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C10CORIE 03-23-2011 01:17 AM

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.

VetteVet 03-23-2011 02:06 AM

Re: 68 C10 Yellow wire from firewall to starter/coil.
 
2 Attachment(s)
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.

Attachment 718651

This is the HEI wire

Attachment 718652

C10CORIE 03-23-2011 02:13 AM

Re: 68 C10 Yellow wire from firewall to starter/coil.
 
I LOVE THIS SITE!!!!!!!!!!!!!!!!!!!!!!

Thank you soooooooooo much!!!!!!!

Wiring gives me a headache!! LOL

Gene 03-23-2011 07:12 PM

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?

ray_mcavoy 03-28-2011 07:43 PM

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.

68_GMC 04-04-2011 08:21 PM

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.

ray_mcavoy 04-04-2011 08:57 PM

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.

VetteVet 04-04-2011 11:03 PM

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.

68_GMC 04-05-2011 12:39 PM

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.

ATrain707 04-19-2011 04:41 AM

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
Posted via Mobile Device

chuchito 09-03-2015 08:57 PM

Re: 68 C10 Yellow wire from firewall to starter/coil.
 
Quote:

Originally Posted by VetteVet (Post 4571651)
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.

Attachment 718651

This is the HEI wire

Attachment 718652

Vettevet, I have been looking all over for info on this subject. Everyone talks about the wire with the resistor and says to replace with a different wire from the terminal block to connect to HEI but it gets confusing on what to do with the yellow wires that tee off with the resistor wire. Everyone makes it seem that there is a yellow wire directly to the original coil from the terminal block. The fact is that the resistor wire goes from the terminal block to a splice where it then transitions into two yellow wires. At least it does on my 72 GMC Suburban. My suburban has HEI and has not run in a few years since working on the body work. I'm trying to get it running and needed to know where those two yellow wires go. Since reading this post, I now know that one goes to the original coil which is not needed anymore but I was stuck on what to do with the second yellow wire. Whether to connect it to the starter or not. Your explanation makes the most sense of all posts I've read. So you are saying that I don't need to connect neither of the yellow wires; one to the original coil, since it is gone, nor the one to the starter. Is this correct?

toolboxchev 09-10-2015 01:11 AM

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.

VetteVet 09-10-2015 09:17 AM

Re: 68 C10 Yellow wire from firewall to starter/coil.
 
1 Attachment(s)
Quote:

Originally Posted by chuchito (Post 7297764)
Vettevet, I have been looking all over for info on this subject. Everyone talks about the wire with the resistor and says to replace with a different wire from the terminal block to connect to HEI but it gets confusing on what to do with the yellow wires that tee off with the resistor wire. Everyone makes it seem that there is a yellow wire directly to the original coil from the terminal block. The fact is that the resistor wire goes from the terminal block to a splice where it then transitions into two yellow wires. At least it does on my 72 GMC Suburban. My suburban has HEI and has not run in a few years since working on the body work. I'm trying to get it running and needed to know where those two yellow wires go. Since reading this post, I now know that one goes to the original coil which is not needed anymore but I was stuck on what to do with the second yellow wire. Whether to connect it to the starter or not. Your explanation makes the most sense of all posts I've read. So you are saying that I don't need to connect neither of the yellow wires; one to the original coil, since it is gone, nor the one to the starter. Is this correct?

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.


Quote:

Originally Posted by toolboxchev (Post 7304195)
So I to still have that silly resistor wire running to the starter with a keyed Hei designated wire in it.

No you can disconnect the resistor wire and the starter wires and remove them or tape them off, but then you have to run a new wire from a keyed on 12 volts source directly to the BAT terminal on the distributor.

For a clean look you can remove the cloth covered wire from the firewall block on the engine side and replace it with a 12 gauge red or pink wire and run it to the HEI distributor. You'll have to acquire a new terminal for the firewall block call a Packard 56 terminal I believe.

Or you can do what most of us lazy guys do and just run the new wire from the IGN UNFUSED terminal on the fuse panel through the firewall to the BAT terminal on the distributor.


I should remove that resistor wire? I have had a few electrical problems.

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




chuchito 09-11-2015 04:16 PM

Re: 68 C10 Yellow wire from firewall to starter/coil.
 
Quote:

Originally Posted by VetteVet (Post 7304373)
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.




