while working on my step van the other day i realized, to my horror, that the output hose of the vacuum pump had become disconnected! I have no idea how long it had been like this, but probably for a long time.
I know the vacuum pump works in conjunction with the transmission, and a friend of mine is always telling me to keep a spare on board in case it goes out because then i will only have first gear and if i try to drive any faster i will ruin the transmission. He said if it goes out while on the highway it is possible the thing will downshift to first gear again.
So first of all, does anyone have an idea of why my transmission was shifting just fine without it? I did notice that when i first started up and drove down the country road i live on, the first time i would speed up a bit it was a little slow to shift - it would rev a bit before clanking into the next gear. But as soon as getting on the main road it shifted perfectly. Maybe it is just a freak thing and the hose came off right before i realized it, but my guess is that it had been that way for months.
Also, what else does the vacuum pump do? I found a line going to the injection pump that looked like it was connected to the same line but was not certain.
thanks,
Paul

The experts can correct me if I'm wrong, but I think the vacuum pump is only used for the transmission modulator and maybe to run accessories like A/C control and stuff. The line to the IP would just be the trans vacuum control that is mounted there because it works off of the throttle position.

I believe that without the pump working right, the transmission would still work okay. If I'm thinking correctly, it would just tell the transmission that you are at wide open throttle all the time so it would wait longer to shift. It would still shift, just wait longer because it thinks you are gunning it. I've wondered whether my modulator stuff is working properly too, since my trans seems to shift late all the time.

I'm a novice so I may be corrected on this stuff.

Yep I agree. Vacuum pump makes the vacuum modulator work on the TH350 and TH400 transmissions, the AC/heater vent doors and the such, and the EGR on the C code engines. I also think youre right, a vacuum line does go to the injection pump to work the vacuum regulator valve thats attached to it. Not too sure what that exactly does though.
Early truck 6.2s had a gear driven vacuum pump and the later models had a belt driven I believe. Also, I think, all vans had the belt driven one?
TH700R4 transmissons dont have a dont have a vacuum modulator, they have a throttle cable in its place.
Like the previous post, I believe if the pump wasnt working correctly(or disconnected) the transmisson would just shift late, but it would still work. Youd also have a problem with anything else vacuum operated of course.
Recently had a modulator go bad in a TH400 here. Transmisson was still shifting but transmisson fluid was being sucked through the vacuum lines. Low tranny fluid was first noticable warning.
Correct me if im wrong with anything.

On my truck the vacuum pump powers the brakes too. In fact, since I have a manual tranny and no a/c, that's all it does I think.

The vacuum pump simply supplies vacuum for any and all vehicle needs. Diesels can't make vacuum because of the open intake, so the vacuum pump generates a substitute signal for whatever requires vacuum to operate.

The THM350/THM400 uses a variable vacuum signal to increase modulator pressure to the shift valves and the clutches depending on throttle position/engine load. The greater the load, the higher the shift pressure needs to be to prevent low speed, soft shifts and burned clutches.

The default position of the modulator valve is full pressure (spring loaded). Add a vacuum signal and it retards or lowers the pressure to soften the light throttle/light load shifts. Without a vacuum signal (i.e. line pulled off or misadjusted VRV) the trans thinks the engine is at full load/throttle and supplies max pressure to the clutches and shift valves. The governor must overcome shift valve pressure to make the valve move and shift to the next higher gear. It must spin faster when the modulator pressure is high. This equates to having to "push" the truck to a higher than normal speed before it shifts. Because pressure is higher, the shift is more firm (read: BANG). No harm is done from a firm shift as long as it doesn't happen forever. The parts that will feel the strain are drive line/rear axle components that are worn or weak. Harsh shifts have been known to snap u-joints. I'd fix it just to be safe.