I've got a 99 F-250 7.3. I am doing a body swap to a 77 Supercab. I am into trimming down the harness. I have started into it and got all the loom off and started cutting out some things that I know I don't need like power windows and such. I have not been able to find wiring diagrams though to go much deeper. I have been looking for a week now and have come across the workshop manual but nowhere in there that I can find is there actual wiring diagrams. there are a number of diagrams for specific systems but nothing overly helpful. I know that there are a few guys on here that have done it successfully and judging by all the threads I've read recently they are sharing very few tips. I am trying to trim it down as much as possible. Do away with the fuse box and the three connectors that pass through into the cab. I do want to keep wires in the cab for a few of the warning lights and the DLC and stuff like that. I have a 4r100 so apparently if I disassemble the cluster I can pull the box in there that translates the signal from the VSS for the PCM. I am terrified though that I am going to cut out some seemingly useless wire and not be able to start this thing when it all goes back together and have no idea where to start looking for the issue. Looking at pictures from one members trimmed down harness it looks like he just has the PCM, IDM and the GEM although the GEM is not connected to anything through it's pins on the back it looks like.
Here's a picture of it mounted on the 99 frame because I think it's sweet.

 

Yah. Its a mess. Itll take you a while to sort out whats needed and not. Hack out the gem module... You dont need it and the ecu wont know its missing. Just work backwards from there and eliminate all the wires that went to the gem as cleanly as possible. Air bag and abs and hvac system can disappear too. The engine system doesnt care. The only thing on the cab side you need is the brake switch, e-brake switch, diagnostic port, accelerator pedal, and instrument cluster. Techically, you dont need the instrument cluster but it throws MIL lights for glow-plug indicator faulty if you remove it. Not a big deal thou... A well placed resistor would work too. Search my posts for my f350 engine swap project.

 

That is going to be a really cool rig when you finish! Good luck with all of the wiring.

 

Thanks Prototypemech. I had a look at your thread and watched the video. I still have a pile of questions but I will just have to figure it out like everybody else did.

 

I have been thinking about it and doing a little bit of reading. Why do I need the E brake switch? What does it serve in the path? Also do I need the wires running up to the overdrive button? I imagine that it ties into the PCM somehow.
I was planning to keep the ABS and probably still will. I imagine that it is a self contained system and doesn't need a bunch of other stuff to keep it operational.

 

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Any help is appreciated. I haven't done much cutting lately. I found a complete vehicle wiring diagram for an 02 but it is not quite the same. I have pulled apart the instrument cluster and am going to keep just the circuit board so that it doesn't throw any Glo Plug codes and the transmission gets the speed signal it needs. I am still looking for any knowledge or help that anybody can offer on the rest of it though. Tips on where to find wiring diagrams for my truck. Anybody that has done this before.