I've been reading a lot of problems related to clutches and transmissions lately, and I thought I'd contribute some things I've learned. My day job as an engineer has me developing clutches and making them work with transmissions for major OEMs, so I have a fair grasp on the system. I am not a mechanic, however, so I haven't seen all of the problems and don't pretend to know how to fix them all. But hopefully this will help.

If you're driving down the road, and suddenly it's difficult or impossible to shift, it's probably the clutch, or the system that connects your left foot to the clutch. Here's what happens when you go to shift, normally, with the clutch:
1) You depress the clutch pedal, forcing hydraulic fluid through the pipes from the master cylinder to the slave cylinder. The slave cylinder pushes on a fork, which has the "release" or "throwout" bearing on its length. This release bearing actuates the diaphragm spring fingers in the clutch, which removes the clamp load holding the disc between the pressure plate and flywheel. The disc is now free from the engine.
2) You move the shift lever into the next gear. In order for the shifting to take place, your transmission has to match the speed of the "input shaft" (the shaft that the disc is attached to) to the speed of the "main shaft" (the shaft that the differential and eventually the wheels are attached to). This is where the synchronizer comes in. The synchronizer is a cone-shaped friction clutch that moves in and accelerates/decelerates the input shaft to match the speed of the output shaft in a given gear ratio. For example, say 2nd gear at 40 MPH has your input shaft turning at 6000 RPM and the main shaft turning at 2000 RPM. Now, if you shift into 3rd gear, for that 2000 RPM of the mainshaft, the input shaft only has to turn at 3000 RPM. So the synchonizer decelerates the input shaft AND DISC to the new speed.
3) You let the engine speed drop to the correct speed, and re-engage the clutch to allow torque to once again be transmitted through the system.

Now one cause of hard shifting is synchronizer wear. Each time you shift, particuarly a hard downshift (4th to 2nd or something like that) causes your synchronizer in that gear to wear slightly. In severe cases, downshifts can increase the surface temperature of the synchro to over 1600 degrees F. Also, old or poor oil can contribute, as it will not allow the shifting components (levers, synchros, gears) to mesh properly. However, both of these causes happen over time and won't suddenly appear, particularly in all gears.

Most of the time, it's caused by the clutch NOT RELEASING THE DISC when you depress the pedal. If the disc is still clamped, even just a little bit, when you shift, the synchronizer must not only overcome the rotational inertia of the disc to change its speed, but also the friction caused by the clamping! Effectively, you're asking your synchronizer to accelerate THE WHOLE ENGINE to the new speed, if the clutch is still engaged when you shift. Not only does this make shifting hard, but it also is terrible for the synchro!

What would cause a clutch not to release the disc? Usually a loss of hydraulic pressure in the system. If the master cylinder, slave cylinder, or piping develops a leak, or air gets in the system (just like brakes) then you could have a problem. Less common would be something in the clutch breaking and preventing the release from happening.

So if you're having trouble shifting in ALL gears, diagnose the release system first, then the clutch, then the transmission.

Hope this wasn't too long and someone learns something.

 

Great post

 

Very good post. I don't think I could put that down in writing as efficiently as that. Good job. I hope people will read this and learn something.

 

Yeah great post... now I know what SYNCHROS are, shoddy ones at least.

My car grinds if I shift very quickly from 2nd to 3rd, and 3rd to 4th.

If I leave my clutch in a little bit longer, or if I wait on shifting just a half a second later, it's fine...

How much is it to fix synchros?

 

The synchros themselves are realitively cheap (guess $10-20 each). The cost is in pulling and taking apart the transmission. Probably would be "book time" of 10-12 hours. Rough guess, but you get the picture.

 

thoughty syunchros were more like $130-140

 

When I replace synchros, would it be a good time to replace the flywheel as well?
I know what a flywheel is and what it does, but no clue where it's located.

 

Yep, drive on a bad clutch, and your transmission will suffer.
Great post !

 

i think i screwed up my snycros or something, first it started out that my clutch master cylinder was leaking and it ended up makeing it hard for me to get it into gear all the time, so i replaced that and then i had a problem with it grinding when i was at the high r's.... I i figured it was my snycros and i took the tranny out only to find a piece of the clutch was broken and causing my clutch to not fully disengage. so i replace the clutch. bleed the system. car works fine for about 2 months. now when i am sitting still it is almost imposible for me to get the car into first or second gear. i have to let the car roll a little and it slides right in. or i will shift to reverse and then i can get it in first without rolling. but driving it shifts perfectly... so what could the problem be. i checked my fluid and its still good.

 

Had almost identical problems as you sloth.
My clutch MC was leaking causing some grinding.
I replaced it maybe after too much time passed.
Clutch had inconsistent engagement - sometimes would not shift at all or pop out of gear.
A clutch spring retainer had broken off and was periodically wedging the plate.
Replaced clutch with 6 puck and HD PP.
Now having probs with grinding when shifting aggressively.
Probably need to replace tranny...sigh...
Moral of the story - fix stuff QUICK - don't wait...

 

The flywheel is bolted to the transmission side of the crankshaft, inside the transmission bellhousing. When your clutch is engaged, it is squeezing the disc between the pressure plate and the flywheel.

If you want to replace your flywheel, then yes, that would definately be the time to do it. Dropping a trans is something you do only when necessary!

But you don't have to replace it: the flywheel is rarely a wear issue.

 

Yeah, It may not be a problem. I recently replaced my stock clutch with 120,000 miles on it, and the flywheel still looked new after I cleaned it.

 

Acutally, syncrho sets are quite expensive. I just replaced my 1-2 and 3-4 set. The cost for both sets was ~$575.00.

As for replacing the synchros, it is a straighforward job. I would have to agree with the 10-12 hour job time. This time includes prep, removal of tranny, teardown and rebuild, tranny install, and clean up. It may seem like a long time, but I personally bag each bolt and label each bag with Page number, step number, and quantity if applicable. Even though I've done trannies before, you can never ever be too careful.

mario-