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03-23-2011, 01:17 AM | #1 |
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68 C10 Yellow wire from firewall to starter/coil.
So I have a HEI and a BBC in a 1968 C10.
I have a wire running to the HEI that is wired into a 12V "key on" hot panel. There is a wire that runs fron the top 1/2 of the outside firewall plug, which is supposed to be the YELLOW wire the goes to the starter/coil. My question to you wiring guru's is...............what is the purpose of this wire and do I need it if I have a wire run to my HEI ignition???? Thanks in advance Corie. |
03-23-2011, 02:06 AM | #2 |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
The GM coils are not designed to run full time on 12 volts like the HEI distributors, so the pre HEI ignitions used this wire with a resistance built into it to reduce the 12 volts from the key switch down to 8.5 to 9 volts for normal engine running. The engineers realized that it would be better to start the engines with a hotter spark so they tied into the yellow wire from the starter solenoid to provide a full 12 volts to the coil when starting.
With the introduction of the HEI ignitions the resistance wire and the yellow wire were no longer needed so they removed them on the 74 models. A lot of the guys when they make the conversions to HEI use this wire and soon find out that their trucks don't run very good and don't relate it to the lower voltage that the resistance wire provides. That is why a new wire needs to be run from a keyed full 12 volt source such as you have. It is possible to just remove the resistance wire from the bulkhead connector and run the new wire from there since it is 12 volts ahead of the bulkhead connector. Here's a couple of diagrams for the readers. The first one shows the stock wiring and the resistance wire is orange/purple from the bulkhead connector and where it joins the yellow wire that goes to the coil. This is the HEI wire
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03-23-2011, 02:13 AM | #3 |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
I LOVE THIS SITE!!!!!!!!!!!!!!!!!!!!!!
Thank you soooooooooo much!!!!!!! Wiring gives me a headache!! LOL |
03-23-2011, 07:12 PM | #4 |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
I installed a new wiring harness that designates it as a HEI harness and it has the yellow wire also? But it has the HEI ignition clip too. My truck runs fine, could it run better if I made modifications?
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03-28-2011, 07:43 PM | #5 |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
Hi Gene,
As long as the harness you used has a regular (non-resistance) wire running from the bulkhead connector to the HEI terminal you'll be all set. Check for full voltage at the HEI with the engine running to be sure. Having that bypass wire going to the starter solenoid won't hurt anything. It's just not necessary with HEI. |
04-04-2011, 08:21 PM | #6 |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
I have a 1968 GMC with HEI now and used the yellow wire for 5 years and had no problems. I went to start the truck the other day and the battery was dead. We put a new battery in and it started up but now when I turn the key off the truck keeps on running. I have to pull the yellow wire on the HEI to turn the motor off or kill it with the clutch. I tested the fuse block and half of them are hot even with the key off. I have to pull the battery cable so it doesn't kill my battery over night. Something really strange is going on and I have no idea what it could be. Any ideas would help thanks.
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04-04-2011, 08:57 PM | #7 |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
Hi 68_GMC,
My first guess would be that your ignition switch has gone bad. In other words, the electrical contacts inside the switch are staying on even though you've turned the key off. That'd explain the engine continuing to run with the key off, as well as the dead battery after sitting overnight. |
04-04-2011, 11:03 PM | #8 |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
Another possibility is the diode in the alternator. Before you pull the HEI wire you should disconnect the plastic plug in the alternator and the external regulator plug if it has not been converted to an internally regulated alternator. If the engine dies than it is most likely the diode in the alternator. It may charge just fine when it's running but it will back feed the ignition circuit if the diode is shorted through.
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04-05-2011, 12:39 PM | #9 |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
Thanks for the info I just put a new alternator on so it could be that and Ill check the key too.
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04-19-2011, 04:41 AM | #10 |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
I just did a v8 swap, the hei has to have a constant 12v going to it while it's running? I had a I6 with a coil setup in it and haven't ran a wire to the hei yet
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09-03-2015, 08:57 PM | #11 | |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
Quote:
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09-10-2015, 01:11 AM | #12 |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
So I to still have that silly resistor wire running to the starter with a keyed Hei designated wire in it.
I should remove that resistor wire? I have had a few electrical problems. |
09-10-2015, 09:17 AM | #13 | ||
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Re: 68 C10 Yellow wire from firewall to starter/coil.
Quote:
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. 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. Quote:
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09-11-2015, 04:16 PM | #14 | |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
Quote:
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09-16-2015, 04:00 PM | #15 | |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
Quote:
I am still an electrical newbie |
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09-17-2015, 09:23 AM | #16 | |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
............
Quote:
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09-21-2016, 03:06 PM | #17 |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
I know the posts are a little old and I may be reviving this... But almost all of my questions have been answered by reading this. I just have a question about my one wire alternator setup. Just a little background info, this is for my k20 suburban, originally an I6 which the previous owner converted to a V8 with HEI and the one wire alternator. (All of it is a hack job that I am attempting to rescue) The HEI wasn't setup even remotely correct (which I can now fix from what I read here, thank you), the one wire alternator is the next issue, and if there's anyone who has advice on clutch linkage from an I6 to a V8 I'd be happy to hear what you have to say.
ANYWAY, the alternator currently just has a 2 gauge wire coming off of the top stud that goes to the positive post of the battery. The regulator, or the hack job and splicing bonanza that's left seems questionable at best as well. Basically, I'm looking for some advice on what to hook up to get a charge going from the alternator and what should still be utilized from the external regulator. |
09-21-2016, 10:20 PM | #18 |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
Nothing wrong with reviving an old post if it's relevant to your problem, There's always new people to read and learn from it.
Using a one-wire alternator is as simple as it gets. You should be charging with it wired like you have it now except the battery gauge won't work like it's wired, with the output wired directly to the battery plus post. In order to correct the wiring you will have to know how it was wired to begin with. The central point in the harness is a junction of the wires that feed the battery, the cab, and the external regulator, all from the alternator. I have a diagram of the stock wiring with the HEI distributor wired. It should help you see how to wire yours. Follow the red wires to the junction in the bottom center of the diagram and you'll see where they originate. The one wire alternator will eliminate all the wires to the alternator and the regulator and you can just run the single wire off the alternator stud, not 2 gauge but maybe 12 gauge I'm thinking, to the junction of all the red wires in the center of the diagram.
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VetteVet metallic green 67 stepside 74 corvette convertible 1965 Harley sportster 1995 Harley wide glide Growing old is hell, but it beats the alternative. |
09-21-2016, 10:46 PM | #19 |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
Here's the same diagram with all you need to charge the battery and supply the system with amperage. The advantage of this wiring is that the alternator can feed the circuits without going through the battery as you have it wired. It will also allow the battery gauge to work whereas if you wire the alternator output directly to the battery positive post it bypasses the sensing system for the battery gauge and it won't work.
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09-23-2016, 09:06 PM | #20 | |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
Quote:
Thoughts anyone, please and thank you for the help |
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09-24-2016, 10:00 AM | #21 | ||
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Re: 68 C10 Yellow wire from firewall to starter/coil.
Quote:
If you have spark and 12 volts at the distributor then you have a timing problem not a charging problem. Or you have a fuel delivery problem. Quoted from your first post. Quote:
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09-24-2016, 12:05 PM | #22 |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
I would like to believe that something as simple as that is the problem, however I just had it running last week
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09-26-2016, 12:14 PM | #23 | |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
Quote:
Some of the early engines used a fiber cam gear and they were known to let the timing chain jump a few teeth which would throw the cam timing and the valves off enough so that the engine wouldn't run. Most of the time the engine would still pop or backfire through the intake.
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VetteVet metallic green 67 stepside 74 corvette convertible 1965 Harley sportster 1995 Harley wide glide Growing old is hell, but it beats the alternative. |
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11-18-2016, 09:39 PM | #24 |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
This thread is priceless. It has been a wealth of accurate information. Thank you all for your input on this subject. I have a perfectly clear idea what is going on with my charging system from reading this. Thanks again!
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11-18-2016, 09:41 PM | #25 |
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Re: 68 C10 Yellow wire from firewall to starter/coil.
VetteVet- You are a fountain of information man..... A geyser of knowledge! LOL!
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