I did the 3G swap, and everything seemed great. My truck sat for a couple days, and was dead as a doornail. I narrowed it down and determined the black with gray or white stripe was drawing the battery down pretty quickly. I had it on the + side of the solenoid. This wire was part of the old alternator system, but I'm not exactly sure what it was for. I do know there is no power in the truck when it isn't connected. Can anyone shed some light?

 

If it was a junkyard alternator (or I guess, a fresh one from the McParts store) the diodes could be bad, and that can cause a drain.

I wonder if you could leave the battery connected in the truck, without running it for a few hours, and look at it with a heat gun to see if there is a warm spot anywhere from the derelict current.

 

Yeah, alternator diodes or even a failing battery can do it. In lieu of finding a quick answer, if you leave the battery disconnected for a couple of days and it still dies, then the drain is the battery itself.
But if it lives to start another day after that, then it's the "other" things.

Does your truck have an ammeter, or an alternator/amp indicator lamp?
I can't say as I've seen a Black w/white stripe wire on our trucks before. And it doesn't look like either a faded yellow or red stripe from here. Any chance you can see under the tape to verify it's still white?

Looks like it might have had a fusible link blow at one time and was replaced with a simple ring terminal. There are multiple fusible link wires attached to the starter relay on most of these trucks. A '78 for sure I would think.

I'm thinking that because disconnecting that wire kills power to the cabin, it's your large Black w/yellow wire (even if the stripe isn't yellow) and runs up over the cowl area to the power terminal near the brake booster/driver's side area. This actually IS how the power gets from the battery to the cab, so that would certainly 'splain why it kills the power.
Obviously you need it whether it's the source of your drain or not.
But it's also easy to test to see if that is the same wire.

With it disconnected like it is now, does the battery still drain? If so, reconnect it to get your power back and check some other things.
You can disconnect the one from the alternator's BAT/CHG post (Black w/red?) and see what happens next. Drain still there? It's not the alternator.

Same for the old voltage regulator. Why is it still there anyway? I've seen regulators cause a slight drain, but I don't know if they had to be connected to the alternator still to do that?
At the very least disconnect the connector from the regulator to isolate it. If the battery no longer drains, great! Get rid of it and safely cap off the wires that need it, or remove them altogether.
Unless you're thinking in terms of versatility for going back to stock, they're completely unneeded. Except perhaps for your ammeter to function. But not sure about that.
I liked the old ammeters with their inductive loop. They were easy to understand, compared to a shunted gauge. But having all that current flowing around the entire cab and engine compartment from the alternator to the battery probably didn't seem like a good idea once they thought about it.

So reconnect that one wire you have with the stripe of unknown color and do those other tests and see what happens.
Ultimately, and with all this fooling around with wires, it wouldn't be a bad idea to have another fusible link in that wire. Or is it connected to what looks like a possible Mega-Fuse in that plastic holder at the edge of the pic? If so, maybe that's enough protection.

And speaking of which... What all is connected there and is that a fuse?

Good luck!

Paul

 

Hmm, on another side note... What are you using as an exciter wire for the new 3G?
Did you tap into the old Green w/red wire from the old regulator? Or something else?

I'm just wondering in case there is some stray voltage coming through all the time, your alternator would be energized and would definitely cause a drain. Not sure how big, but that would definitely be worth verifying.
Should only be constant voltage to the Yellow w/white wire in the new 3G's 3-wire plug.
Green w/red wire is switched in RUN only.
White w/black stator wire is "usually" connected to it's own plug on the side of the 3G body.
Is that how yours is wired?

Paul

 

- The black/white turned out to be black/yellow after all. I peeled back some tape until the color was easier to see. I was just confused because there was another black/yellow wire there that's about one size smaller. The black/yellow in question goes straight to the ignition switch and drains the batter when connected to + solenoid.

- I left that wire disconnected for more than two days, and my battery still shows about 12.6v. When I connect the heavy black/yellow, the voltage starts going down immediately.

