Diagnosing Manual Transmission Problems

by Don Bowman
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Manual Transmissions Were First

A manual transmission car was the very first in existence and have evolved through many stages over the years. Today, manual transmissions are used in every type of vehicle. Manual transmissions started off as three-speed and advanced to four-, five- and even six-speed transmissions in cars. Trucks have advanced to transmissions with dual ranges from low to high in each gear by using two different-sized reduction drive gears. For the most part, manual transmissions are trouble free. However, like with anything mechanical, there can be component failures.

Pay Attention to the Details

The way to diagnose transmission problems is to first take note of when the problem is present and under what conditions. If a manual transmission makes noises just when the clutch pedal is all the way up or engaged and a grinding or rattling noise is heard, the throw-out bearing for the clutch is bad. If the transmission makes a rattling noise that is loudest in neutral, although it is always there, and the noise changes when it is put into gear, but shifts well, the input shaft bearing is bad. This is the shaft in the front of the transmission; the clutch is attached to it by the splines on the shaft.

Listen for the Noise

If the transmission makes a loud whining or grinding noise continuously while running, check the oil in the transmission. If it is not full, fill it with the approved fluid and test drive it again. If the transmission makes noise when it has the correct fluid, the bearings are faulty and the transmission requires a complete rebuild. It would very likely be more cost effective to buy a used or new transmission.

How Does It Feel?

If the transmission sounds and works well, but is hard to shift, the linkage is faulty and not a difficult repair in most circumstances. If the transmission is a top loader, which is the type where the gear shift enters the top of the transmission, the shifting fork or the ball at the end of the gear shift is the problem. When the gear shift handle is sloppy, it is just a matter of the ball end being loose in its socket. A top-loader transmission uses a gear shift handle that has a spring under the plate, and it pushes down on a ball at the bottom of the shifter. This ball fits into the shift forks in the center of the gearshift hole into a round pocket. When the plate is bolted down, the spring forces the ball into its socket in the shift fork, which moves the gear bank to change gears.

Physically Difficult to Shift

If the transmission is running quiet, but is difficult to get into gear without grinding, the synchronizers are bad. They are the same circumference as the gears, but are narrow and in place of teeth they employ triangular-shaped short teeth designed to slow the main gear, making it easier to mesh the gear bank. When both gears to be meshed are moving at drastically different speeds, they will not mesh without grinding badly. The synchronizers bring the speeds closer together for a smoother shift. This type of problem requires a complete rebuild as well.

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