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Old 11-19-2016, 02:31 AM   #26
SonomaComa
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Re: 68 C10 Yellow wire from firewall to starter/coil.

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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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Old 11-19-2016, 03:18 AM   #27
VetteVet
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Re: 68 C10 Yellow wire from firewall to starter/coil.

The cloth covered wire is the ignition wire from the key switch to the coil (OEM)
It comes into the cab through the firewall terminal block behind the fuse panel and becomes a pink wire which runs to the key switch after teeing with two other pink wires as shown in the first diagram and then the second.

The green wire is the temperature sending wire which goes to the key switch if you have the idiot light dash, and then to the dash cluster to the no. 5 pin on the cluster plug. If you do not have the idiot light dash but have the gauge dash then the green wire goes to pin 6 on the cluster plug and then to the temperature gauge in the dash. It sounds like this wire may be shorted to a hot wire at some point since it is grounded through the sender to the engine block.

Look at the two diagrams and on the lower left you will notice the firewall block and the pink ignition wire coming out of it in the top section. Follow this wire up to the tee and on up it turns right and heads to the ignition switch.

Just to the left of it is the green wire for the temperature sender. and below it in the second diagram. Again this wire goes to the dash cluster plug if you have a gauge dash.


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Ignore the arrows, they point to the brown accessory wire which goes back to the fuse panel to power the turn signal flasher, the wiper switch and the hazard flasher.
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Old 11-19-2016, 04:07 AM   #28
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Re: 68 C10 Yellow wire from firewall to starter/coil.

Awesome. So I guess I don't need to connect anything to the 'R' terminal on the starter with an HEI distributor then.
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Old 11-19-2016, 08:38 AM   #29
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Re: 68 C10 Yellow wire from firewall to starter/coil.

Yes that is correct. VV
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Old 11-21-2016, 03:57 AM   #30
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Re: 68 C10 Yellow wire from firewall to starter/coil.

When re-wiring is it better to use thick strand wire or thin multi strand wire?
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Old 11-21-2016, 01:30 PM   #31
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Re: 68 C10 Yellow wire from firewall to starter/coil.

Well most of the current runs along the surface of the wires so the more wires you have the more current can travel through the wire. This is not much of a factor in automotive use as long as you use AWG wire designed for vehicles.
The strength factor may come into play where movement can cause broken wires and loss of conductivity.I might use a larger strand in that case but I think I would just go with a larger gauge for less resistance than worry about broken strands. Case in point would be the alternator conversion to 100 amp or higher. Then go from 12 gauge to 10 or 8. JMO.
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Old 11-21-2016, 01:32 PM   #32
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Re: 68 C10 Yellow wire from firewall to starter/coil.

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VetteVet- You are a fountain of information man..... A geyser of knowledge! LOL!
Hey I'm not just another pretty face you know.
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Old 07-16-2017, 12:17 PM   #33
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Re: 68 C10 Yellow wire from firewall to starter/coil.

So my 1968 GMC 1500 had the old distributor set up, so I go and do the HEI like I have in countless other cars and trucks I have, I run the HEI from the fuse panel and have power to the coil, but can't seem to get any spark, I rebuilt the distributor with all new parts thinking that was the issue and still nothing, could this be a grounding issue? I'm Stumped and about to just reinstall my old set up with a petronix conversion, and yes I ran a jumper from the battery to also see if that was missed power issue and nothing, any ideas? Thanks!
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