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I've been having a problem for a while on my MTX Nissan Almera with only being able to engage fifth gear at specific speeds, around 68-72 km/h.

I recently noticed while changing an inner tie rod dust boot that it looks like I've got oil leaking from one of my axle oil seals. It's not a huge leak, but it's likely that it's been there for a long time, like maybe more than a year.

So I'm wondering if my gear oil is low ( probably ) and if that could cause the above mentioned problem?

EDIT

While this is a general question, here is a picture of my particular model of gear box ( but not mine from my car ):

enter image description here

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    Synchronyzers are friction devices. Lack of oil would cause them to work better at first but burn up faster due to the lack of cooling from the oil. If 5th gear was the first thing uncovered when the oil went low then the synchro may have burned up by now. Refilling the transmission with oil will probably not bring it back. – vini_i Jun 22 '16 at 12:14
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    @vini_i That sounds an awful lot like an answer :-) – Robert S. Barnes Jun 22 '16 at 13:41
  • Hmm, wouldn't lack of oil eventually ruin all the synchros, though? – I have no idea what I'm doing Jun 22 '16 at 15:01
  • @IhavenoideawhatI'mdoing I'm positing a small slow leak. If #5 is highest it could go dryish first. – Robert S. Barnes Jun 22 '16 at 17:30
  • @vini_i I agree with Rob, post your comment as an answer, if not for the rep, then to get this off the 'unanswered' list! – MooseLucifer Jun 22 '16 at 17:53
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Synchronizers are friction devices. They are most commonly made out of brass although other materials may be used. Brass is a soft metal and as it wears down the power it produces would not damage other components.

The synchronizer resembles two cones that fit one inside the other. One cone is attached to the blocking ring and the other to the gear train. In neutral none of the synchronizers are touching. As you begin to shift and the blocking ring moves toward a gear the first thing to make contact is the synchronizer. The synchronizer inside the blocking ring is spring loaded and as more pressure is applied to the shift more pressure is applied to the synchronizer. When enough pressure is applied the synchronizers stop slipping and start rotating as a single unit matching the speed of the great train and blocking ring. Finally the blocking ring engages the dog teeth of the gear train and the transmission is in gear.

The metal chosen for the synchronizer, the surface area that touches inside the synchronizer, and the pressure of the springs are all factors that are chosen keeping in mind that everything will be swimming in gear lube. Not only does the gear lube provide lubrication it also provides cooling by transferring heat from inside the gear train to the transmission outer case.

If 5th gear was at the top of the transmission then it would go dry first if the transmission has a slow leak. At first the increase in friction from lack of gear oil inside the synchronizer would improve performance but at the cost of increased wear. Also with a lack of oil inside the synchronizer cooling would be diminished further speeding up the wear process. If 5th gear is at the top of the transmission it may have failed by now and refilling the transmission with oil may not bring it back.

Side comment.

To further illustrate the importance of the correct friction inside the synchronizers. I worked on the manual transmission where the customer added Slick 50 oil additive. The transmission began grinding in every gear. The synchronizers could not do their job because the friction inside them was reduced to such a low level. Several drain and fill of carb cleaner and a refill of clean gear lube fixed the problem.

Specific to the transmission of the original poster.

According to the parts explosion that was provided the transmission has two shafts, input shaft and main shaft. The main shaft has two blocking rings. One of the blocking rings serves 1-2 and reverse (how it also serves reverse i don't know but i'm willing to bet that it has something to do with the gear that's built into it). The other blocking ring serves 3-4. The final blocking ring is on the input shaft and is labeled OD (overdrive) this is 5th gear.

Looking at the picture provided in the post, the shaft in the foreground the main shaft as indicated by the two blocking rings. The shaft that's peeking out from behind the main shaft is the input shaft. The shift fork can be seen on the left. Extrapolating the orientation of the transmission the input shaft is the highest shaft in the transmission. This makes it plausible that 5th gear would go dry first damaging the synchronizer.

  • How can the fifth gear be "at the top of the transmission", above the rest, though? Do you have a schematic? – I have no idea what I'm doing Jun 23 '16 at 9:32
  • @IhavenoideawhatI'mdoing This is the best picture is could find to illustrate the point. Note how there are two blocking rings on one shaft and two on the other. depending on the orientation one set of blocking rings could sit above the other. Everything depends on the specific transmission that you have. I was only putting forth a hypothesis. rapid-racer.com/resources/… – vini_i Jun 23 '16 at 12:52
  • If you could relate specifically to my transmission also, I'll accept your answer. There is a picture of it in the OP, and here is a breakout diagram of the tranny internals: nissanpartsdeal.com/parts-list/1999-nissan-sentra-b14u/… – Robert S. Barnes Jul 7 '16 at 8:38
  • @RobertS.Barnes I updated my answer. – vini_i Jul 7 '16 at 23:17

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