Bad Igniter?
 

New here and looking to learn.
I have an 83 base model (S?) with no mods. When I first start up my Tach does not work, but once the engine is warmed up the tach slowly begins to start working (in the summer it took about 5 mins. Now in the cold it takes about 20 mins). Also if I restart the car while still warm the Tach works fine????Can't be a loose wire, otherwise it would be intermittent? Do igniters come to life once the engine is warm? Lastly, it is ALWAYS like this, once warm the tach works, when cold it does not?
Anyone got a suggestion?

Igniters have been known to be temperature sensitive. Tachs, too. And dirty wiring connectors can also act that way.

If the only problem you have is the tach (no performance issues), then I would use a timing light to quickly verify the trailing ignitors are firing, then focus on the tach itself, on possibly dirty ground points, and on wiring.

All the searches I've read on this problem either say it's the grease in the pivot point of the needle on the tach, or it's maybe an electrical soldering problem (known to be shaky). I have yet to go through and re-solder all of the dash unit, but I've thought about it. Keep us up to date, maybe you'll come up with something we haven't thought of yet!