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Automatic transmission shudder

Gluteus Maximus

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1974 620 with automatic...My truck is running great for the most part, but I've noticed a "shudder" when it shifts from 1st to 2nd gear, especially when under a load or going uphill.  Fluid looks great...Is there anything a non mechanic can do to help this, or should I just drive it till it dies at which point I'll consider having a 4 or 5 speed conversion done?



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This is free.... Inspect your carrier bearing rubber surround for deterioration. This thing on the rear of the front driveshaft...






If rotted torn or worn the driveshaft will become off center under load and vibrate. As in a shift of gears.



Looking at the trouble shooting chart for the 3N71B...


1/ Oil level is correct? Is dip stick damaged and reading incorrectly? After driving check stick for any foam indicating over filling.


2/ Vacuum diaphragm or piping leak. This is the small hard line from the brake booster fitting on the rear of the intake that runs down behind the engine to the left side of the automatic. There is a rubber hose at each end connecting it to the intake fitting and at the transmission end, to the modulator valve. Engine vacuum is a good indication of throttle position and engine load. The vacuum diaphragm tailors hydraulic pressures for shifting. Look for cracked hose.


3/ Control Valve*


4/ Band servo*


5/ Brake Band*


* Least likely as the 3N71B is a very reliable automatic.

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