Wiring harness question

[Hoodbj]

I recently purchased a 87 r10 with a 305. I also purchased a 91 suburban as a donor truck for parts. The 91 suburban has a 350 but has the serpentine belt system. I am in the process of putting the 350 from the 91 into the 87. Would it be best to remove the engine wiring harness from the suburban with the engine? I'm not sure all of the differences between the engines as far as electrical plugs go. If replacing harness is best what all is required to remove harness? Thanks

 

[RetroC10Sport]

It took me all of 20 minutes to remove an entire TBI harness for a member. Take everything, computer and all. You might need to move some wires around at the C100 connector on the driver's side of the firewall near the fusebox.

 

[HotRodPC]

And if you would Billy, keep us updated on this. There is a strong possibly I might be doing the exact same thing. I have a feeling I'm going to be transferring the whole harness from a 91 Burb that is TBI 350/700r4 to an 85 K10 that was originally 6.2 Diesel/700r4. So for me, it'll be out with all the Diesel harness and transferring all the 91 harness into the cab too including the Electronic Speedo Cluster.

For me, if I can get this project off the ground, this will be the truck I also run an A833 4 speed manual trans, so I'll need to use a Tcase with the electonic speedo provision with reluctor wheel for the speedo. You might want to check into that for yourself also if you don't have the speedo, which I think you do, that has the manual speedo, but also has the adapter box for the ECM. Retro will certainly be able to verify that for you.

 

[kleedus]

the 87 uses the old style plug on the firewall and the 91 uses the newer small style plug will not interchange

it would be easier to keep the 87 harness they both should be tbi

 

[HotRodPC]

the 87 uses the old style plug on the firewall and the 91 uses the newer small style plug will not interchange

it would be easier to keep the 87 harness they both should be tbi

So that means for my 85 K10 OEM Diesel truck, I will have to swap all the behind the dash wiring too. Well so be it, just might be worth it so I know it's all GAS compatible. I've got the whole 91 Burb, so it shouldn't be a problem.

 

[Hoodbj]

Thanks everyone, I will keep everyone updated as I go. I just finished the core support swap onto the pickup. I plan on starting the engine tomorrow. I already have the 305 out of the pickup, I only have the 350 from the burb to pull. They are both tbi but the location of accessories, ac, alternator etc are in different spots and it looks like alternator may have different plug(just going from memory). If I do leave the wiring harness in the 87 what would I have to splice/convert to fit? Thanks again!

 

[Hoodbj]

Ok, got the engine stabbed yesterday and everything went pretty smooth other than Hercules used a cheater to tighten all bolts in the suburban and then emursed them in acid to rust them really well. Going to start plugging in connectors and installing tranny tomorrow. Everything looks to plug in the same so far exept I will have to extend alternator and ac wire due to them being on different sides than before. I made the ac hoses work but will probably be looking out for a set off of an 89-98 pickup since they came out with the ac on that side. The suburban had rear ac so the hoses wouldn't work. I'll update when up and running.

 

[Jims86]

Ok, got the engine stabbed yesterday and everything went pretty smooth other than Hercules used a cheater to tighten all bolts in the suburban and then emursed them in acid to rust them really well. Going to start plugging in connectors and installing tranny tomorrow. Everything looks to plug in the same so far exept I will have to extend alternator and ac wire due to them being on different sides than before. I made the ac hoses work but will probably be looking out for a set off of an 89-98 pickup since they came out with the ac on that side. The suburban had rear ac so the hoses wouldn't work. I'll update when up and running.

And that alt and AC wiring is the only thing you need to modify. No need to move harnesses from one square body to another. Use the EcM from the burb, it will plug right into the 87 harness.

 

[HotRodPC]

Whoa, Jim is here.

Our buddy kleedus is saying the 87 and 91 harnesses are different. I hope to be having to deal with a similar issue later. The 85K10 I have, I don't have an underhood harness at all. The asshats took it. The truck was originally 6.2 and I hope to be putitng a TBI or maybe even a SFI Vortec engine in it.

 

[Jims86]

Whoa, Jim is here.

Our buddy kleedus is saying the 87 and 91 harnesses are different. I hope to be having to deal with a similar issue later. The 85K10 I have, I don't have an underhood harness at all. The asshats took it. The truck was originally 6.2 and I hope to be putitng a TBI or maybe even a SFI Vortec engine in it.

Harnesses are different between the 91 gMT400 1/2,-3/4 ton and Squarebodies, and unless the 91 burb, whitch is still square, is a 3/4 or 1 ton with a THM-r2(early 4l80 E) the TBI hanesses and ECMs are the same. And the only difeerence is really the harness length,mbecaus they routed the GMT 400 harness under the HVAC box, instead of over and to the pass side of the trans tunnel.

 

[HotRodPC]

So you're saying GMT400 91 will be different and not match up to a 91 Squarebody harness, but the Squarebody 91 harness would still plug into a 85 Square???

 

[Jims86]

No,i meant an SB TBI truck. 86 and down are not worth messing with to incorporate an EFI harness, just run it stand alone. The 86 and earlier harnesses pretty much only handle the charging system. In the OP's case, he is just swapping an 87 305 for a 91 sub 350. only difference is the ecm engine wise, and a slim chance that the TPS plug is different. And then theres the modded wiring for the ac and Alt, just because they sawapped sides for the serp setup.

 

[Hoodbj]

I noticed on the harness that where it plugs in on the firewall the light harness plugs into the bottom on the burb and the top on the pickup, not sure if it would make a difference but not much modifying needed to make the old harness it appears. I'm hoping to get back on it this weekend. I also noticed the 350 didn't have the oil psi sending unit directly above the oil filter, the port was there but it was plugged, should I install mine here or did they update this to a different place?

 

[HotRodPC]

I noticed on the harness that where it plugs in on the firewall the light harness plugs into the bottom on the burb and the top on the pickup, not sure if it would make a difference but not much modifying needed to make the old harness it appears. I'm hoping to get back on it this weekend. I also noticed the 350 didn't have the oil psi sending unit directly above the oil filter, the port was there but it was plugged, should I install mine here or did they update this to a different place?

Many times the Oil Pressure sendor was behind the Dizzy on top of the block at the back.

 

[Jims86]

I noticed on the harness that where it plugs in on the firewall the light harness plugs into the bottom on the burb and the top on the pickup, not sure if it would make a difference but not much modifying needed to make the old harness it appears. I'm hoping to get back on it this weekend. I also noticed the 350 didn't have the oil psi sending unit directly above the oil filter, the port was there but it was plugged, should I install mine here or did they update this to a different place?

No reason to change the harness just to install the 350. Just swap the ecM, as there are different chips for the 350 and 305.
The oil pressure sender on the 350 should be behind and to the drivers side of the distributor, tapped in with the oil pressure switch for the fuel pump.