Tach won't work in cab

[raysr11]

Hello.  I have an '85 C10 with a 350 and an early 4 pin module distributor(hei).  I have been having one heck of a time getting a tach to work.  I hooked a known good wire and new connector to the "tach" terminal and the tach will not work in the cab.  So, I took the cover off the coil and clipped the tach signal wire directly to the yellow wire coming out of the coil and the tach worked.  So, the tach won't work in the cab with 6 ft of wire connected but it will work clipped right to the coil wire/connector.  What could be causing this?

[bd]

Is it a factory style tachometer mounted in the instrument cluster or an aftermarket standalone model?  Does the tach signal wire have an inline filter installed?  Tach heads require three electrical connections to function:  1) 12-volt ignition, 2) ground, and 3) tach signal.  Standalone tachs may have a backlight connection to the dash illumination circuit, in addition.

Symptoms suggest that one or more of the tach head electrical connections is/are open.  It isn't totally clear from your description whether the only difference between the tach 'not working' and 'working' was disconnecting the existing tach signal wire from the TACH plugin on the side of the distributor cap and then reconnecting it by jumping directly to the yellow coil negative such that you simply bypassed the factory style connector  OR  if you temporarily relocated the entire tach head under the hood during your test, establishing direct connections using completely different wires.  Clarification of the details will provide better insight for assistance.