Just installed the 52mm tach in the 1978 Volvo 244- purchased off of eBay. After install seemed to work well for a few drives……but now…basically some times it will just go flat to zero then it will pop back to working then some times it gets stuck at 4K rpm? Then stop working again- getting the positive and negative from the pins in the main board- and used a wire that had already been attached to the coil-

Any ideas- the fact that it sometimes works makes me think it’s a short of some kind- and not a faulty gauge….

 

The tach is a nice addition. Unfortunately once installed there is virtually no way to tell if the problem is in the tach or the car's wiring. I would remove the tach and hook it up to the correct signals and power with wires totally external to the car's wiring. Run the wires from the engine compartment out the corner of the hood and into the driver's window.

You should also look up the correct connections in the Volvo Wiring Diagrams Manual available to download for free from https://ozvolvo.org/archive. While you are there download any and all manuals and literature for you year. You never know how long a good thing will last on the internet.