OK here's my deal I have a 91v3500 it has the 4l80e and 205 transfer case. Problem is I want take my 5.9 out of my 84k3500 with a th400 adapter. The 4l80e needs a TCU which I have two one from TCU and another from a 93k3500 6.5/4l80e.

Main problem I need a tach signal, I can tap into the alt but how do you make this signal work with the TCUs that I have. I also have a tps and the speed sensor on the 205 is electronic.

I'm thinking the TCU from tci is my best bet cause if I have a problem I can call someone who made it.

 

Heres my guess most of the chevy diesel run off the alternator I would run the chevy tcu and tach but one question I couldnt be sure of to tell you is are the pully ratios the same from the cummins to chevy I know many have used alternators for tachs if I havent helped with a straight answer I hope I got you closer with questions to look for

 

i did just get a hold of the tci guy that i been talking to for the past months. he said "Once you have the setup to converter the alternator pulse over to tach signal then you should be able to wire in our RPM module and our harness and it will read the signal. "
but searched their website and cant find it. i know dakota digital makes one and its $60. but the way you're talking makes sense and would be cheaper. so ill have to do a little more research on the gm tcu

 

Well good luck I will for sure be waiting on to see what you find out