Im working on a 57 truck 235 motor stock wiring. I noticed that on the stomp starter switch there is a threaded stud with a nut on it. After doing further research, I read that a wire runs from that stud to the positive side of the coil.
My truck has been running without it since I bought it about 10 years ago. I don't drive it much, but wanted to start getting it on the road more often and I'm giving it a full tune up thought this would be a great time to get some information on that missing wire.
It is the ignition resistor by pass circuit, sending full B+ voltage to the points & coil when cranking. 12v systems have resistance wire or resistor to bring down voltage to keep points alive. This disconnected wire will result in low voltage to the coil primary windings = weak spark when cranking. If you were in a cold climate it would have caused issues by now.
BC 1960 Chevy C10 driver 261 T5 4.10 dana 44 power loc 1949 GMC 250 project in waiting 1960 C60 pasture art Retired GM dealer tech. 1980 - 2022
Since the amperage load during cranking drops the battery voltage somewhat, the bypass wire allows the coil to get approximately the same voltage while cranking as it does during normal running through the resistor. That's somewhere in the neighborhood of 9.5-10 volts, which assures a good spark while the starter is running. Jerry
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Since the amperage load during cranking drops the battery voltage somewhat, the bypass wire allows the coil to get approximately the same voltage while cranking as it does during normal running through the resistor.
There is a few pitfalls that can get you here. The original system was setup to use ballast resistor, a ignition resistor bypass circuit (attached to your starter switch) and a non resistor coil (usually marked on the coil as to whether it needs an external resistor or not).
Allot of backyard mechanics are under the impression if you run a coil with an internal resistor you can eliminate the ballast resistor. As Jerry pointed out, the system was designed to supply full voltage to the coil while starting then switch over to reduced voltage while running. This will allow full starting voltage and protect your points from burning up to quickly.
First check to see if the coil does not have a built in resistor like the photo below. You want one that requires the external resister then make sure either resistance wire or a ballast resistor (like the photo below) is installed inline to the wire supplying voltage to the coil. Finally attach that new wire from the starter switch to the coil. Now you’ll have two wires attached to the positive terminal on the coil.
When I bought my truck it was wired without the resistor and without a bypass starting circuit. I found out thru this forum that allot of these old trucks have been running without one installed for years.
Last edited by Phak1; 02/02/20208:53 PM.
Phil Moderator, The Engine Shop, Interiors and Project Journals
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