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I dropped the motor and harness in the truck but now I'm going through the actual truck wiring and it's all kinds of screwed. Basically I know what wires come through the firewall and where they are suppose to go. Problem the previous owner(s) spliced in so much crap I don't know if there is suppose to be a fusible link? an inline fuse? Duct tape????
Can I run the bulkhead wire (powers the fuse box) straight to the starter lug or is there suppose to be some sort of fuse inline there?
The actual ignition looks like it goes through some relays on the firewall and probably the neutral safety switch. I plan to clean this up a little bit and let it run as is.
Here is what I am having to unscrew.....
Yes these are all power wires that have had splices put in. The fusible link I have no idea where that went too.
No problem man! Looks like a cool ride. Who made the stand alone harness?
I did from the stock harness and instructions from LT1 swap. It's kind of a mess at the computer because I want to fire it up before I tuck it all away.
You only need four wires connected to get it started and running as long as the Powertrain Control Module (PCM) has been flashed to remove the Vehicle Anti Theft System that prevents starting. There are other things inside the PCM that will need to be shutoff to prevent OBD II codes but with VATS suppressed it will at least let it start and run.
1 - Full time 12 volt "keep alive" power to the keep alive wiring port on the PCM. This wire only needs very low amperage support. This makes for easier starting and better operation because the PCM "remembers" its learned parameters, especially fuel related things.
2 - Ignition key on 12 volt power that actually boots up and operates the PCM. It needs to have 12 volt power on at "Run" and during cranking on the engine. This circuit should need no more than perhaps five amps.
3 - 12 volt power to the starter from the ignition key for cranking. Gonna' need a lotta' amps. Don't forget the neutral safety switch here.
4 - 12 volt power to the fuel pump during Run and during cranking. Perhaps five amps needed here unless you have a very large fuel pump. For best operation and what most people do, allow the PCM to control fuel pump priming (two seconds or so on) and then shut off if no RPM is detected. This is also a safety feature. If RPM is detected, the PCM kicks the fuel pump into operation to support starting and operation. Most people also run the pump from a relay because the PCM is unable to control this directly - it is only controlling a ground that a relay uses to control the fuel pump.
Rick
Last edited by B52bombardier1; May 26, 2019 at 03:36 PM.
Ten (10) gauge wire might be a little thin for a starter motor cable. I'm thinking four (4) gauge wiring for the least voltage drop and best spinning power to the starter motor. The 12 volt ground wire from the battery to the block, chassis, radiator core support and the 4L60E also needs to be large and thick. You can never have too many good ground wires.
Ten amps to operate all power through the firewall bulkhead into the fuse block is also WAY . . . WAY too small. Think about the amps needed for lighting, heater blower, radio, wipers, etc.
Some of those photos are factory wires. At best they are 8 gauge and that is all that fed the bulkhead fuse box. A 4 gauge would be over kill and there would be issues with running that thick of a wire to the harness.
Some of those photos are factory wires. At best they are 8 gauge and that is all that fed the bulkhead fuse box. A 4 gauge would be over kill and there would be issues with running that thick of a wire to the harness.
I suggest getting a factory wiring diagram. It will show if there is supposed to be a usable link in the wire that feeds the fuse box. I suspect there is supposed to be one there.
I'm only suggesting four gauge wiring to the starter. Just to the starter - ten gauge to the starter is too thin. Eight gauge might be sufficient to supply the bulkhead fuse box.
I'm still collecting parts for my conversion but I was at the wreckers earlier this week and you might consider what I got for use for your truck. I went to an 09 Silverado and got the positive cable from the battery to the megafuse on the firewall. The megafuse has three heavy positive cables going from it to the starter, the alternator and a long one going across to the under hood fuse/relay box. I managed to remove it in one piece. The long one going to the fuse/relay box could be used as a main source for your trucks fuse box although you might swap it out for a lighter gauge.
(Don't forget the plastic cover for the megafuse and the one for the battery clamp.)
Also keep in mind that the stock LS alternator puts out well in excess of a 110 amps so your wiring needs be able to handle the amp load safely. The wiring from the starter stud to the alternator stud needs to large enough to handle the load.
Personally I hate sorting out hacked wiring on projects. The truck is 40 years old and the wiring 40 years old.
I would consider replacing it entirely with a kit from Painless or AWW.
In lieu of that in the Painless kits I've used all have a "Maxi" fuse to protect the fuse block instead of fusible links like in the original harness. You can buy similar Maxi fuses at NAPA.
As said above a wiring diagram is your new best friend. I believe on a 79 C10 with an auto trans the neutral safety switch is mounted on the lower part of the steering column. It's part of the start switch? Typically the start run wire is pink? coming out of the switch. And the start wire that engages the starter solenoid is purple? It's been some time since I've worked on worked on one.
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C10 starter/ignition wiring help 
