Stock wire harness to battery and starter

Elliot W

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Joined
Apr 13, 2021
Posts
14
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Location
Cville
First Name
Elliot
Truck Year
1974
Truck Model
C10
Engine Size
5.3
I’m having a tough time figuring out my stock wiring harness (engine bay motor side) for a 74 c10. When I got the truck, the motor was already taken out so I couldn’t label the wires. The lighting side of the firewall wiring terminal is straight forward, but the engine side is not so much. To make matters worse, I have a 5.3 in it now (I believe it was a 305 that was in it before).

My question is, where does the positive battery terminal need to go, what wires need to go to the starter and on what post, and do any other wires need to go to the two post positive battery terminal that is located on the fire wall?

I have a stand alone wire harness for the 5.3 that needs one 12v connection to the starter, and obviously the starter needs 12v and ignition, but is there anything else that needs power? Not sure if it helps but I have a couple pictures of the harness that need to find terminals.

Thank you in advance.
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75gmck25

Full Access Member
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Oct 13, 2016
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Location
Northern Virginia
First Name
Bruce
Truck Year
1975
Truck Model
K25 Camper Special TH350 NP203
Engine Size
5.7
Stock setup is this (IIRC)
- large battery cable runs down and over to a wire retainer screwed to the top of the frame, then to the vicinity of the mechanical fuel pump, through another wire retainer that is attached to one of the oil pan bolts, and then to the large terminal on the starter.
Starter then has one wire connected to the solenoid terminal and running up to the key switch.
Two smaller power wires are also connected to the large terminal on the starter
- one wire with a fusible link that runs up to the wire junction on the firewall next to the brake booster. This is your 12 volt power for the truck.
- one wire with a fusible link that runs up to the back of the valve cover and down the top of the cover to connect to the alternator in the passenger side. This is your alternator charge wire.

All three small wires run up from the starter and then through a metal tube that is bolted to one of the transmission housing bolts. This tube keeps them away from exhaust heat.

You can use a more direct path for the alternator charge wire, and run it directly from battery positive over to the Bat terminal on the alternator . I chose to run mine from the battery positive to a junction on the passenger side fender, and then over to the alternator. That junction gives me a place to pull 12 volt power for my headlight relays and other aftermarket accessories. I pulled the junction from another squarebody truck in the junkyard.
 

Elliot W

Junior Member
Joined
Apr 13, 2021
Posts
14
Reaction score
13
Location
Cville
First Name
Elliot
Truck Year
1974
Truck Model
C10
Engine Size
5.3
Thank you very much for the response this is helpful. Sounds like all I have to do is figure out which wire on the harness is from the ignition and figure out the other cable that needs 12v, then run the battery to the starter. My alternator is wired through the stand alone harness on the engine.

I think there were some other accessories getting power in the harness, so I’ll have to see if anything in the truck needs power before putting 12v to them.

Thanks again for the response and help!
 

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