tachometer interchage

I have an 82 and an 84 300d. Tach on 84 works ok, tach on 82 doesn't. The 82 has the front pickup and the black cylinder on left fender wall. 84 picks up on rear of engine, no black cylinder on left fender wall. I've tried all the cigarette butt tricks in the 82 cylinder fitting.

The question is, are the gauges interchangeable? Would the 82 work in the 84 if all else were ok. If, for example, I take out the gauge from the 84 and put it into 82 and still doesn't work and the 82 into the 84 does work, then I know its the circuit to the gauge, not the gauge itself that is failing.

Thanks for any help.

[BoostnBenz]

84 picks up on rear of engine, no black cylinder on left fender wall.

I believe they are the same systems, both my 83 and 84 have the tach amp in the left front fender wall. The tachometer on my 83 & 84 would swap if needed so I suppose your 82/84 should but I really doubt this will help. Why not just get another used tachometer amp off of ebay? You could probably get one pretty cheap.