Vac system for T400 trans

[Dvldog8793]

Howdy
Just an update with information that I figured would be useful for others. I got my vacuum system figured out and my T400 shifts pretty good. A shift kit to firm things up a bit more is in the works. The following information was for my truck and should not be considered standard but I think it is pretty close.
- Vacuum pump output was 22in idle
- Vacuum after the injector mounted switch was 16in idle and would drop to 4-6 at high RPM
- vacuum Feed from vac pump to the switch is the bottom port.
- My hard line to Trans mod was full of nasties that I cleaned out with brake parts cleaner.
I replaced the switch and had to open up the adjusting slots about 1/16 of an inch to get the vacuum that I thought right.
Vacuum switch has a small rubber gasket inside that I would guess is the failure point. Not sure why I see a loss at the switch but I couldn't make it go away. I expect it is just due to system/connector loss.
- Tie wrap the vac line rubber connector at the trans mod/hard line to ensure no loss.
Thanks! Hope this helps someone!

[More Power]

Thanks for the tips!

For those who may not know, the vacuum system, as it pertains to the TH400/6.2L diesel, was designed to emulate manifold vacuum produced on a gasoline engine.

Manifold vacuum on a gas engine becomes more negative at low throttle angles near idle (approaching 22 inches) and close to zero (no or very little vacuum) when the throttle is near wide open. The vacuum modulator is used for shift scheduling and shift quality, depending on throttle position.

There is also a governor assembly within the transmission that is the final determination for upshifting. Without vacuum, the transmission will shift as though you've got the skinny-pedal to the floor, meaning upshifts into the next higher gear will occur at high engine speeds regardless of how far the accelerator pedal is pressed.

So, the transmission's governor assembly is specific to either gas or diesel because of the lower governed engine speeds seen in a diesel. Jim

[Vin82k3500]

I got a detent cable setup for a th 400 that goes in place of the vac modulator not using it I converted to an nv4500 5 speed