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  • karjunkie 01/24/10 10:45 am PST

    A synchronizer is almost certainly the culprit. The synchronizer is a bronze helitical cut gear that is positioned in front of a drive gear. Every gear except reverse has one and its primary purpose is to line up the teeth of the gear you are in with the next gear up or down. The gears are cut at an angle so when they are aligned at different speeds of rotation they will be able to mesh. Now, the second function of the synchronizer is to keep the car in its selected gear. Because the gears are helitical "teeth cut at an angle" the direction the angle faces can have a dramatic effect on how the gear reacts. The gear is designed to act in a way that when you engage it the rotation of the gear and the direction of the teeth force it to disengage. If it were the other way around the disengagement of gears would be next to impossible at any speed. This is where the second function of the synchronizer comes into play. It prevents the gear from backing out of gear unless the clutch is engaged and there is no load on that gear. So, your synchronizer is internally worn and is no longer tight enough to prevent the gear from slipping out at or above a certain load or RPM. You can probably drain the tranny to get any metal shavings out and continue to drive it as a 4 speed with no problems. The worst that will happen is lower fuel economy at higher speeds on the highway. You can certainly do it yourself with a good shop manual if you are handy with a wrench.

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