TH350/400 Shift Point Adjustment

 

TH350/400 Shift Point Adjustment

 

 

The 3 speed Turbohydramatic 350 and its big brother the TH400 were some of GM's most widely used automatic transmissions from the mid 60's through the early 80's. The TH350 was mainly factory installed with small blocks and the TH400 with big blocks and high performance and heavier duty vehicles. They also find their way into many Chevelles and other GM classics if not originally equipped as such, as well as street rods, race cars, etc. They are inexpensive, reliable, and easily modified for performance and racing use.

If you have a vehicle equipped with one of these and the automatic shift points (engine rpm/vehicle speed) in Drive range seem too high or too low, it usually can be rectified without even pulling the trans pan. If your trans is not original to the vehicle it's currently in, or the rear end gear ratio has been changed, the trans governor is probably not calibrated properly to the rear gear ratio. The governor is a pressure valve that contains centrifugal weights and springs much like a distributor and is geared to the output shaft of the trans. If the rear end ratio is changed, the governor spins faster or slower in relation to vehicle speed than originally intended. The speed of the governor causes its weights to move out or in against springs and move a valve that alters fluid pressure in the trans, which in turn affects the shift points. Fortunately, there are spring and weight kits available to fine tune the shift points. They are available from performance trans shops such as TCI (www.tciauto.com) and others.


Another possible cause of errant shift points could be the vacuum modulator or a vacuum leak in the line connecting it to the engine. Be sure to ensure a sound vacuum line and connections before wasting money on parts. The vacuum modulator senses engine load from manifold vacuum and moves a fluid pressure valve in the trans accordingly. Aftermarket vacuum modulators have an adjustment screw inside that can
slightly change shift points and can definitely alter shift firmness. They are available at local auto parts stores for about $11 and are easily changed. If you have a modified engine with longer duration cam and lower vacuum, special low vacuum modulators are available for use with those. It's easier and cheaper to replace the vacuum modulator and play with its adjustment before diving into the governor, which still isn't too terribly expensive or difficult. If no vacuum leaks exist and the modulator isn't defective, the governor affects shifts points much more than the modulator.


It's always possible the cause of errant shift points lies deeper in the trans but if the trans isn't slipping and fluid isn't burnt, and other symptoms of trans trouble don't exist, chances are either replacing the vacuum modulator or governor springs/weights will fix the problem.


In the next issue I'll explain how to remove and recalibrate the governor.