You will need less than.01 voltage drop on the positive cables to the starter. You will need less than.1 volts on the negitive cables all the way to the engine frame. You also may have a voltage drop at the control wire with that age of a unit I would figure your going to find a reading of 1 volt or higher in that circut. if so use a remote starter to try and crank the engine. If that works i would install a bosh relay to activate the starter with the imput from the key switch.