Hi John,
The system was designed to switch a 3 Ohm load (12 v coil). The coil is the load.
If you run a 1.5 Ohm coil (6 v coil) the transistor and the internal resistor (I'm pretty sure one would be fitted), will be switching a much higher current and the dissipation will be very high.
The 12 volt feed to the unit just powers the electronics.
It's late, but i'm pretty sure I am correct?
Cheers,
Iain.