Posted by Sherman in Idaho [108.162.246.17] on Tuesday, October 18, 2016 at 12:23:16 :
In Reply to: Re: speaking of coils posted by TGP (IL) [172.68.58.165] on Tuesday, October 18, 2016 at 11:23:29 :
There seems to be a lot of misunderstanding of the purpose of a ballast resistor. It's there so that you can bypass it while starting. Seems kind of weird, but it's actually clever. The battery voltage can drop by half during hard cranking. By cutting out the ballast resistor during cranking, you ensure that it gets full voltage to the spark.
Things have to wired up properly for this to happen, though. If the ballast resistor is just in series with the coil with no way to bypass it, you can have starting issues. One symptom is that it doesn't start while cranking but sometimes it starts (or tries to) the moment you let up on the starter. If the engine's still spinning from inertia when the starter load is removed and the battery voltage jumps back to normal, that can be enough to get it going. Had a mickey-moused M37 that did that.
In general, there should be no ballast resistor if you're using a floor-pedal starter because there's no way to bypass it. You can do it with a relay or a diode but I just put a spring-loaded toggle switch on the dash. It'll confuse the thieves even more. But if you have a starter relay (the cylindrical kind about the size of a shot glass) it should have an extra terminal for bypassing the ballast resistor. Don't try to hook the coil direct to the starter motor, thinking that will give it full juice while cranking. It will, and the engine will start, but as soon as you let off on the starter it will stall because the starter motor will ground out the coil.
A lot of old rigs with "modified" wiring end up with the coil/resistor circuit mis-wired, so that the resistor is in the circuit all the time. It causes frustrating intermittent starting issues that can result in chasing a lot of ghosts. It's always smart to check the voltage at the hot side of the coil primary (the terminal that doesn't go to the distributor) and see what happens to it during a starting sequence. With a 12V system, you should see no less than 6V at the coil (to ground) while cranking, and there should be essential zero voltage drop from the battery ungrounded (usually +) post to the coil hot terminal while cranking, but that should increase to 3-6V or so while running if the resistor is doing its thing.
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