You need 2 relays. Both can be connected to the same power source of course.
Can you tap power off the alternator post. WOW, What a can a worms. This very question has been argued back and forth many times on this forum.
I absolutely wouldn't do it. I tapped off the solenoid with a motorcycle sized battery cable to a strip fuse, fuse panel mounted on the forward side of the passengers footwell. And then run power for headlight relays and electric fans, etc.from there. Many say absolutely, tap off the alternator. It's easy.
it's extremely dirty power and very sloppy wiring at best. But it's easy.
I would ask you to go to "Mad electricals" Web site. Go to there tech pages. These guys have wiring for our older cars writen for us average guys.
Read up on the correct way to install headlight relays, and other good electrical upgrades for our 60's and 70's cars.

 

No I’m not opening that can of worms. If there is debate, then it’s not universally accepted, and I don’t want to show my car and need to defend it. That said, I do know that alternator power is dirty, but for headlights…. Who cares. Be that as it may, I can just run another line from the starter.

So if I do this… it does save headlight power from running through the old wiring and old switches, but how does it prevent the voltage drop which is now the top suspected cause of this bucking?

And 2 relays… if I’m understanding, this is because I need one for headlights and the other for high beams… correct?

 

Alright. Looking for some input again.

I am thinking that I should try running relays for the headlights and high beams. And see if that hopefully solves this problem.

I don’t want to run another line from the starter.
I don’t want to run a line from the alternator, since that is controversial… let’s not debate that here.
I also don’t want to run a line through the firewall.

Here is my question:

Could I grab my power by connecting into the horn relay?
So, headlight switch to new relay1, high beam switch to new relay2, and loop the power over to new relay1 and new relay2 from the power terminal on the horn relay.

Now the breaker in the headlight switch is 30-amps I believe.
The fuse for the horn is 20-amps I believe.

I believe with both the high and low beams on, I’m drawing …. What…. Almost 20-amps?? Right? I guess I should look and see what the watts are on the lights, but they are probably 55x2 low and 65x2 high….
And the horn… probably draws 5-10 amps???

So I’m thinking if I have the highs and lows on, then lay on the horn, I’ll blow that fuse…. Right?

So I wonder if I replace the 20-amp fuse with a 30-amp if there would be a problem. None of the 3 individual relays would ever draw that much individually, so I’m not worried about the relays… but the line from the fuse block to the horn relay… does anyone know if that wire is ok for 30-amps?

 

guess you havent checked those grounds yet

 

No I’ve done nothing yet. Still gathering options and coming up with my list of things to check and try in an order of least involved to most involved.

So when I get a day or night to work on it, I have a bunch of things to do/try…. Cause I only have that time once every few weekends.

 

Good to hear you're a planner. Try unplugging the headlight bulbs one at a time and see if there's a change. Also with the motor off, disconnect the battery and see what your amperage draw is through a voltage meter while turning on the lights and using the hi/lo switch before you start changing anything. You can check again as you check things off your list and this will tell you if there's been a change...

Did I mention to clean your battery cables, check the connections to the frame there and also the resistance of both your cables, they can corroded under the plastic sheath

 

OK, your gathering information.
PLEASE, go to Mad Electrical s Web site.
Please !
I simply can not take the time to type this much when they already have!
Please, top of there home page, tech pages. Read all about charging systems, read about headlight relays, and how and why to wire them.
I am done here. I tried.

 

This is very helpful. Thank you.

 

Thank you. Read the articles not once, not twice, but three times. Very helpful information.

Now with regard to a voltage drop I may be experiencing…. My MSD6AL is drawing its power by being directly connected to the starter terminals. Which in of itself, based on what I am reading, would not experience any voltage drop from the headlights. The voltage drop occurs at the fuse block. However, the ignition switch is connected to power from the fuse block. So it is possible for my ignition system to have a hiccup when turning on the lights because of low voltage at the ignition switch. Is this a good hypothesis???

That said, the main takeaway I got from this article is that the horn relay is a main junction, this junction is where the alternator is connected, and where the voltage regulator is sensing. This junction is a good place to run my line for the two headlight relays.

Am I on the right track here?

Any chance you know where I could find a nice easy to understand full wiring diagram for a 69 vette?

Thank again!

 

Wait hold on…. Just re-read this. With the motor off and battery disconnected, how are the headlights, or anything else, getting power to test anything? Typo… did you mean something else?

 

Wiring diagrams.....your in luck! A fellow forum member is offering them, all you have to do is follow the instructions.

https://www.corvetteforum.com/forums...or-anyone.html

 

Thanks for sending me that resource.

 

you connect the multimeter between the positive post and the positive cable so the amperage is read through the meter. It should show the amperage change when turning the lights on and off. You can do the same for the voltage to see if there is one light or bank of lights or the switch itself that is the culprit if indeed that is the issue

 

Where does the new MSD box get power and ground from?

 

So your saying your aftermarket ignition system is getting it's power from the main battery cable at the starter solenoid. OK, that's new information.
How are you then turning it on and off? Powering your ignition straight off the solenoid battery post would give you fairly clean power. But no way to turn it off. Are you pulling power from the R terminal on the solenoid as the really old C3's had?
Then that's not the same.
How I power my aftermarket ignition and EFI. I run 6G. wires from the battery posts. With a 30amp fuse right in the battery box. I run these 2 wires along with the main positive battery cable to the starter area. Then up with the starter harness to the firewall. I have the positive wire feeding 2 relays. One supplies power to my ign. system. And that relay is triggered by the factory power wire that once was connected to the coil. Thus supplying clean battery power to my ign. The other relay on this dedicated line is the relay for my EFI. Power for my ECU, and also a clean ground for the ECU.
I have no issues.

 

MSD has a trigger wire

 

connected to the starter

 

Yes the MSD box is connected directly to the starter for its main power. But the MSD box has a separate wire that connects to the original wire from the ignition switch… so the MSD box actually only on using the power from the main battery cable when the ignition switch is on. Make sense?

Again though…. This MSD setup was in the car before this issue ever appeared.

The only thing I can think changed was the MSD box previously was a very very old MSD6… it burned out. I replaced it with a new MSD6AL box…. And the issue “I THINK” appeared after that. I did not change the wiring route… it’s the same, just connected to a new box. But maybe the new 6AL box is more sensitive to voltage drop? And the only drop I can think of would come from the the connection to the ignition switch…. So if the voltage drops from there, perhaps the MSD6AL shuts off for a split second like a hiccup…. And giving my the buck.

Idk… I have a lot of things to look into from this post.

 

Obviously when you turn on your high beams there is a spike or surge in your wiring that your ign. box doesn't like.
thus the suggestions for relays to isolate the surge.
best of luck.

 

I requested a copy of the 69 wiring diagram to take a look. Meanwhile I have a 71 diagram and I suspect the ignition switch and headlight switch portion is not much different from the 71 if not identical. So Im using that diagram until I get the 69 diagram to verify but I suspect your problem is an interruption in the wire that comes from the ignition switch. The power TO the headlight switch and the ignition switch comes from the same source. If the high beam switch is malfunctioning where for that brief moment when you select high beams on then off then it would affect the same power to the ignition switch. I think your problem is not the main power to the MSD box, I think its the ign wire to the MSD that is getting interrupted for a brief moment.
To test this, (temporarily) disconnect the +12V ignition line to the MSD and run a jumper wire to the red wire that powers the MSD from the starter. Fire up the car and test the high beams and see if you get the stumble or if it goes away. At that point you will know for sure if that is what is interrupting your ignition.
I suspect the high beam switch is failing.