1990 K3500 Duly 4x4 Trany swap to 4 spd auto

[StewartB]

Looking to swap trany and; 1 keep towing and torque capacity, 2 get an overdrive for better gas mileage. Best ans cleanest swap info would be greatly appreciated.

[CamGTP]

A 4L80e swap would be what I would do.

 

I don't know what transfer case you have and whether or not it will be a straight bolt on for the 4L80e. You'll have to modify the transmission crossmember or make a new one because the 4L80e is longer than a Th400 and likely longer than whatever 5 speed manual was used back then.

 

Your drive shafts will have to be shortend or made longer for the front.

 

And you will need a transmission controller for the 4L80e.

 

https://www.tciauto.com/ez-tcu-gm-4l60e-4l65e-4l80e-4l85e-transmission-controller.html

[StewartB]

Thanks for the info.

[spikeitin]

i changed a 96 from automatic to manual and if you change out your transfer case with the automatic tranny it should all bolt up... however you are going to have electronic issues unless you pretty much change out your entire wiring harness with computer... going the other way is much better but i have a problem with mine as well cuz i didn't change harness or computer... so my check engine light is constantly on but i like it... who knows anything about dually tires and max size k3500?

 

[Noah Allshouse]

Max size if your gonna haul is 265-75-R16! That's what I have on my 91 k3500 4x4 dually and if I get a good bit of weight on her or tow my 28ft prowler the inside and outside walls touch!! Lol!