Hi everyone I have what I think is a problem on a 5 speed '90 535i.
On occassion when I start off after being stopped there is a loud bang
and the shifter is thrown out of first. This happens as I am letting
the clutch out.
If I give sufficient throttle and let the clutch out quickly it
Is this normal?
IIRC the synchronizer or synchromesh purely aligns the teeth to allow
a smooth transition. Once the drive gear is engaged it plays no
further part until the next gear change.
If the synchromesh is worn, you'll get a grinding gear change from
teeth not messing as you try to select a new gear.
Jumping out of gear under load either means you've been grinding too
many teeth and have now damaged the gears, or your selector
fork/linkage has become damaged and isn't pushing the drive gear fully
In a synchro transmission all of the gears are ALWAYS in mesh so there is
never any opportunity to 'grind the gears'. The gears that are not engaged
are however free to turn on their respective shafts and so are unable to
transmit power. The way a gear is engaged is to slide a splined coupling
into the gear. This coupling is always splined to the shaft and is splined
to the gear when it is engaged thereby making it possible to transmit power.
The spline teeth that engage the gear have two sections to them; a tapered
section that aligns the spline teeth as it engages and a parallel section to
transmit the torque after it is engaged. When you 'miss a gear' it is the
tapered section that tosses you out of the gear that you didn't quite get.
If the gearbox is abused by numerous instances of releasing the clutch
before the splined coupling is all the way home, the engagement teeth can
become worn and the parallel section becomes tapered thus tossing you out of
the gear even though the coupling is all the way engaged. There is no cure
short of replacing the sychros so it is probably time to start shopping for
a used gearbox. This kind of damage is often caused by a leaking clutch
master or slave cylinder that releases the clutch prematurely even though
you haven't taken your foot off the pedal so it would be a good idea to
check that before replacing the box.
I agree. If the synch rings are the fault, then muchmore wear has happened
that is causing this problem. The synch rings shouldn't play any role in
keeping the gears engagde. If the gears are slipping apart, then they have
been abused to the point that they need to be replaced. The synch rings
could be seriously worn at this point as well, but that isn't why the gears
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.