This is formerly Allison XR700, now Crane/Allison or Crane XR700. Click on image at right to download a 20-page installation instruction in pdf format. More information for this unit can be found on the Crane Cams web site.

Diagram above is for negative earth connection (showing impulse tach).
Diagram below is for positive earth connection (showing inductive tach).

The same unit can be wired for either negative or positive earth setup. Compare the two diagrams above (from the PDF instructions). The module only needs + and - battery connections, and it cares not which one happens to be grounded to the chassis. The new pickup module in the distributor does not ground through the distributor body, so the entire electronic system is "floating" until one of the power leads is grounded.

The XR700 unit is designed for ballasted ignition system, a needing 1.6-ohm coil and 1.6-ohm ballast resistor. The electronic unit normally runs on 6-7 volts and tolerates a momentary 12 volt input during crank starting (same as the ballasted ignition coil).

The XR3000 unit is designed to operate continuously on 12-14 volts. This unit would be better suited for the older cars with no ballast resistor and 12-volt coil.

To use the XR700 unit in the MGA you need to use a 1.6-ohm coil and add the ballast resistor so the coil and electronic unit run continuously on 6-7 volts. There will be no bypass wire for full system voltage during cranking. System voltage may drop to 10 volts during cranking, and coil & module input may drop to 5 volts, but it should still spark okay during cranking.

With the XR700 unit in the MGA, if you want to use the full voltage bypass for cranking you can connect to the output side of the starter switch, but then you need to add a diode (10 amps capacity) in series to prevent drain back through the starter motor during normal operation while running. Alternately you could install a relay triggered by the starter switch to connect full system voltage to the coil momentarily during cranking.