Thanks Vettevet

toolboxchev 09-16-2015 04:00 PM

Re: 68 C10 Yellow wire from firewall to starter/coil.
 
Quote:

Originally Posted by VetteVet (Post 4601002)
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.

So if I wish for my electrical system to be working properly with an HEI and standard externally regulated alternator both the resistor wire and the plug on the alt with blue and white wire can be eliminated?

I am still an electrical newbie:lol:

VetteVet 09-17-2015 09:23 AM

Re: 68 C10 Yellow wire from firewall to starter/coil.
 
............
Quote:

Originally Posted by toolboxchev (Post 7310852)
So if I wish for my electrical system to be working properly with an HEI and standard externally regulated alternator both the resistor wire and the plug on the alt with blue and white wire can be eliminated?

Keep in mind that you have two separate resistor wires in the truck circuit. One of them is for the ignition and the other is for the charging system. When you convert to the HEI ignition you can eliminate the resistor wire that goes to the coil and the starter from the bulkhead block. The cloth covered one.

You will still need resistance for the alternator whether you have an external or an internally regulated alternator. The brown wire from the bulkhead connector that goes to the external regulator and the white wire from the regulator serves this purpose on the externally regulated alternators. The blue wire is the voltage sensing wire and also supplies current to the fields of the alternator for greater or lesser output as needed. So yes you need the blue and white wires for an externally regulated system.

I am still an electrical newbie:lol:


Mikestone 09-21-2016 03:06 PM

Re: 68 C10 Yellow wire from firewall to starter/coil.
 
Quote:

Originally Posted by VetteVet (Post 7311537)
............

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.:chevy::chevy::chevy:

VetteVet 09-21-2016 10:20 PM

Re: 68 C10 Yellow wire from firewall to starter/coil.
 
1 Attachment(s)
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.

Attachment 1573149

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.

VetteVet 09-21-2016 10:46 PM

Re: 68 C10 Yellow wire from firewall to starter/coil.
 
1 Attachment(s)
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.

Attachment 1573168

Mikestone 09-23-2016 09:06 PM

Re: 68 C10 Yellow wire from firewall to starter/coil.
 
Quote:

Originally Posted by VetteVet (Post 7720626)
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.

Attachment 1573168

Thanks for getting back to me. At this point I've hooked everything up just as it is in the diagram, alternator turned out to actually be a 3 wire and I set it up like everyone has suggested. However.... now when I go to start it I can't get it to fire (I checked the plugs and I have spark. I also have 12 volts at the distributor). Either the purple wire or the main lead to the starter began to smoke after I attempted starting it. I disconnected the purple thinking the connection may be bad, tried again, still smoking.

Thoughts anyone, please and thank you for the help

VetteVet 09-24-2016 10:00 AM

Re: 68 C10 Yellow wire from firewall to starter/coil.
 
Quote:

Originally Posted by Mikestone (Post 7722417)
Thanks for getting back to me. At this point I've hooked everything up just as it is in the diagram, alternator turned out to actually be a 3 wire and I set it up like everyone has suggested. However.... now when I go to start it I can't get it to fire (I checked the plugs and I have spark. I also have 12 volts at the distributor). Either the purple wire or the main lead to the starter began to smoke after I attempted starting it. I disconnected the purple thinking the connection may be bad, tried again, still smoking.

Thoughts anyone, please and thank you for the help

Cranking the engine for extended periods will send a lot of amps through the wires. The battery cable carries as much as 200 amps and the small purple wire can carry up to 10 amps. This generates a lot of heat. If your wires aren't burning the insulation off then it's normal for them to get warm under these conditions.



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:

The HEI wasn't setup even remotely correct (which I can now fix from what I read here, thank you)
Re check your distributor to piston timing you may have the distributor installed 180* out.

Mikestone 09-24-2016 12:05 PM

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 :confused:

VetteVet 09-26-2016 12:14 PM

Re: 68 C10 Yellow wire from firewall to starter/coil.
 
Quote:

Originally Posted by Mikestone (Post 7722850)
I would like to believe that something as simple as that is the problem, however I just had it running last week :confused:

Pull the air cleaner and pump the accelerator while looking down the carb. You should see a good squirt of fuel from each front barrel. If you don't see any squirts., try some starter fluid or a little bit of gas down the carb and crank it.
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.

396C-10 11-18-2016 09:39 PM

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!

396C-10 11-18-2016 09:41 PM

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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