of this forum as it has been an excellent resource for me up to this point!

I recently purchased a 1984 300SD after quite a search. The car is running reasonably well, but the paint is pretty ugly, it needs an hood, and has some issues that I’ve been slowly tracing. I’ll be chasing oil leaks for the foreseeable future, but I’m enjoying bringing the car back to its former glory. I hate to see these cars neglected. I’ll have about a thousand more questions as I fix it up, I’ve already found a few older modifications done to this car that have been making me scratch my head and I’ll need some help making them right.

Now that my long-winded intro is out of the way, here’s the issue I’m dealing with that I can’t seem to sort out. I am troubleshooting an inoperative tachometer, and after checking a few likely suspects and electrical connections that all look good, I decided to test the tach connector at the back of the gauge cluster with a multimeter. I seem to be getting power to the instrument, which led me to believe that I likely had a bad capacitor in the instrument itself. Once I pulled apart the cluster, I noticed that somebody had replaced a capacitor before, as well as the two capacitors for the clock. Through reading on this forum and others, I have surmised that the correct values for the clock capacitors should both be 100 μF. Whoever replaced these capacitors used one 100 μF and one 47 μF. This makes me wonder if they used the correct capacitance in the tachometer, or not. There is a 22 μF cap on the tachometer, just above a blue cylinder (another capacitor?) and that appears to be the only newer solder on the board. I can post photos if necessary. Is this the right value for that cap? If not, could it be the source of a totally dead tach, no movement of the needle at all? Does anybody know the correct value and anything else I might need to replace in that circuit? I don’t see any burnt resistors or bad solder joints.

Thanks in advance for your help! I’m a bit of an electrical novice, I know some basic safety and can solder pretty well but I’m an absolute hack when it comes to reading schematics.