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Old 11-19-2016, 02:31 AM   #7
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Join Date: Sep 2015
Location: Sonoma County California
Posts: 83
Re: 68 C10 Yellow wire from firewall to starter/coil.

Originally Posted by VetteVet View Post
Yes this is correct, the color of the wires may vary but their purpose is the same. On the stock trucks with coils there are two sources for ignition.
The wire off the ignition switch (pink) that runs to the firewall block inside the cab, and the wire from the starter terminal that joins it after it comes through the firewall block.

When the wire from the ignition comes through the firewall block it changes to a special resistance wire that drops the ignition switch voltage down from 12 volts to around 8 volts to protect the coil during normal running.

This is enough for a good spark once the engine fires but more is needed for starting. This is where the wire from the starter comes in.

When the solenoid engages it acts as a relay to connect the large starter cable voltage directly from the battery, to the other wire in the circuit. This sends a full 12 volts to the ignition coil, to provide a hotter spark during cranking.Once the starter disengages, it breaks the circuit and the ignition current reverts back to the resistance wire from the firewall block.

In the original harness the resistance wire was a cloth covered wire colored orange/white/ and purple.

Attachment 1445421

When GM went to the HEI ignition they needed a full 12 volts to the coil anytime the ignition was on. They simply removed the resistor wire from the circuit and ran the ignition straight to the HEI coil distributor, eliminating the wire from the starter.

Yes I know and we are here to help with those.

I recently acquired a 69 Chevy C20 with an HEI already installed. One day it wouldn't start, the bendix would kick out but wouldn't turn over. I went to change the starter and didn't know what that yellow wire connected to the 'R' terminal was for. I traced it back to that white colored cloth wire you have in the picture.
That cloth wire does not go into the fuse box on the fire wall, it is broken. The green wire you have pictured is melted from the inside out, I traced it to a sensor in the block on the driver's side. I am wondering what happened.
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