Transmission troubles

OK. I've been given a 1996 Grand Am SE 2.4L with 139,000 miles. I know enough about automatic transmissions to be dangerous.

A couple of years ago, my niece was driving this vehicle and I discovered one evening as she was leaving our house that the car wouldn't move until it was completely warmed up. I looked at the car and found that there was a check valve in the vacuum line to the vacuum modulator that was installed backwards. I turned it around, and everything cleared up.

So, now I've been given the car, and my sister-in-law told me that the car will very sporadically "go out of gear" when sitting at a stoplight. She said that you had to shift to Park, then to Drive, and the problem fixed itself. While driving the car, I noticed that it slams into reverse when cold, and that it downshifts very hard from 4th to 3rd. Also, the ETS OFF light was on.

I took the car to a transmission specialist yesterday, and he discovered that there was a hole in the vacuum line to the vacuum modulator. He fixed that and said that it cleared up the reverse slam and the hard shifts from 4th to 3rd. He said, "with the vacuum modulator not working, the transmission was shifting with full pressure."

He also showed me a book that says that this particular transmission has an internal problem once it gets a lot of miles that causes the car to go into a neutral condition after decelerating from 4th gear. He said that fixing that would require a rebuild, and he suggested that we just put in a used transmission.

We decided that we would go ahead and take the car and see if we can re-create the "neutralizing" thing.

Well, when I went to pick up the car today, it wouldn't back up. Complete opposite of the reverse slam we were experiencing before.

So, we've decided to go ahead with the used transmission.

However, here's where I'm confused...

When the check valve was installed backwards before, the vacuum modulator wasn't getting vacuum and the car would not move until it was completely warmed up. When I turned the check valve around, the car shifted normally into reverse.

Now, when the vacuum modulator had a leaky hose, it slammed into reverse. When the leaky hose was fixed, it now won't go into reverse.

The transmission guy's explanation is that it WAS getting full pressure, but now it isn't. That make sense.

But, that doesn't explain what happened when I fixed the check valve before.

This whole thing has me stumped. I just don't understand this enough to make sense of it.

That is not a real check valve like the one you can find on power booster line, (you can blow air from one side but not from the other) that is more like first line of defense in the case your modulator goes bad and starts to allow transmission oil to leak, orifice will slow down oil (notice one hole is very small) to be sucked into intake which could lead to major trans failure. What type of modulator you have? does it look big and grey or it is smaller and gold. The small ones can be adjusted with small screw driver. When you tighten your screw inside the modulator you will have firmer shifts when you do oposite you will have soft shifts. You can test modulator with vacuum pump it should keep 15 inHG of vacuum if not replace it, or if you see oil coming from the modulator replace it. Sometimes other lines lijke AC vacum lines are connected to "T" fitting that also conects to your modulator line, if they are disconnected you may not have AC and you will only get air from the top of your dash board since that is default mode in the case something goes bad with vacuum you will at least be able to use defoger and see where you are driving.