TH400 Question

[cwlehner]

My truck has some pretty stiff shifts and they always occur at the same vehicle speeds (1-2 at 30 MPh and 2-3 at 50 MPH). As you can imagine this isn't good for driveability or fuel economy, the only time it feels smooth is during a WOT launch. I was thinking about this while driving to work this morning and was wondering how the trans line pressure is adjusted as a function of engine load. I know the 700R4's have a TV cable, but I am not that familiar with the 400's.. Haven't had a chance to pop the hood and take a look, but if anyone has any ideas I would appreciate it.

Thanks

[BobND]

The SPEED at which a THM-400 shifts on a diesel is determined by the weights and springs in the governor, combined with the vacuum fed to it's vacuum modulator by the vacuum control valve operated by the I.P.'s throttle shaft.

If there is no vacuum, due to a failed pump, or a leak, shifts will be late and HARSH.

The angular position of the valve in respect to the I.P. will "tune" the shifts, if all else is working OK.

[catmandoo]

first pull the hose off the modulator if fluid comes out its junk,if not check vac pump.or hoses