I had kind of a strange thing happen. A friend with lots of experience working on the kind of step van i have (P-30 chassis) has always told me the transmission will not work if the vacuum pump goes out so i should keep a spare on hand. I had noticed that it was slow to shift out of first gear - it would rev a bit high and then get into the next gear kind of rough - but usually that was when it was just warming up. Once it got on the highway it seemed to shift fine. Then when i was working on it i found that the vacuum hose had gotten disconnected right at the pump - so the pump was doing nothing but it was still shifting. I believe it is a TH400 HD transmission (automatic).
Anyway, so i put the hose back on and at the same time installed a new injection pump, and now it shifts very easily especially coming out of first gear. It happens so soft that you don't even notice it.
But the strange thing here is that when i tried to test to see if the pump was working, it did not seem to be. On mine there is a T right near the transmission dip stick tube, i believe the line comes from the Injection pump and goes to the trans, and there is a little rubber cap on the T where you can take it off. When i take it off, while idling, and put my finger over it, there does not seem to be any vacuum at all. I reved it up a little and still none.

the reason this concerns me is that the friend i mentioned has the exact same van, and said that there were times in the past when his vacuum pump went out, like on the highway, and the whole thing downshifted back to first gear on him. That's what i am trying to avoid, getting stuck like that.

so it is really hard for me to go and buy a new vacuum pump when it is shifting so well, and on mine the vacuum pump doesn't affect anything else anyway. But still there does not seem to be any vacuum coming from that line.
Any advice on how to test it?
thanks,
Paul

start the truck and let it idle remove the vacuum line from the transmission modulator on the pasengers side of trans and install a vacuum guage there you should have around 15-18 inches of vac and it should drop as you rev it up the trans will shift with no vacuum it will just be late and harsh you can also manuly shift that trans up with no vac

Paul, Disconnect the main line (3/8") from the fitting on the vacuum pump. Put a vacuum guage on the fitting at the pump (metal line coming out of the pump). The vacuum reading should be near barometric pressure. Meaning, roughly 28" of vacuum at sea level. The lower the baro reading for your altitude, the lower the vacuum reading at the pump will be.

If you do not get a strong vacuum signal, or you get no (zero) reading, the pump is bad. Start at the pump and work your way out and down the vacuum line(s). If the pump is good, look for a leak elsewhere. The reason to start at the pump is to eliminate all the tubing, lines, plastic tees, potential leaks, etc.

If you eventually get to the switch on the injector pump and have a strong vacuum signal at the inner vacuum hose bib, put the vacuum guage on the outer hose bib and you should see 8" to 12" at closed throttle (idle) position. The vacuum should drop as the throttle is opened. You can loosen the two 3/8 hex head screws down on the side of the IP holding the black plastic vacuum switch (regulator valve) and move the valve toward the front of the engine to increase the vacuum to the modulator. This causes earlier, softer shifts. Move it back toward the rear of the engine (firewall) to lessen the vacuum signal to the trans modulator. This causes later, firmer shifts.

Check Fire!! Reverse the above statement about which way to move it, Move the switch toward the front of the truck to make it shift harder and at a higher speed. Move it toward the rear to soften the shifts, sorry for the confusion. Fred