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Author Topic: Powerglide, rough downshift  (Read 8611 times)

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69FBCONV

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Powerglide, rough downshift
« on: November 22, 2006, 03:07:24 PM »
I just bought a 69 FB with powerglide, 61k original miles, fully restored incl. trans with new master kit, filter and fluid 8K miles ago.  Car is mechanically perfect except for rough downshift when reach about 10mph.  Seems to lessen when engine/trans warmed up and car driven a bit.  Fluid perfectly clear, no burning.  Any suggestions, thoughts, other than me having trans pulled?  Any problem just leaving alone?  Thanks.
69 FB CONV

Todd Stahr

  • Firebird & Trans Am Club Tech Staff
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Re: Powerglide, rough downshift
« Reply #1 on: December 07, 2006, 09:31:26 AM »
Sorry its been a couple weeks.

Fist things first. You dont have a powerglide if its the stock transmission. You would have a Super Turbine 300, or ST300. It was Pontiacs 2 speed, works similar to the PG, looks vaguely like one, but very little will interchange between them.

By downshift do you mean the engine is running/sounding slower? or when you are coming to a stop and it shifts to first making the engine speed up?

Either way the thing you need to check, and its about the only thing you can do anything about, is the vacuum modulator. Make sure it is connected and there are no vacuum leaks to it, also you need to find out if its ruptured interntally. Oil in the vacuum feed line would be an indicator, you could replace it as well.

Otherwise the least it could be is a band adjustment, more than adjusting the band or replacing the modulator/hoses, its time to take it to a shop.

My opinion is if it works even after checking/replacing the modulator, just run it untill it dosnt move anymore. if it really bothers you, a TH350 is an easy swap and would give you another gear for better low speed driveability and acceleration.

The two speeds have roughly the same gear ratio as 2nd and 3rd on a three speed auto. So its like you are taking off in 2nd gear with a TH350/400. First in a 2 speed and 2nd in a 3 speed are roughly around 1.76:1 - 1.52:1 and first in a Th350 is 2.52:1 so you get more effective gear off the start with a three speed. Three speeds and two speeds all have a 1:1 top gear ratio, so the cruise rpm would still be the same, it would just get there quicker.
Performance and general repair tech. Traditional Pontiac V8 is my specialty, as well as automatic transmission repair/diagonossis. 2nd gen speciality, and with the addition of the 98 Formula 4th gen LS1 I am getting to know them real well. Holley and Qjet advice as well.

Chalk mark restoration and date codes arent my thing. W.O.T. is my thing..

Is it strange to have 9 firebirds?

69FBCONV

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Re: Powerglide, rough downshift
« Reply #2 on: December 08, 2006, 08:28:43 PM »
Thanks
It is original, so must be ST300, just always had in my head that it was a powerglide.
Problem when coming to a stop, when slowing to <10mph, jerky ("clunk") downshift.
Will check vacuum and if no help, will leave it alone and hope for the best.
69 FB CONV

miketransamkid

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Re: Powerglide, rough downshift
« Reply #3 on: June 19, 2007, 10:57:32 AM »
Only on downshift? Accumulator spring???? Ususally harsh shift concerns have to deal with a mushy accumulator spring. thats the old trick to make your tires chirp everytime the thing would shift. They would remove the accumulator spring so it wouldn't dampen the fluid going to the servo or clutches and it would slam the apply.


Another to consider is the electrical ok to the trans. The 300-ST has an electrical kick down switch. Might wanna make sure that its hooked up right.

National Firebird and Trans Am Club Message Board

Re: Powerglide, rough downshift
« Reply #3 on: June 19, 2007, 10:57:32 AM »


 

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