Starter and Alternator wires

Rooster73 · Jan 26, 2025

Looking for some help or at least a direction to go to find an answer. I picked up a '79 GMC k1500 Suburban. The previous owner started to pull the engine out and then gave up. The wires to the starter and to the alternator were cut. I'm not sure which wires and how many wires there should be and where they all go to and come from. I'm finding lots of wiring diagrams online, but they are confusing to me, or they conflict with each other.

Another question related... Can I solder in splices or am I going to have to pull new wires? I'm not sure if any of these wires will have fusible links and if they do, I'm not sure if they've been cut out too.

Any suggestions or direction where to find this information?

Thanks.

Ricko1966 · Jan 26, 2025

Did they cut all three wires to the alternator? Starter should have two wires with fusible links going to the main post along with the positive battery cable. A purple wire going to the s terminal on the solenoid. Alternator needs 1 wire to feed 12 volts to the battery from the B+ terminal a brown wire to terminal 1 to work dash light and as exciter wire. Third wire is voltage sensing wire. Is the harness still in the truck and are the cut pieces? I can't imagine someone would cut them and disconnect them,so I'm thinking all the wires should be there. This should be a simple splice it back together situation,or splice on extensions where the harness was cut sitiation

Rooster73 · Jan 27, 2025

The cut off portions on the starter are gone. The starter was sitting inside the cab. The alternator was gone completely. I think I’ll be good to splice some sections in. I just wanted to make sure the right wires were there and I wasn’t missing any fusible links or anything. I haven’t done much rewiring so I’m not experienced with what wires exactly go where. The other square body I have the wiring is kind of cobbled so it’s not a great reference.

The harness is still in the truck. It’s almost as if they just cut the wires when removing the starter and alternator. Weird. Not how I would have thought anyone would remove the starter or alternator.

75gmck25 · Jan 27, 2025

Your truck is probably wired exactly like my '75 and Rick1966's '75.
One quirk is that you may have an extra wire on the starter that runs up to the wiring loom for the in-dash ammeter (ammeter was used up through about '76). This wire has a small fuse in the harness, usually in the wiring above the passenger side valve cover.

The in-dash ammeter gauge shunt has one side connected to the full 12 volt side of the wiring at the starter, and the other side of the ammeter shunt is connected to the power junction near the brake booster. The difference in voltage between these two points is used to calculate the amps displayed on the gauge. Kind of kludgy, since an ammeter shunt needs to have a known resistance to be effective, and I'm sure the resistance of the wiring degrades over time. However, it's better than running the full amperage up and through the dash gauge.

mano · Jan 27, 2025

I don’t know if this will help you out or not. These pics are out of a K 10 1979. After going through four different alternators, I went with the tough stuff, one wire alternator, and haven’t had any problems since. Just ran the one wire right to the positive side of the battery.

Jgonick · Jan 27, 2025

Here is a link to the wiring diagrams
Link-wiring diagrams

Rooster73 · Jan 27, 2025

Thank you all for the help. I’ll go take a look and see if I can make sense of it now.

Jgonick · Jan 27, 2025

Just a FYI- On the wiring diagrams the first number (before color) is wire size in metric. The number after the color is the circuit.

Example 3 PPL 6 would be 3mm Purple 6 = 12 gauge Purple 6

Rooster73 · Jan 27, 2025

That’s awesome. So helpful. Thank you.

spedigo · Monday at 1:24 PM

Ricko1966 said:
Did they cut all three wires to the alternator? Starter should have two wires with fusible links going to the main post along with the positive battery cable. A purple wire going to the s terminal on the solenoid. Alternator needs 1 wire to feed 12 volts to the battery from the B+ terminal a brown wire to terminal 1 to work dash light and as exciter wire. Third wire is voltage sensing wire. Is the harness still in the truck and are the cut pieces? I can't imagine someone would cut them and disconnect them,so I'm thinking all the wires should be there. This should be a simple splice it back together situation,or splice on extensions where the harness was cut sitiation

clarification please. i’m checking all wiring from previous owner and want to be sure it’s correct and understand what options i have.

as i understand it the starter will have

1. connection to battery
2. 2 wires (that i believe both go to fuse box)
3. purple wire to S terminal that comes neutral safety switch

10 gauge or better from battery to alternator

i’ve seen some jump the voltage sensing wire to the battery connection on alternator but is this the best option? is running it to the junction block on firewall a better option?

Jgonick · Monday at 4:43 PM

spedigo said:
i’ve seen some jump the voltage sensing wire to the battery connection on alternator but is this the best option? is running it to the junction block on firewall a better option?

This is how it was explained to me-
- sensing voltage at the alternator output - seriously hamstrings the system - it's like a self-fulfilling system - it's supposed to maintain 14.0 volts, it outputs 14.0 volts, therefore everything must be OK, right? But it completely ignores voltage drop in the rest of the system - and that's bad for performance
This is where the concept of REMOTE voltage sensing comes into play. Instead of sensing system voltage at the alternator or close to it (which is like congratulating yourself on a great job regardless of the results!) - we sense system voltage where we need full voltage -. That way, when the voltage sensing circuit detects 13.0 volts at the junction block, it says: " I'm supposed to produce and output sufficient to maintain system voltage at 14.0 volts. I sense system voltage as 13.0 volts. Better increase output to 15.0 volts to bring system voltage up to 14.0 volts."

Originally it is spliced to the red wire running from starter and going to light switch. (See attachment.)

Alternatively- I've also seen it ran to the junction block.

spedigo · Monday at 5:20 PM

Jgonick said:
This is how it was explained to me-
- sensing voltage at the alternator output - seriously hamstrings the system - it's like a self-fulfilling system - it's supposed to maintain 14.0 volts, it outputs 14.0 volts, therefore everything must be OK, right? But it completely ignores voltage drop in the rest of the system - and that's bad for performance
This is where the concept of REMOTE voltage sensing comes into play. Instead of sensing system voltage at the alternator or close to it (which is like congratulating yourself on a great job regardless of the results!) - we sense system voltage where we need full voltage -. That way, when the voltage sensing circuit detects 13.0 volts at the junction block, it says: " I'm supposed to produce and output sufficient to maintain system voltage at 14.0 volts. I sense system voltage as 13.0 volts. Better increase output to 15.0 volts to bring system voltage up to 14.0 volts."

Originally it is spliced to the red wire running from starter and going to light switch. (See attachment.)

Alternatively- I've also seen it ran to the junction block.

ok. makes more sense now.

so is the sensing wire from the alternator one of the two wires that are joined together on the starter post with the battery wire, correct?

If I’m remembering right (my truck is actually in a workshop out of town and it’s been a couple of weeks since I’ve been able to go out and take a look ), there are two red wires that come through the firewall, one red wire leaves the junction block and goes down to the starter. the other red wire comes from the fuse box and makes a straight run to the starter. those two join up with the battery wire and the sensing wire on the starter.

and the output wire from alternator goes to the junction block as well - in the diagram.

I know it would all make sense if I was able to just go look at my truck. But I’ve been on the road for the past couple of weeks. and my one day off a week I haven’t had a chance to go out there and sort it out. Trying to figure it all out online only has been conflicting information.

Jgonick · Monday at 6:00 PM

spedigo said:
so is the sensing wire from the alternator one of the two wires that are joined together on the starter post with the battery wire, correct?

Yes - originally is is spliced to a red wire (at the fusible link) that attaches to you starter with another red wire and black battery wire.

Compare the diagram above when you get to look at your truck.

GTX63 · Monday at 6:34 PM

Seems like this guy might have what you are looking for.

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Starter and Alternator wires

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