In about 1973 the current sensing tachometer was replaced by a superior design that senses voltage pulses from the contact breaker. At about the same time, the coil was changed to one requiring a ballast resistor, current flows from the batteries or battery on a large cross section area cable to the starter motor/solenoid which is used as a securing point. There it connects to a thinner brown (N) wire that goes to the fuse block. At the fuse block another brown wire takes current to the ignition switch. When the ignition switch is on, current can flow from the ignition switch on a white (W) wire and back to the fuse block. A white wire spurs off from the fuse block to supply current to the coil + terminal via a ballast resistor and white/light-green (WLG) wire.
However, during cranking, the coil is supplied full voltage via a white/light green wire from the starter/solenoid, effectively jumping the ballast resistor out of circuit. The ballast resistor may be either a length of resistance wire or a discreet electrically resistive component. The – side of the coil is connected via a white/black (WB) wire to the distributor, which makes a connection to the contact breaker.
The later MGBs had a 9 volts coil and a ballast resistor wire in the loom. Cars with ballast resistors have 9v coils. There is an extra terminal on the starter. When you are cranking the motor the extra terminal is live and sends 12 volts to the 9 Volt coil, bypassing the ballast resistor. Coil gets hot, but not very,as cranking doesn’t take long. When the motor is running the terminal is off and the voltage to the coil goes through the ballast resistor which reduces the voltage to 9 volts. The voltage is not 12v due to the starter load. Even if it were, the coil (and points) can easily take the 35% overload for the brief interval during cranking.
It is true, however, that if you bypassed the ballast resistor (and continued to use a 9v coil) then the extra current (and the even higher voltages formed by the coil’s larger field collapsing) would burn the points prematurely.
In 1977/8 the ballast resistor is a very long special wire that runs out past the radiator, ( but you won’t find it without a lot of digging ) in the light wiring bundle, across to the left side of the car and doubles back to the coil. It’s made that way to spread the resistance, and thus the heat, over a large area instead of being concentrated in one small “hot spot.
How can you tell if you have a ballast resistor ?
If your starter motor has two small wires attached to two seperate small posts, and your coil has two wires to the +ve post, and one of them matches the colour code of one of the small wires on the starter you have a ballast resistor somewhere. That extra small wire bypasses it while the starter is engaged. Some ballast resistors are a ceramic piece with a coil of wire in it and others are long wires of calibrated resistance hidden in a wiring harness.
In a 1977/8 MGB, for instance, the resistor is in the forward wiring harness. It goes out front near the right side of the radiator, across the front of the car, and loops back to the coil. By spreading the resistance over a large area there is very little heat build-up and the and the ballast resistor is not likely to ever fail. Small ceramic ballast resistors concentrate the resistance, therefore heat, in a small area.
The classic symptom of a failed ballast resistor is:
The engine will run while the starter is engaged, but will not when it isn’t.
A bad connection at one of the wires to the plus post of the coil will display the same symptoms.