The other day I found and fixed a vacuum leak on the 3800 engine in my white car. It was a little T connector on the throttle body, with one branch going to the fuel pressure regulator, and the other branch going to the transmission modulator. I couldn't find that T connector anywhere, so I jury rigged up a fix.
Yesterday on the trial run, my transmission wasn't shifting correctly, and at a gas stop, I noticed that exhaust heat had softened the new plastic vacuum line going to the transmission modulator, and it had flattened.
Today I replaced that vacuum line, but the transmission still didn't shift right. So I tried disconnecting it, and plugging it so that there was no vacuum leak. I went on a test drive, and the transmission shifted perfectly. So what good is the transmission modulator anyway? I'm just going to leave it disconnected for now.
I ordered an "adjustable transmission modulator" from Rock Auto, and I'll see what that does when it gets here.
Edit: A minor problem noticed with the modulator disconnected is that the torque converter lock-up happens at a very low RPM. it seems like it should be a little higher. I'll see if I can fix that with the adjustable modulator.