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1985 6.2 Diesel vs Gas engine underhood wiring harness for conversion.

Discussion in 'Electrical & Audio' started by HotRodPC, Oct 22, 2020.

  1. HotRodPC

    HotRodPC Administrator Staff Member

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    I'm trying to figure out which way I want to go about this conversion from my K10 that was originally a 6.2 Diesel Truck. It looks like the underhood harness is way to short and been buthered all to hell. Sooo I'm going to have to find an underhood harness for this truck.

    I do aslo have that 85 C20 cab that has all it's wiring still in it. Of course I have that underhood harness but it's been cut in half so I'd have to splice all those wires together to use it. I think I'd rather just use a complete harness that doesn't need spliced.

    The BIG QUESTION is, and asking for opinions.... How much different is the underhood wiring on a 6.2 Diesel to a Gas engine. I'm aware of the controller for the glow plugs etc. It's my understanding the ignition wire on a Diesel went to the injector pump and you just move it to the coil on a gas engine. That part is easy enough. But I think I want a proper gas conversion. Does anyone if I was to find a Gas truck underhood harness, will it work plug and play just to plug it in on the firewall and the pinout knows where everything should go like AC, gauges, igntion power to the coil etc? If I want it to be a true gas truck should I swap fuse blocks and all from the 85 C20 gas truck and put it in the 85 K10 Diesel truck? Seems like that would be a ton of work.
     
  2. HotRodPC

    HotRodPC Administrator Staff Member

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    Also, I'm planning on taking the big block out of the 86 C20 gas truck for the K10/20 conversion. Pretty sure I seen somewhere where the fuse blocks are different and changed in 86. So this likely means my underhood harness on the 86 wouldn't fit or wire up right to the 85 year model fuse block right? I was hoping it would be just as easy as plugging in the gas harness at the bulkhead on the front of the firewall, but I have a feeling it's not going to be that easy.
     
  3. HotRodPC

    HotRodPC Administrator Staff Member

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    Hey self, the fuse blocks didn't change from 85 to 86. They appear to be identical.

    Top Pic, 85 Silverado K10 Diesel 6.2

    Bottom Pic, 86 Silverado C20 Gas 454

    FuseBlock 002.jpg

    FuseBlock86C20Slvrado7b.jpg
     
  4. MrMarty51

    MrMarty51 Full Access Member

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    I`m watching this thread. Maybe a repair manual for the two units might show differences.
    I`m thinking more to the possibility that if the outside the firewall harnesses was changed that there would need to be some changing on the fuse panel side harness too, Being that there is a possibility that some slots between the two are used on one and not used on the other.
     
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  5. HotRodPC

    HotRodPC Administrator Staff Member

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    You're 100% on target Marty. That is what I did too. I pulled the CK Wiring Manuals for both years and looked at the engine harnesses pinout. So they do appear to be the same for both years, BUT, there is differences for the 6.2 and in my case since it's a LE8 454 considered Heavy Duty vs Light Duty for the LE9 305 and the 350. The 454 has the aux cooling fan and all that good stuff, so I'll use the engine harness off the 86 C20. I also hope you're right that I'll be able to figure out the pin difference and just swap some pins on the fuse block.

    Otherwise, I think my other choice would be to remove the whole fuse block from the C20 and install it in the K10/20. I really don't want to do all that work but I might end up doing that. I'm kinda picky about how things get done and don't like redneckin' crap. Which is a bad thing. That in itself is what keeps me overwhelmed and overthinking things and why I never get plans carried through. Sometimes the want to do it right will hinder it getting done at all.

    So hopefully some others might have some input or knowledge. That wiring diagram online is very small and I'm pretty impatient to follow all those little lines like a maze across 3 or 4 pieces of paper to see exactly where they go. I'll have to do some incorporating of the 4wd harness too for things like the 4wd light on the dash since the C20 harness obviously won't have that.
     
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  6. AuroraGirl

    AuroraGirl Supporting Member Supporting Member

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    go to a 78 glass fuse block. youll regret it instantly and have everything you need and so much to be desired!
     
  7. MrMarty51

    MrMarty51 Full Access Member

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    Lift the current fuse block enough to release the fuse connectors/wires, set in the 454 block and plug back in what is applicable, use pigtails from the 454 chassis and add the new wires and just string onto those pigtails to complete the necessary circuits. Keep from having to unwrap and strip harnesses.
     
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  8. HotRodPC

    HotRodPC Administrator Staff Member

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    I likes the way you thank. I'm hoping it works just that way too. Exactly what I'm hoping. Now I might have to pull a wire or 2 from the C20 harness. I won't tear the harness appart. I'll just add a wire or move them around. I'm really hoping the 85 Diesel block is the same on the front bulkhead under the hood so I can just take the harness off the C20 and put it on the K10 after moving those wires around to match the C20 like you're saying. That's my hopes anyway. If not, I'll likely pull the whole interior harness from the 85 C20 cab I have sitting out at my dads. It was a small block truck though. Might even just pull the fuse block from the 86 C20 454 truck and then I know for a fact it'll fit and all work perfect just as if it was originally a C20 454 truck then all I have to add is harness that's in the K10 for the Transfer case to the dash to light 4WD. In all honesty, I could live without that if I had to. I am going to keep the dual batteries though since I have both trays in the K10 diesel truck and the 80PSI Oil Pressure guage since my current truck goes right on up to about 58-59 PSI at anything over about 2000 rpm. The gas trucks get the 60PSI gauge.
     
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  9. custodian

    custodian Full Access Member

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    I was lucky when I switched from the 6.2 to a 350. I just cut the wires out I didn't need and used the glow plug wires for the HEI power and the electric choke power. I kept the wires I needed such as the oil pressure, temp, alternator, trans and left the cruise control and AC wires intact.
     
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  10. HotRodPC

    HotRodPC Administrator Staff Member

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    This was pretty much info I was looking for @custodian. Thanks for that. So now I can look at the wiring diagram and figure out what color those are. I know things like the dk green is temp sender, tan is the Oil pressure sender, heavy guage red goes to the junk on the firewall, heavy gauge purple to the starter IIRC and things like that.

    I'd still rather prefer made the wiring like factory C/K 20 gas.
     
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