VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc Powerglide CLUNKS down shifting during slow down

I have a 1961 Powerglide behind a 235ci 6.
Trans goes into all gears fine, accelerates fine w/ no noticeable noise, slippage, etc.
Fluid level appears to be fine, and fluid appears clean.
Problem comes when slowing down. When the trans downshifts there is a fairly major "CLUNK" as it engages. No issues or noise when accelerating away from the slow down or stop.
Any thoughts on what is causing this? Are there any adjustments that can be performed w/ the trans in the car?
Any thoughts appreciated.

Easy things to try first.
Remove hose from vacuum modulator and see if their is vacuum at the hose or PG fluid dripping out of the hose.
If it up-shifts too late adjust TV linkage to delay shift a little. Make sure linkage is working freely. See shop manual for proceedure on a 6 Cyl.
Idle speed set too high.

If none of these things make a change it is probably the low servo piston ring or spring is broken.

Thanks
Will check the vacuum
Upshift feels fine, at least it's not noticeably late.
Also, engine idle speed seems fine. Not racing, or stuttering.
Will sit there and idle easily, without any fuss.
Sounds like those other options mean an excursion into the box?

Chevgene,
I have good vacuum @ the vacuum modulator.
Did get some fluid out when I disconnected the hose.
Does that mean that the vacuum modulator needs to be replaced?
Does the modulator valve come with that when you buy the modulator? Depending on how long that is I'm not sure there is enough room between the trans and the frame to remove/install it.
Have also drained the trans fluid, and will replace that.
Dex III/M ? Right?
Thanks for your help

If there is fluid in the line it means the diaphragm in the modulator is bad.
There was a thin off-set wrench made for removing and replacing the modulator.
The modulator valve stays in the transmission....it bumps against the inside of the modulator(diaphragm).
Any of the lesser Dexron fluids if fine, the synthetic newer is OK but not necessary.
Not sure about the room problem.
The clunk when slowing down and shifting into low indicates the internal pressure is too high when it should not be and could cause band breakage.

Are clean fluid and a new vacuum modulator likely to reduce the high pressure?

Also, could you suggest the best place to pick up that wrench and the vacuum modulator?
Thanks, Bill

The "working" modulator regulates the pressure. When the vacuum is high like at idle or a steady speed it reduces the internal pressure. When vacuum is low like under power the pressuure is increased so the clutches grip faster and harder during the shift from low to drive.
May be better to find a good transmission shop to replace the modulator and you can change the fluid at home.
With a change you drin the P.G. sump only and the fluid in the torque converter can not be drained....but do change the fluid.
They would probably have to order the modualtor. Make sure it is for an cast iron Power Glide and not an aluminum case unit that was used with 327 engines in 1962.
Don't let them sell you a band adjustment, etc.

After looking that thing right in the eye this morning I think that's something I can replace if I can get wrench on it and find the vacuum modulator.
I will keep you posted!

If you are sure it's 1961 PG, Fatsco (.net) has the modulator. Best I can tell there are three different modulators for the cast iron PGs. 50-54 55-57 58-62
There's even small pdf d/l with instructions to do your own band adjustment.

There are two different modulators.
The 1950-1954 was an open diaphragm. Looked like a big fuel pump diaphragm.

The 1955-1957 DID NOT have a modulator.

The 1958-1962 had a can type.

Due to the popularity NAPA problem has them.

They might but their database (AZ and AA also) came up empty.

Thanks for the clarification. I wondered why the tri-fives where strangely absent.

Boardwalk,
Have you had dealings with Fatsco?
I found them in Hemmings. Called and spoke with Bob yesterday
He seemed knowledgeable, and I ordered the part. He said the part is the same for '58 thru '62 cast iron PGs.
Websites like Advance Auto and NAPA did not give much confidence on the part being correct, and I'm not sure the young folks on the counter know what a power glide is.
Thanks to both of you guys for your input.
I will let you know how this works out.

It's been a long time but never had any problems with them. Likely will go back soon. My wife's car has a 65' aluminum glide that needs freshening up and occasionally downshifts hard too.

Installed the modulator today. A relatively thin 7/8" open end wrench worked fine.
Replaced the fluid, started it up,put shifter in all gears, then let it idle in neutral.
Re-checked fluid level, topped up, and went for a 17 mile ride.
Happy to report that it is down shifting MUCH better.
Want to thank both of you guys for your suggestions and comments.

There's one final thing you might want to check. If the modulator gets its vacuum from a Tee off of the distributor vacuum line, you're fine, but if it gets it from a fitting on the intake runner, you should pull #8 plug and look for fouling. That ATF will foul the plug and wash down the cylinder.

Verne

Did you check the U-joints and the hanger bearing? I had a similar problem years ago and the cage on the hanger bearing was broken, caused the shaft to bounce and there was a noise most noticeable on the downshift while the engine was coasting. Just something to look at.