- After I first hooked up the new 3G, I was getting over 14.3v. I performed a diode test as shown on YouTube, and it what they said it was supposed to do. Place one lead on housing, the other on the positive post. It should show no value one way and a "high" reading the other way. I got no reading one way and .508v the other way. Not sure if that's good or bad because the videos don't really say what an actual acceptable value is.

 

Paul

I just connected the old red/green to the 3G red/green. I hooked up the heavy positive and the yellow/ white the + solenoid (via 175a fuse).

I also do not have an ammeter. I have the Rocketman voltmeter conversion.

 

Do you have another vehicle which you can use as a test-bed for your battery, to see if it holds its charge over a few days of use?

 

Sounds like it does from what he just said about leaving the wire disconnected and the battery keeping it's charge.
So in theory, sounds like the battery is still good to go.

Now you just have to find out what is drawing the current. And though I don't know the acceptable levels either, that half a volt is usually not good when testing for voltage losses.
Usually you want in the low tenths of a volt I think. But whether you were measuring a drain or something else I'm not sure.

So you can still disconnect the regulator's connector to see if that gets rid of the drain.
You can also pull the fuses while testing for current draw with the old method of disconnecting the ground cable from the battery and putting a meter or light in between it and the battery. If it lights up, you have a drain. Pulling fuses one at a time, or disconnecting things one at a time until the light goes out is how you do it.
Never done it myself luckily, so maybe wait for someone to confirm the method and fill in the blanks to make sure you do it right.
The only problem I remember hearing about with the test lamp method vs a meter is that the lamps are usually very lightly fused and delicate anyway, so using it as a bridge to test loads could damage the test lamp from passing too much current through it.

Paul

 

So, I hooked up the main ignition switch black/yellow back up early this morning to try to use my meter to find the source of the draw. Suddenly, there was no draw, and the battery was holding a charge. It was 12.70v this morning and 12.70 when I checked again this afternoon. I've done nothing in the meantime but fiddle with the ignition switch to try to identify some wires.

Is it possible I have a bad switch? It's not that old. Seems like I changed it less than 5 years ago. Gotta love intermittent electrical bugs.

 

I don't know about the switch, but definitely check out the wires going into the plug for the ignition switch. That plug is probably brittle, been overheated, and who knows what evil lurks in there...

 

Definitely. Our old plastic connector bodies are getting pretty long in the tooth nowadays. Seems like especially the headlight switch and the ignition switch connectors/strain reliefs.

But frankly, brand new stuff is absolute crap! Seems like the stuff you get nowadays (in the last four to five years anyway) has about a 50% chance of being bad right out of the box. Never had to worry about that when I first started working on cars. Didn't occur to us when this first started cropping up (crapping up?) to suspect the new parts. Didn't take long to figure that out though.
In your case, a new switch that has lasted five years is practically a miracle!

That said, I still think if it lasted this long, you have a better than average chance that it's still good.

Something's been bugging me since you said this the other day and I'm just now getting around to commenting on it.
My feeling is that there is no way that large wire goes straight to the switch. Maybe it does and I'm just overthinking it, but normally the large wire goes everywhere else first. It has splices taking power from it to things like the fuse box, headlight switch, and other things powered by the battery. But there's no way that the switch itself can pass the current that wire is capable of flowing. Which means to me it's unlikely Ford ran it to the switch directly.

Yeah, I know they keep surprising me with all sorts of "strange things" that don't follow the diagrams. But they do follow some normal patterns. And running the big wire to the switch, just like running any other higher load from the switch, is not normal. They don't build the switches to pass that much current, so don't need the big wires.

Anyway, just rambling on to get it off my chest. When you're digging around double check that and make sure it doesn't go somewhere else.
But the bottom line is that the larger of the Black w/yellow wires powers everything in the truck at one point or another, so if the battery drains when you connect it, it could still be just about anything at fault.
I know you said it's not doing it right now, and that's great. But in case it starts doing it again, you know to trace that wire again just to make sure.

Good luck. Crossing fingers that it just doesn't come up again.

Paul