The cars runs? That means the tacho is letting the current through to the ballast resistor and coil. The pass-through on the tacho is working.
According to the pictures, the wire which supplies the tacho itself is a female spade. The supply that gets to the coil is fed from that spade into a female bullet. The car runs, so barring that the spade is dirty and not making full contact with the male spade on the tacho, the tacho is getting a live supply.
This may not be readily apparent, but is the cluster earthing properly? From the thread, it seems you have tried everything bar that. On the picture there is a black wire with an unsheathed female spade, this must be the earth for the cluster. Check that is actually earthing.
The warning lights and gauges all work (bar the tacho, of course!), but they all have a 'constant from the ignition switch' live supply, and are activated/controlled by giving them an earth (the warning lights (bar the choke warning light and alternator/IGN lamp)), or a resistive path to earth (the gauges, the choke warning light and alternator/IGN lamp).
The only other thing that depends on the earth of the cluster is the clock and (possibly, I am not sure how they are fed/earthed) the back illumination lights (the lights that illuminate the dashboard at night!). Are either of them working?
If it is not an earthing problem, it may well be you have 2 faulty tacho units. (A bit unlikely, but not impossible)
For the record and as a matter of interest:-
Any ideas? I don't need an RVC counter do I?
No, as Brian says, RVC senses the voltage fluctuations. Brian's tacho, like yours, senses current fluctuations. If you were to wire a RVC counter into a RVI loom, the current reaching the points would be zero (or insignificant). The car would not start!
Conversely, if you were to wire a RVI counter into a RVC loom, the points would be shorted, no current reaching them, so car would not start then either!
My (Brian's) 1971 model has the rev counter on the -ve side of the coil in series with the dizzy. <=== RVI
Your 1972 model appears to have the rev counter on the +ve side of the coil in series.<=== RVI
in 1973, they put it back on the -ve side of the coil, but NOT in series with the distributor!!<=== RVC
The RVI counter measures the number of times the
current swings from 0 to full current. Though Brian's tacho taps current from the coil to dizzy wire, and yours from the supply to the ballast resistor (and then to the coil), the number of times the current swings at a given rpm at any given interval of time is the same.
I hasten to add the full current may well be different on Brian's LT system than yours (I am not sure if Brian's car has a ballast resistor?), but the number of times the current changes in a given time at a given rpm is the same.
An RVC counter measure the number of times the
voltage swings from 0 to alternator/battery voltage (even with a ballast resistor [With the points closed, the voltage on the dizzy wire is 0V. With the points open, no current flows, therefore I x R = 0, there is 0 volt drop in the ballast resistor, so alternator/battery voltage is 'sniffed' by the RVC tacho])
