Still perplexed by tach not working

[stevef]

Anybody shed some light on why my tach won't work. I'm an electronic neophite so help me. I have a Marrelli Distributor in my 71 2.2T. The purple and black lead is connected to the points as per the photo

I disconnected the purple/black lead and bypassed the distributor and connected it to the LT side of the coil and get a good signal. The tach needle moves to about 4500revs at idle. So I know something about the circuit to the gauge but I don't know what. When I move ot back to the dizzt....nothing, back to the coil....4500rpm indicated

The rear electric panel has a couple of boxes on it and I can't tell if these are the speed switches that the wiring diagram tells me I have.



I can't understand why the distributor is not producing a signal. I've checked and reset the point gap at .016". Curiously I can't do a static timing check. My bulb does not light up when connected across the points to ground when the points open. The car runs great and I'm getting a good spark and have set the timing with the timing light with no problem.

What am I missing, anybody?

[Early_S_Man]

Steve,

The distributor is producing a signal good enough to trigger the CDI unit or your engine wouldn't run!

Here is a simplified diagram of the speed switch, CDI, and tach wiring for a '69 MFI car that used the old style tachometer and ballast [a form of Voltage divider] unit, which reduced the 400 Volt pulses to the coil down to usable levels for the tach. You may want to remove the electrical panel and trace/examine the path of Black/Violet wire to the tach ... if you find a strange cylindrical aluminum can about 1" in diameter and 2" long with three wires going to it ... you may have the answer! I have heard of other '70/'71 cars that used the 'ballast' unit and tach ... even though the '70 wiring diagrams don't show the ballast unit in the tach circuit! I suspect that existing stocks of tachs and ballast unit were used until exhausted, and it may not have exactly coincuded with commencement of '70 model production when the later tachs and circuit were phased into production. I also suspect that different phase-in dates occurred for 911T, 911E, and 911S models because of the different tachs in each model!

[stevef]

Warren, Thanks, I'll look for the ballast unit. My sense is that it doesn't have one. In the picture of the rear electric panel there is a black relay sticking up through the panel. I wonder if it's some type of relay someone has plugged in there mistakenly? From what you tell me here it could be that I might have to run the wire from the coil through the ballast unit to the tach. I have pieced this car together. The panel is not from the original car. I installed the 2.2 engine with a panel from 2 different cars. The PO ran a 2.7 in it until it seized so I can't be sure what tach I have. If it turns out I need a ballast unit are they available?
Thanks