William Buckley

My '66 coupe was jerking going from park or neutral into drive or reverse, so after reading here and talking to people that knew more than I did, the conclusion was that the vacuum modulator on the powerglide transmission case was going bad. I replaced it with a new one that was termed "adjustable". The jerking was worse with the new one, but the unit was termed "adjustable". The question is how do you adjust a unit that has two openings? ..... one for the vacuum tube from the motor and the other end screws into the transmission case. There are no adjustable places to do anything on the outside of the modulator housing. Checked google and the local parts store. No one seems to know how to perform the "adjustment". Here's hoping that someone has replaced one of these or is experienced enough to tell me how to perform the "adjustment". Thanks for your help.

Vet65te

If you're saying that the trans shifts hard with a bit of a clunk or 'jerk' as you described it, when going from park into gear, have you double checked the idle rpm? If it's a bit high, putting it into any gear could create that kick you're talking about. As for the vacuum modulator, they are adjustable. Remove the vacuum hose and look into the tube that the hose was connected to. You should see a shaft with a slot in it for a small straight blade screwdriver. The modulator typically either works or it doesn't and you'll know when it fails because there's a vacuum diaphragm inside and when that cracks or let's go, the engine vacuum will draw trans fluid up into your intake manifold and your exhaust will look like you're laying down a smokescreen like a WWII Destroyer. So, if you double check the vacuum modulator with a vacuum pump and it holds, the adjustment screw-shaft will allow you to tune how long it holds the gear before shifting up and on the new one I just added to a T350 trans, the adjustability is only maybe 2-4 mph difference, not a lot.
Mike T - Prescott AZ

leif.anderson93

William,
I have never used an adjustable vacuum modulator but I would be willing to bet dollars to donuts that the adjustment screw will be found inside the vacuum tube. You can probably access the screw with a long, thin screwdriver. Adjust in quarter turn increments and see how it goes. If the shifting becomes harsher with your adjustment, adjust in the opposite direction.

BTW...the few automatic 'vettes that I've owned over the years...they all shifted rather hard from park to drive or reverse...kinda the nature of the beast.

William Buckley

Thanks to Mike and Leif. It was apparent that I could not find a way to adjust the "adjustable" vacuum modulator, especially when the new unit produced a more pronounced "jerk" than the old unit did. Will pull the incoming vacuum line and make the inside adjustment with a thin screwdriver. I have only had this car for ten weeks, and Leif may be correct that the "jerk" is part of the powerglide syndrome. I am going to try to minimize it though. Makes me think than any jerk is doing damage to the drive train. Thanks to both of you.

leif.anderson93

Just be sure to have your foot firmly on the brake when shifting

William Buckley

That's a given!

Critter1

All original GM modulators for aluminum powerglides were adjustable. (3905487, if I remember correctly)

Using a thin (1/8"?) rod about 3" in length, make a mark on the rod at it's center. Insert the ends of the rod into the old and new modulators and push the two together against the spring pressure. Adjust the screw on the new unit so the old and new modulators push evenly on the rod and your mark remains stationary, centered between the two.

William Buckley

Fantastic idea, Critter! Thanks very much!

W Guy

Not all of them had a separate pin.