Wiring Questions C6 & Tcase

[IDIrty]

Hello Everyone!

I have a 1986 6.9, C6, 4x4.

I am wondering if the C6 and T case actually need any wires running to them to be functional? Or as long as I have vacuum will they be functional?

I have deleted almost all “unnecessary wiring” on my truck as I plan to use it as a purely off road vehicle and won’t need lights or anything.

I cut the speedometer cable to Tcase but I wanted to make sure the other wires on Tcase were ok to cut too? And what wires do I need for the C6 (if any) to make it work?

Thank you!

 

[Clb]

It's been a c o o n s age but...
C6 has a vacuum module.
You need it.
The t case wires are just idiot lights

 

[IDIrty]

Thank you!!! Just made my life a lot easier!

 

[Brian VT]

You'll still need the neutral/reverse safety switch, I think.

 

[franklin2]

You'll still need the neutral/reverse safety switch, I think.

Unless you cut the two red/blue wires and twist them together. That will let it crank.

 

[hacked89]

Depending on the type of off roading your doing you don’t need vacuum to the c6 either

 

[franklin2]

Depending on the type of off roading your doing you don’t need vacuum to the c6 either

It will stay in 1st gear all the time.

 

[hacked89]

It will stay in 1st gear all the time.

I’ve tried unhooking the vacuum line and the results were - 1st gear shifts high rpm, 2nd gear, no shift to third gear. This is close to how I drive manual off road in my terrain. Crawling and holding 1st, sometimes second, not using third.

I’m going to be trying this again in the project stress test thread, I’ll let you guys know if I can duplicate it?

 

[Cubey]

It will stay in 1st gear all the time.

Nah it has a mechanical setup that won't allow it in 1st if it's going over a certain speed. I think it forces upshift. I do know you can't force a downshift to 1, even with the shifter, if you're going faster than around 20-25. I know this because of driving the slow as sin RV in the mountains and having to control egt. I'd put it to 1 and back off the throttle to slow on a steep grade if egt was getting too high. It wouldn't go to 1 unless it slowed enough.

 

[Clb]

Disconnect the vacuum module and report back.

 

[XOLATEM]

I am not a fan of deleting stuff unless you want to work on it a lot...you will find out why it is there...

Bypassing the neutral safety switch will learn ya all about running into and over stuff that you don't want to...and maybe run yourself into the back wall of the garage...

Not running a vacuum system for the vacuum modulator will learn ya all about wearing out the pump body and gears...and eventually strip the hub on the rear planetary annulus gear unless you have it built by someone that knows the difference and puts an '88 setup in there...and that is only prolonging the inevitable...

When you delete the vacuum system you force the unit to run max mainline pressure and it will bang into gear and break things...

An exception is to run a manual valve body designed by a company that has a good reputation for quality...not just a splashy website...

With a stock setup...the not going into manual low unless you slow down to a lower speed is a safety feature for road driving assisted by the governor system...keeps you from skidding on slick roads and learning all about bodywork and cops, judges, and lawyers...

If you are off-road...probably would not matter...I'd go with a good manual valve body setup if it were me....that would help with a bit of longevity...

Good luck on it...

P.S. I have seen the back of a C-6 case break in the Low-Reverse piston area...from high line pressure...and...at the time...it was super tough to find another case to fix the guy up...maybe now it will be easier since the C-6 is falling out of favor because of no overdrive...

I am more of a fan of firm, but not brutal, shifts into gear and then...once it is in gear...romping on it...

I hope you have a good transmission man close by...

 

[IDIrty]

I’ve tried unhooking the vacuum line and the results were - 1st gear shifts high rpm, 2nd gear, no shift to third gear. This is close to how I drive manual off road in my terrain. Crawling and holding 1st, sometimes second, not using third.

I’m going to be trying this again in the project stress test thread, I’ll let you guys know if I can duplicate it?

I will be keeping vaccuum line in but, am still curious to see what disconnecting vaccuum does

 

[IDIrty]

I am not a fan of deleting stuff unless you want to work on it a lot...you will find out why it is there...

Bypassing the neutral safety switch will learn ya all about running into and over stuff that you don't want to...and maybe run yourself into the back wall of the garage...

Not running a vacuum system for the vacuum modulator will learn ya all about wearing out the pump body and gears...and eventually strip the hub on the rear planetary annulus gear unless you have it built by someone that knows the difference and puts an '88 setup in there...and that is only prolonging the inevitable...

When you delete the vacuum system you force the unit to run max mainline pressure and it will bang into gear and break things...

An exception is to run a manual valve body designed by a company that has a good reputation for quality...not just a splashy website...

With a stock setup...the not going into manual low unless you slow down to a lower speed is a safety feature for road driving assisted by the governor system...keeps you from skidding on slick roads and learning all about bodywork and cops, judges, and lawyers...

If you are off-road...probably would not matter...I'd go with a good manual valve body setup if it were me....that would help with a bit of longevity...

Good luck on it...

P.S. I have seen the back of a C-6 case break in the Low-Reverse piston area...from high line pressure...and...at the time...it was super tough to find another case to fix the guy up...maybe now it will be easier since the C-6 is falling out of favor because of no overdrive...

I am more of a fan of firm, but not brutal, shifts into gear and then...once it is in gear...romping on it...

I hope you have a good transmission man close by...

Agreed. Keeping everything that is functional as is. Only gotten rid of stuff like lights, radio, air con, etc (as this will be a mud truck). I was more asking to see if there were dummy lights on the Bruce Jenner as there are on the Tcase.

 

[franklin2]

I will be keeping vaccuum line in but, am still curious to see what disconnecting vaccuum does

Like the others said, it will eventually shift without the vacuum being hooked up, but you will think it's going to blow the engine before it does finally shift.

They do make manual shift valve bodies for the c6. At least they used to. These transmissions are getting old, into the antique realm. Aren't we all getting old?

 

[KansasIDI]

Only thing that I’m glad I deleted is the glow plugs on Duchess. I also don’t have a clutch switch, but of course, all big rigs with manuals are that way. Wiggle the shifter, then crank her over… I am also planning in getting rid of the power steering, but replacing with a true manual steering box. Once again, not too worried about that.

Perhaps I am one to simplify, rather than delete? Most of what my truck is equipped with, I am in favor of.

I probably wouldn’t delete the system that shifts an auto tranny, or the system that boosts my brakes…

The service truck will keep power steering, even though I think its pump is on the way out, but, being hydroboost, and 2 1/2 times heavier than Duchess, power steering and the crummy luck I’ve been having with pumps may have to stick around for that truck…