Body swapping a 94 gmc onto a 98 Chevy Chassis

[the_orange_gmc]

Hey everyone, I have a 1994 gmc c1500 sl with a tbi 305 that my dad bought from CalTrans back in 2004 with 160k on it. The truck now has 316k on it and it’s tired and in desperate need of a new engine. Right now I have a 1998 Chevy K1500 vortec 5.7 that had 126k on it but I rolled it a couple years ago and I put a new cab on it but I’m wanting to downsize my “fleet” and I want to fix my dads truck to use it. So here’s my grand idea, I’m wanting to take my dads truck and put it on the low mile chassis and keep the vortec engine management but keep the 94 interior in the truck. I don’t want to switch to a different injection system yet because I’m trying to do this as cheaply as possible. So mainly I’m just curious if anyone has done something like this and if anyone has any information on how to make the wiring work between the engine bay in the 98 and the dash in the 94. I know there’s a lot rambling sorry about that but appreciate any advice anyone has, thank you.

 

[AK49BWL]

Well I can tell you most of the electronics between 98 and 94 are the same, save for one thing - the engine will not run without a tune to remove VATS - 98+ uses PassLock, and there's no way to make that work on a 94 column. Alternatively you could get the module to "relearn" a new resistance value and just put a permanent resistor on the module. As far as the cluster tho, everything is the same, save for maybe a couple wire color changes. I have a 97, I swapped the interior to basically a 94 and custom-rewired everything, making my own wiring diagram as I went. It's very time consuming but my 97 has been riding around fully functional with a 94 interior since May or so of 2021.

 

[the_orange_gmc]

Well I can tell you most of the electronics between 98 and 94 are the same, save for one thing - the engine will not run without a tune to remove VATS - 98+ uses PassLock, and there's no way to make that work on a 94 column. Alternatively you could get the module to "relearn" a new resistance value and just put a permanent resistor on the module. As far as the cluster tho, everything is the same, save for maybe a couple wire color changes. I have a 97, I swapped the interior to basically a 94 and custom-rewired everything, making my own wiring diagram as I went. It's very time consuming but my 97 has been riding around fully functional with a 94 interior since May or so of 2021.

Oh awesome, did you use all of the 94 wiring in the cab? Because half of my issues with the 98 I have are wiring and I still can’t decide if it would be easier to tbi swap the engine or mess with the wiring

 

[AK49BWL]

I kinda went to extremes with mine in that I ran every single wire myself... End to end, with new terminals, some new connectors, etc etc ... There's literally nothing factory about the wiring at all in my truck, for any year lol... And that includes the pin-out for the firewall connector and I also have the 0411 PCM so nothing under the hood is really factory electrically either.

In order to do what you want to do (94 interior with 98 engine), you're definitely going to have to de-pin a couple connectors, particularly the firewall connector as they're not compatible with each other. The windshield wipers and cruise control connectors are the same so I don't think you'd need any changes there, but you will need to replace your DLC with the OBDII connector, and the passlock/variable steering control module will need to go in that new interior. The cluster wiring will have to be matched. The A/C request wire no longer goes directly to the A/C clutch, but the PCM. Oh hell lol, I almost forgot the 94 has the PCM in the cab. Yeeaaahhhh.... I mean if you were looking for easy, a 350 TBI engine swap on the 94 would be simpler than mixing the two years, for sure... There's a lot involved. If you want to look, the last page of my project thread in my signature shows basically what I did... I pulled out everything and started from scratch with individual wires to everything. You want to do this thing, I can get you the wiring diagram sets for both years and we can mix and match this thing til it works lol.

 

[the_orange_gmc]

from what I’ve seen on here it’s seems like that it might be easier to replace the intake and swap the motor over to a tbi setup I’m just worried about losing power. Did you by chance write down your wiring diagrams or take pictures?

 

[Komet]

I put an L31 in my '92, but I cheated and used aftermarket engine management. As long as you defeat passlock and give power to the factory ecm, it should work fine though. The TBI ecm can be removed entirely. You'll need the factory service manual for both years to splice it all together.

 

[AK49BWL]

from what I’ve seen on here it’s seems like that it might be easier to replace the intake and swap the motor over to a tbi setup I’m just worried about losing power. Did you by chance write down your wiring diagrams or take pictures?

My truck's entire custom wiring diagram is here - I still need to finish the exterior/engine stuff but the entire interior is complete. https://www.ak49bwl.com/other/wires/1997K1500/MySierra/SierraWiringSchematic.png

 

[the_orange_gmc]

That’s a lot of wire to go through man that looks great thank you!

 

[the_orange_gmc]

I put an L31 in my '92, but I cheated and used aftermarket engine management. As long as you defeat passlock and give power to the factory ecm, it should work fine though. The TBI ecm can be removed entirely. You'll need the factory service manual for both years to splice it all together.

I definitely want to go to a different EFI one day when I can justify dropping the money on it lol

 

[Erik the Awful]

Put in too big of a cam, and that justification comes around real quick.