So I replaced the original alternator with a 150 amp that requires a 6 gauge positive as well as a ground that matches. I’m trying to figure out where this wire runs so I can replace it. I’ve traced it to the engine wiring harness but I don’t know where it runs from there and how to go about replacing it. I’ve searched YouTube but found nothing and wiring is my biggest weak point

 

This one is easy. I am assuming you bought a "one wire" Alternator. As opposed to a 3 wire Alternator. The positive wire go's to the the solenoid, same terminal that the battery cable is hooked up to. The negative wire should go to a good,clean chassis ground. Just run it along with the factory harness. I kinda wrapped mine around the harness in a spiral. Behind the engine and down the right side to the starter. Get yourself a good solid ring terminal to install it to the solenoid stud.

 

Right side wiring harness

I don’t know much about wiring but I don’t think my wiring harness should look like this.

 

Just old dryed up sealer. Don't freak. Cars didn't get good weather proof electrical connectors till the mid to late 90's. Common to see a tar like sealer on them to keep the water out.

 

This will work, but it will also bypass the (limited) protection of the stock harness by removing the fusable link(s) from the circuit. If you have a short to ground, you could have a fire. What year is your car? Later cars had a few extra wires going to the solenoid.

A better solution is to add a fuse close to the positive terminal of any high current source. When I added my 150 Amp, 3-wire alternator, I put a fuse at the alternator terminal, another the breakout for my high current accessory wires (near the heater/AC box), and preserved the fusable links on the starter solenoid extension to complete the charge circuit and feed the rest of the car. I'll replace these all with MRBF style fuses after I prove it out on the 2nd battery installation on my truck.

No doubt @Richard454 will post a link that shows an even better way to do this, and hopefully the diagram that I couldn't easily find.

 

She’s a ‘78
For now I just crimped a larger ring connector to the OG wire that ran from the harness so it would fit onto the new alt. If I were to run the positive wire straight from the solenoid to the alternator (w/ a fuse along the line) then would I need to do anything to the wires that run through the harness and fuse box?

 

Hey Parker-

See wiring diagram-

The factory alternator wire needs to be connected and PROTECTED.

Just jumping a large gauge wire from the alternator to the starter- then connecting the factory wire- FULL battery power is available unprotected ALL the way to the fusible link at the firewall (on diagram says "thermo")

If you put protection at the alternator- you have FULL battery power running from the starter to the alternator UNPROTECTED.

Caveat- if the alternator shorts out- smoke comes out of the alternator and it quits producing power. HOWEVER- if you short the battery- It has over 700A and it doesn't stop till it burns the wire up.

Unless adding electric fans- stereo other stuff- no real need to jump up to a larger alternator.

No additional electronics -FUSE at the battery- I like the Blue Sea Marine piece- 225A should be sufficent

If adding stuff- see the diagram below

fuse I like -

 

See how that worked out? Thanks @Richard454!

 

78 is a year you really don't want to keep relying on the original wire. The wire goes to the "engine" firewall plug and then jumpers under the dash to the "chassis" firewall plug where it then continues under the hood to the starer solenoid. So, it goes through 2 spade connectors that have been known to melt-down with the stock alternator.

Why did you upgrade the alternator?

What you do depends on why you're doing it. You basically need to either remove the wire from the alternator to the firewall plug or the wire from the starter solenoid to the firewall plug so that charging current can't flow on the original wire. Then, run a new wire or wires to properly connect the alternator to the solenoid with a fuse or fusible link at the solenoid.

You put fuses or fusible links on the battery end of wires. The battery is the component that will provide the short circuit current which melts wires. The alternator can't do this, so a fuse at the alternator end of the wire is pointless.

The alternator grounds through the brackets to the engine block which is then grounded by the big block to frame ground wire on the passenger side. The ground wire on the alternator case is to ground the rad support to give a ground for the lights and such up front. It's not there to ground the alternator.

 

I upgraded the alternator to support the dual electric fans that are going in with a new radiator and to support any new electronics that I plan on adding in (ex: Gauges, radio, speakers, headlight conversion, etc)

 

Here you go- I made this up for another forum member-

If you do add/amp/aftermarket radio- I'd run it directly off the battery- least likely to have noise issues-

 

OK, I'd run the new wire to the firewall and put a terminal block there. Then, continue it to the starter solenoid. Put a fusible link or fuse at the solenoid.

Bring one or both of the old wires from the firewall to this new terminal block. Put a fuse or fusible link at the terminal block end of these wires.

One of the terminals on the alternator is a voltage sensing wire so run this voltage sensing wire to the terminal block. This will get the alternator to maintain full voltage at the terminal block. Put a fusible link or fuse at the terminal block end of this wire.

You can then connect the fans to the new terminal block.