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I was having an issue with what I thought was gear lash noise (on deacceleration), so I bought a low km gearbox and had my mechanic install it. The deacceleration noise is still there (must be my diff?); and now there's a bigger problem: it grinds when "power shifting" to 3rd.

It usually only starts to do it when downshifting to third at over 100 km/h. But after a couple of hours of highway driving, it grinds at all speeds, downshifting and upshifting. Always to third gear only.

The previous owner claims it didn't grind. Motocraft Full Synthetic hasn't made a difference.

Is it definitely a bad synchro, or could something else be causing this?

If the replacement box does not have any fault (depending on the validity of the previous owners word), maybe it is your diff. Also, it is worth checking PPF alignment first.

Have a look here for some specs and suggestions:

http://mx5cartalk.com/forum/viewtopic.php?f=29&t=57217

Thanks. I actually gave a copy of that diagram to my mechanic when he changed the box, to make sure he set the PPF correctly after.

It's more the grind that concerns me now. It seems a bit unusual to me (though I don't know much) that it gets worse as it heats up, rather than the other way around. And the noise sort of reminds me of my old box when I failed to disengage the clutch fully. Could the clutch or clutch cable have a role in this?

Took a drive tonight and made some observations. At 100 km/h, downshifting to third (5-3 and 4-3) always grinds, and downshifting to second doesn't. Rev matching alone doesn't help, but double clutching does (though not always). So I guess it's definitely a bad synchro?

Any leads on good gearboxes in WA?

Almost certainly a gearbox issue. I would not have thought it a great idea to be changing down to 3rd at 100km and I would not shift from 5 to 3rd either. It may well be bearing wear. Worn synchros tend to crunch but a grinding or regular noise indicates wear in a rotating part. If your diff was not noisy before the gearbox change its a pretty fair bet its the gearbox- you can also ask the mechanic to drive it - they can pick things.

Try pumping the clutch pedal before you change, if that solves things than you might need new clutch fluid and perhaps a new clutch slave.

Hey, thanks for the tip about pumping the clutch. Interestingly, it actually makes the grinding much worse and to happen at lower speeds (i.e. 60 km/h). Is there some sort of clutch problem that is made worse by pumping, rather than better?

1990MarinerBlue wrote:Hey, thanks for the tip about pumping the clutch. Interestingly, it actually makes the grinding much worse and to happen at lower speeds (i.e. 60 km/h). Is there some sort of clutch problem that is made worse by pumping, rather than better?

Worse in what sense? If when changing gears, and a crunch occurs more often after multiple pumps, maybe the clutch slave cylinder is leaking (lack of pressure) and needs to be replaced.

That still doesn't explain your declaration noise. I still put my money on it being your diff (considering you experienced it both before AND after changing the gearbox)