I've pulled a few junkyard tachs from 240's in the past. To check before installing (or putting them on eBay), I place the tach on the slam panel by the battery, and use little jumper wires with alligator clips to connect the tach's proper terminals to battery pos and neg, and its #1 terminal to the coil's #1. This of course bypasses all the car's internal wiring and the instrument cluster's copper traces and pins, and leaves the tach as the only suspect.

Then make sure nothing is touching chassis metal and start the engine. I've had about 1 out of 4 prove bad! I also check accuracy using an automotive multimeter with an inductive pickup on a spark plug wire. I've learned that those old 240 tach's are quite accurate - when they work.
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Bob: Son's XC70, my 83 244DL, 89 745 (Chev LT-1 V8), 98 S90 (recently sold) and XC60. Also '77 MGB and four old motorcycles