Noisy (wearing) Reverse Synchro (reverse gear brake)

[Calan]

Anyone ever heard of this?

We just rebuilt my tranny (and tore it apart again to check things out; see my other thread).

The reverse synchro is wearing on the side that rides the wavy washer, and you can hear it "rattling" in the end case at idle, basically at any time the input shaft is being driven. It's really noisy during deceleration.

The tranny was either rebuilt or ordered from a factory replacement type shop before I bought the car. It made a similar noise before I had it rebuilt, but it wasn't as noticable as it is now after the rebuild.

Our only theory is that the end case (cover) has a shaft center that doesn't exactly line up with the input shaft, causing the rev synchro to bind and wear a bit. There is no out-of-tolerance shaft play or anything visual that can explain it.

The plans now are to just remove the synchro, using the replacement hub/slider washer.

Any ideas on why/how this can happen?

 

[marcmsj]

Reverse has no synchro

 

[Calan]

some do.. (like mine).

It would be kind of hard to see a worn reverse synchro if it didn't exist

 

[marcmsj]

...Then I can't help you

 

[Calan]

Maybe I should fix my nomenclature to avoid confusion... it is the reverse brake synchro, that sits on the "case" side of the 5th hub/slider.

 

[pneumo]

All I can say is to check everything that touches the reverse syncro for signs of irregular wear, or any oddness. I've heard the noise in a couple other DSM's but haven't looked inside their trannys for the exact cause.

I wouldn't remove the reverse syncro, it keeps the 3 keys in place inside the hub/slider when you shift into reverse. Without the syncro the keys would pop out.

 

[Calan]

They make a special washer that replaces the synchro to keep the keys in

 

[MyBlack94GST]

Hey, I think I have the same noise. I have yet to pull the end cover off to check it out, though. Who makes the washer?

 

[Defiant]

Use fifth's synchro to brake the gearbox before shifting into reverse.

 

[Calan]

Well... We pulled the end case and removed the reverse brake synchro, replaced the 5th gear, 5th intermediate and shift fork. BUT I STILL HAVE THE SAME DAMN SOUND...

It is definitely coming from the tranny near the passenger side, not up by the TOB. A "loose" rattling while idling in neutral, louder when decelerating in any gear. (it's also there while cruising or accelerating, but not as loud). Push the clutch in and it goes away.

While I had everything torn down for a few months, I used Window Weld on the driver side motor mount and the tranny mount. When I was putting the tranny back in, it was a PITA to get the tranny mount to line up and the bolts started. (BTW - the holes on the tranny for the mount were stripped and had to be heli-coiled).

Could the tranny mount be causing this some how? The bell housing is still up nice against the motor, and the mount seems tight. As stiff as aluminum is, I don't see how the mount could be causing any binding in the tranny with the bell housing bolted up snug, but I'm running out of ideas. We've torn the tranny down twice and removed the rev brake synchro. There was no metal in the fluid (Synchromesh), and shaft play/gear mesh looks fine. Other than the extremely annoying and loud sound, the car drives and shifts fine.

The one good thing out of all this is that now the tranny easily shifts into reverse, instead of the notchy stiff feeling it had before...

FYI -

For anyone wanting to remove the rev synchro and replace it with the dished washer that some (most?) tranny's use...

As far as I know you can only get the washer for the smaller diameter 5th hub. So you may have to replace your 5th gear with one that uses the smaller hub (and shift fork). And if you do this, you'll probably need a new intermediate because the pitch of the two 5th gears is different, even though they are the same size.

 

[Calan]

Took the car to a shop and had them pull/check the tranny and everything else. Turns out that the input shaft was shimmed improperly when it was rebuilt.

They put it back together with the correct shim, and the rattle is gone. I still have some gear lash on decel however do to the bevels wearing a bit while it wasn't right.

Live and learn