Re: vacuum regulator valve (VRV) = rough shifting
All ATs shift below the engine's peak torque at light accel, so what you feel at the seat-of-the-pants is more-or-less constant rate of acceleration, tapering off when the HP required to move the vehicle down the road meets available HP.
If the AT shifts higher up at the same torque delivery (ie the accel pedal is not moved), the truck will accel at a decreasing rate at the engine RPM rises. Then, when the AT finally gets around to upshifting, the accel rate rises again. Gives a surgy/choppy ride because instead of a smooth accel curve, the truck is surging forward, the holding back, then surging forward again.
That's what I mean by "shifting high". The shift point is higher than it should be for a given accel position.
The vacuum modulator is supposed to sense how hard the engine is working and indirectly how hard you're pushing on the accel pedal, via the vacuum level. On a gasser, this is intake manifold vacuum, and when you put your foot down the throttle opens, manifold pressure rises (that is, vacuum "drops" or falls off) and the AT's vacuum modulator uses that signal to raise the line pressure (changes the clutch/band apply pressure, preventing slippage), move the upshift point higher against governor pressure (or perhaps precipitate a part-throttle downshift), and change the next shift's apply rate -- that is, speed up the amount of time to accomplish the next shift.
Therefore, when the vacuum is weak the trans "thinks" that there is a lot of torque being applied to the trans, and it modifies its hydraulics and timings to suit.
On your diesel, if you have no or low system vacuum (failing vacuum pump, large vacuum leak at the brake booster, cruise control servo, HVAC vacuum motor, broken vac. line), the trans is going to "think" that you've got your foot down further than is the case, and will shift late (high) and harder than it should for the actual torque applied via the engine. You get a late shift, and it's sudden and harsh. This is not hard on the transmission, it's what some shift kits do (like the TransGo and B&M Shift Improver kit) except that shift kits don't move the shift points significanly higher at light loads.
Because a trans that is controlled via a vacuum modulator uses it to modify both the shift point and the shift application speed, it's a compromise -- but one that has been tailored to work pretty well. But you can see why a misadjusted VRV or low system vacuum will affect both the shift quality and shift point. The blunting effect of an unlocked torque converted masks some of this, but when mfgrs universally went to locking torque converters in the early 80's, more sophisticated controls became necessary to control the shift point separately from the shift application speed, as many torque converters were in a locked condition as low as 2nd gear, never mind OD, and when the rate of shift application is wrong on a solid gearset, people complain! That's why, for example, my A4LD in my Aerostar uses the PCM to control when OD or TCC lockup occurs, by providing (separately from the vacuum modulator, which it still uses) that shift point information separately from engine manifold vacuum.
Nowadays, of course, this is all old tech, because everybody's automatic transmission are under computer control to some extent, and I haven't seen a vacuum modulator on a new car in years.
Sorry I took so long to get there.
Al S. firstname.lastname@example.org
1984 F-250 4x4 C6, BW1345, 3.54 D70, XLT extra cab, box-stock so far.