Ignition nightmares
4 pin ballast and an ohm meter
OK. Look at the number of pins in the connector on the ECU; if it is 5 pins you MAY need the 4 pin ballast (dual ballast). If there are 4 pins in the ECU connector, then you don't need the 4 pin ballast anymore. What is the part number and source of the new ECU? Look to see if it listed for many years; if so, it has to be able to run in a 4 pin configuration (single ballast).
Everyone please check me, but my understanding is that no one is actually making a true 5 pin, 1973 ECU, anymore. They are all really the 4 pin ECU that does not need the 2 part ballast resistor. You could use a 0.8 ohm MSD ballast for the coil circuit.
Now the next question is if the 4 pin ballast is installed backwards. This will put the high resistance half in the coil circuit where the low resistance half should be, and vice versa. This would effectively starve the coil of current (and may be what is burning out the ballasts). One half of this 2 part ballast is around 5 ohms and the other is < 1 ohm (cold). The low resistance half should be in the circuit going to the + terminal on the coil. (And BTW, during cranking, the ballast is bypassed.)
There is some useful info for you in this thread for another '73 system on the connections and ballast resistances:
Fluctuating neg. to ground on ignition coil
And I'd like to ask again what is the part number (PN) of the ballast you have now. Just for the record....
