5th Gear Grind on High Speed of 80mph above

[edangs]

I have a 2010 Mazda3 Touring with 250miles. I noticed that the 5th gear grinds on high gear.

Heres what happens; going 80mph, i take it out of 5th gear and put it in neutral,wait till the RPM goes back to 1k , I then step on the clutch all the way and shift into 5th gear -this is when i get the grind.

I have had this problem on old cars where the syncros failed, but i find it hard to believe that that is the case on this brand new car.

Anyone other Mazda3 owner experience this problem?

Can you test the same scenario i provided above and post back your finding

[ntechnic]

Couple of diagnostic questions for you:

1) Just driving and shifting normally, getting onto a freeway, when you shift 4th to 5th and then cruise, is there any noise from the tranny?

2) Just double checking, you have the 2,0L and 5-speed manual, right?

3) I'm not sure why you would take it out gear and then put it back in, but whatever, when you do that, are you first matching the revs? Meaning you note what the rpms in 5th (let's just say it's 2,500 rpm), put it into neutral, let clutch out, push clutch in, bring engine rpm back up to 2,500 (matching revs) and then engage 5th again?

4) You're saying "Grind". You mean the actual grinding sound of gear teeth not engaging? Like the sound it makes when you try to start a car with the engine already running--that horrible gnashing grinding of the starter teeth grinding on the flywheel. That kind of grinding sound?

[ntechnic]

Let me explain something about manual transmissions if I may. I'm old enough that when I started racing back in the 70's racing trans usually didn't have synchros. So you had to double clutch and rev match or the box wouldn't go into gear. Because of that training, I'm very easy on trannys and don't put much wear on the synchros--I automatically do their work for them.

Synchros are designed to spin gears up or down in speed to match revs and save the gear teeth/dogs. When you upshift, the engine drops in revs and you will engage the next gear at a lower speed then the one you shifted up from. The synchros compensate for this by changing the gear sets speed.

When you downshift, you will engage the next gear at a higher rpm then you when you shifted out of gear. Here, the synchros really work hard as they spin the next gear set up in rpm in just a second or so.

This is why "speed shifting" or slamming the gear shift through the gears as fast as you can (with or without the clutch) is so hard on trannys--you're forcing the synchros past their long term design duty cycle. It's fine to make fast shifts when you're playing--now and then. Do it all the time, and you'll be rebuilding the trans much earlier than you want to.

Assuming you've got a 5-speed, the transmission is designed to synchronize the upshifts from 1-2, 2-3, 3-4, and 4-5. It's also designed to synch on downshifts from 5-4, 4-3, 3-2, and 2-1. Note that there is no UPSHIFT into first, and there is no DOWNSHIFT into 5th--this never occurs.

However, if you are shifting from 5th into neutral (thus dropping the engine revs), and then back into 5th without rev matching (with the engine revving higher than the state you're leaving), you are essentially DOWNSHIFTING into 5th gear. And of course in a 5-speed, that's impossible, there's no higher gear to down shift into 5th from of course.

If this is the case, you are asking the transmission to do something it was never designed to do, and then it could very well make noise and damage itself.

[edangs]

Originally Posted by ntechnic

Couple of diagnostic questions for you:

1) Just driving and shifting normally, getting onto a freeway, when you shift 4th to 5th and then cruise, is there any noise from the tranny?

2) Just double checking, you have the 2,0L and 5-speed manual, right?

3) I'm not sure why you would take it out gear and then put it back in, but whatever, when you do that, are you first matching the revs? Meaning you note what the rpms in 5th (let's just say it's 2,500 rpm), put it into neutral, let clutch out, push clutch in, bring engine rpm back up to 2,500 (matching revs) and then engage 5th again?

4) You're saying "Grind". You mean the actual grinding sound of gear teeth not engaging? Like the sound it makes when you try to start a car with the engine already running--that horrible gnashing grinding of the starter teeth grinding on the flywheel. That kind of grinding sound?

1. No

2. Yes

3. Sample: on long downhill freeway, i take it out of gear to 'glide'. No, i do not match the rev. I dont think thats normal in driving a stick -especially going at high speed. If so, does that mean i would have to do the same on all the other gears when shifting out of it.

If you have'nt shifted out of 5th gear on high speed before, the rpm usually picks up as you release the clutch -therefore no need to "rev match"

4. Yes the 'Grind' is the actual grinding sound of gear teeth not engaging. So i shift trough the grind to get into 5th while pressing on the clutch.

[ntechnic]

Don't shift into neutral from 5th while coasting at speed. The owners manual should even warn you against coasting in neutral. I don't know of any transmission that is designed to downshift into its highest gear. You will destroy the tranny rather quickly doing this.

[edangs]

Originally Posted by ntechnic

Don't shift into neutral from 5th while coasting at speed. The owners manual should even warn you against coasting in neutral. I don't know of any transmission that is designed to downshift into its highest gear. You will destroy the tranny rather quickly doing this.

nothing in the 2010 Mazda3 manual mentions anything about coasting in neutral -i think its because this is the sole reason why modern gearboxes are also called Synchronized transmission. The synchro will correctly match the speed of the shaft rotation to that of the speed of the gears prior to the "teeth" locking into place.

At least thats what i got from doing a Wiki on "Manual transmission" or "Synchronized transmission"

From what i gather, any "Transmission Grinding" while you have the clutch engaged is not the norm -regardless of the speed.

[Mazda_Knight3]

OK, First the manual does not say anything about going into neutral on a downhill, actually its quite ok do so. It keeps your RPMs down and there is no problem with resyncing the transmision. Although I have done it a couple of times with a couple of my cars I dont really like to because sometimes you have to find the right gear for the right speed as you slow down, thats the only real problem. Second the Manual says, quote, unquote.. Never turn your engine off as your coasting because this can cause damage to your engine and transmission. See theres a difference between going into neutral and shutting your engine off!

[Nooishmee]

edangs

Hey I tried your scenario with my 3s at 80 and 85 mph and looks like theres no problem with mine?

[glennk]

problem with the synchros?

[JadeButterfly]

don't u use more gas when ur coasting in neutral?