FWD, 6spd MTXs and mild clunk on downshift...

[badtlc]

I have noticed this phenomenon in a few different cars I have driven recently. All were FWD, 6 spd MTX smaller cars (focus ST, mazda3 & GTI). This is the process that produces a slight clunk feeling/sound:
- Driving in 4th, 5th or 6th gear at speed
- Put in neutral and coast to slow speed
- if shifting from neutral directly to first gear, there is a clunk/thunk feeling and very quiet sound.
- If downshifting through the gears and releasing the clutch in each gear, it doesn't happen.
- It feels like a large mass that is sort of dropping into alignment when the sound/feeling happens.

Does anyone know what seems to be causing this? I'm assuming it is normal since I have seen it in a few cars now.

 

[SwampSurvivor]

The only time I have felt a similar "clunk" ... if it's the same clunk is on bigger trucks with worn out clutch brakes. The shafts in the transmission are spinning and trying to put it in gear stops them or changes their speed quickly.

I'm guessing it's something completely different in a light FWD manual transmission.

 

[aquariuscsm]

Are you shifting into first while still moving?

 

[scurvy]

Could be a bit too much for the first gear synchronizers. Sometimes shifting into second gear, keep the clutch disengaged and then shifting into first gear before engaging the clutch stops it from happening.

 

[badtlc]

Originally Posted By: aquariuscsm
Are you shifting into first while still moving?


Yes, just slightly.

 

[Hokiefyd]

I suspect that the clunk when shifting back into gear from neutral is the transmission having to synchronize certain shaft speeds, since some may still be spinning at a higher speed from when you took it out of gear. They're generally designed to work best as intended -- leaving them in drive.

The 4T80-E in Cadillacs would "clunk" when slowing in neutral, at about 10 mph. They'd clunk when leaving it in neutral and not touching it... The clunk was the transmission downshifting into first gear (it was always monitoring road speed), but it didn't have the mass of the engaged engine to dampen the sensation. After hearing about it, I tried it in my own car and, sure enough, you'll get a "bump shift" at about 10 mph when slowing down in neutral. It was something you never felt leaving it in drive.

Could be something similar going on in the Mazda transmissions.