350 Turbo Transmission Troubleshootingby John Stevens J.D.
The Turbo-Hydramatic 350, commonly referred to simply as the "Turbo," was a three-speed automatic transmission first produced by General Motors in 1969. The Turbo 350 was known as a durable transmission and was used extensively by GM. However, over time the Turbo 350 will develop problems, as with any type of transmission, but troubleshooting this transmission is easier than many other types due to its basic design.
No Drive in "D" Position
If the transmission fails to propel the vehicle forward when the transmission selector is in the "D" position, the problem is probably a lack of transmission fluid, a linkage which needs adjustment or low fluid pressure. Check whether the transmission has enough fluid in it first, since this is most common reason and because the fluid level is easy to check. Withdraw the transmission's dipstick underneath the hood with the engine running, then check the fluid level on the dipstick. If the transmission has enough fluid, check whether the manual linkage on the driver's side of the transmission requires adjustment. Finally, the transmission may not be able to produce enough pressure to circulate the fluid throughout the engine. A failure to produce sufficient pressure is typically caused by a damaged or missing O-ring seal or a blocked oil strainer.
No Drive in "R" Position
If the transmission fails to propel the vehicle backward when the transmission selector is in the "R" position, the problem is probably a lack of transmission fluid, a linkage which needs adjustment, or a problem with the valve body. As in Section 1, check the transmission's fluid level first, followed by the manual linkage on the driver's side of the transmission. If the problem persists, the control valve body gasket may be either leaking or damaged. If the valve body has recently been removed from the transmission, make sure that the low-reverse check ball was installed. Due to its small size, it is easily forgotten.
If the transmission slips, meaning that it does not accelerate the vehicle smoothly, check the fluid level first. If the transmission has enough fluid, the vacuum modulator could be defective, the modulator's valve may be sticking, or the oil strainer may be plugged or leaking. If the problem persists, the forward, intermediate and direct clutch plates may be slipping. If so, the face of the plates will appear burnt. To avoid damaging the replacement plates, make sure that the seals and oil rings are in good condition, as these are the most common causes of burnt clutch plates.
If the transmission produces an irregular noise, the noise is likely coming from the transmission's pump. The pump will produce noise if the transmission's fluid level is either too high or too low, if the fluid is contaminated with water, or if the transmission's driving gear teeth are damaged. Also check the planetary pinions, the sun gear, and both the front and rear internal gears for tooth damage. Finally, check the condition of the engine's two motor mounts. If one or both of the motor mounts are damaged, the engine will cause the transmission to shift, which will in turn cause the transmission to produce noise.
- "Motor's Auto Repair Manual;" Ralph Ritchen; 1969
- "Turbo Hydra-Matic 350 Handbook;" Ron Sessions; 1987