Improper clutch adjustment is the #1 cause for synchro failure in the 3S 5-6 speed trans. The original synchros are a good design and can withstand a lot of abuse, but once a clutch starts to drag and the driver starts to force the trans into gear, the synchro rings will be badly damaged in short time.
When a trans blocks you from going into gear that means the synchro is having a difficult time synchronizing the gear. When this happens it usually means something went wrong with the clutch system. The master or slave could be bad or leaking, the clutch disk hub could be broken, the clutch pressure plate could be warped, or the clutch throw out bearing could be damaged. The synchro is trying to tell you something when it does not go into gear. Forcing it or to continue to drive the vehicle is just going to destroy them.
There is a simple test for this. If you suspect your clutch is dragging, try this, but please be very careful:
1) Have the car running with the e-brake off so it can roll.
2) Push the clutch pedal all the way to the floor and hold it there.
3) Turn off your launch limiter, if equipped, so the engine can hit the rev limiter.
4) Put the car into 1st gear.
5) Rev the engine up until you feel the car start to move forward.
If the car moves forward below hitting the limiter your clutch is dragging. You absolutely MUST fix this or you will not only have clutch problems but a trans that is grinding into gear and dead too.
Please note, if your trans is grinding when you go into gear you are doing serious damage to expensive parts very quickly. Every time it grinds more $$ is crunching away. Fluid will not help. If it grinds it’s over.
Here are a couple of pictures of worn out synchros that were forced to go into gear when the clutch was dragging terribly badly:
1-2 gear synchro rings. Bad on top, good on bottom:
3-4 gear synchro rings. Bad on top, good on bottom:
Here is a gear and synchro slider with damage due to the owner continuing to drive the vehicle while it was grinding into gear: