MSD Tach adaptor troubleshooting

[hemi68charger]

Hey gang..
Trying to troubleshoot why the tach in my buddy's '71 Challenger isn't working with his MSD tach adaptor. This is what I did to isolate parts of the system:

1. I hooked up the feed from my '70 T/A with standard ignition to the feed terminal on the back of the '71's tach and used the 71's 12v feed on its own tach. I turned the ignition on the '71 and cranked up the T/A, the tach on the '71 worked..

2. I fired up the motor in the '71 and grounded the white wire from the MSD box which is the tach feed and the motor died ( so I am assuming this is what should happen ).

3. I connected the tach feed ( white wire ) from the MSD unit on my Daytona and fired up the Daytona with the ignition on in the '71, the tach didn't work.... mmmm? The tach works with the MSD tach adaptor in my Daytona.

Is there something different between a '69 B-body tach and a '71 E-body  tach? I would think fundamentally, there isn't... Any suggestions?

The four wires of the TACH adaptor are wired properly:
1. white to the tach feed from the MSD unit
2. black to ground
3. violet / purple to feed post of tach
4. red to 12v feed

Cheers,
Troy

[Charger-Bodie]

Why do I have myself thinking you need the purple wire for our tachs?

Edit: sorry just saw your chart.

[BLK 68 R/T]

Is it possible the tach in the 71 has a bad/loose ground and when you were under the dash messing with wiring you just happened to bump it into the right place when you had the 70 t/a connected and it worked? I know the 68-70 b body tachs have a metal housing which I believe grounds to the instrument cluster via the mounting screws. Some of the reproduction tachs are a plastic housing.

EDIT: I was looking at the MSD wiring diagram and it shows the grey wire as the tach wire, not the white wire. Think this might be your problem.