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Automatic transmission shifting at high RPM

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Thierry:
 Hells17, a member from the French section has a problem with an automatic transmission on his 67 Mustang. The shifting is brutal and only happens at high RPM. He thinks it's his converter but in my opinion it would rather be a bad clutch pack. Do you have any idea what to check for this problem?

Soaring:
Find out if this transmission has always done this since he has owned it.  If so, he most likely has a high stall converter. 
If not, then I would suggest a transmission fluid and filter change. Don't flush it, just drop the pan, replace the transmission filter and replace the fluid that came out. Usually a hard shift like this is due to a dirty transmission filter.
That is the first thing I would do.  Let's see if that fixes it before we tear into the transmission. 

rmodel65:
im assuming its a c4? does he have the vacuum modulator hooked up? and has he tried adjusting it

Thierry:
 I told him what you suggested and we'll see what happens. Can you tell me how you adjust the vacuum modulator? I know the hose has to be in good shape but I didn't find anything about how to adjust it.
In case you are interested here is his collection of US vehicles




Soaring:
Stock factory modulators are pre-set and most are sealed.  Special built modulators such as TCI StreetFighter modulator-type transmissions are all equipped with adjustable modulators. By turning the adjusting screw (found in the vacuum nipple) counterclockwise, you will lower the spring pre-load. This will cause earlier shifts. By turning the adjusting screw clockwise and raising the spring pre-load, the shift points will go up. The range of adjustability is only a few mph either way.  Now, I doubt seriously this guy has anything but a stock C-4 that is pre-set and sealed.  That's why it is important for us to know exactly what this guy has.  He may have a high stall converter, or he may just have shot converter.  But, the shift points are determined by the converter and how it is set. 

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