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Gearbox pops out of 4th

Discussion in 'T3 & T4 Tech Help Clinic' started by Dusty Waves, Apr 12, 2019.

  1. Dusty Waves

    Dusty Waves Member

    Messages:
    70
    Location:
    Qld
    My 89 syncro pops out of 4th (and sometimes 3rd) when I decelerate! I hear this is a common issue. Can anyone tell me what parts cause this please?
     
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  2. hairyman

    hairyman Active Member

    Messages:
    391
    Location:
    Sydney, NSW
    I read a worn synchromesh in the 3-4 selector hub is the root cause of this problem. It affects all high mileage vw boxes unfortunately.
     
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  3. Syncro27

    Syncro27 Well-Known Member

    Messages:
    6,076
    Location:
    Terrigal, NSW Central Coast
    Unfortunately it sounds like re-build time.

    From my understanding the gears wear on the edge which allows them to pop out - syncros or gear wear means a rebuild time.
     
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  4. desminton

    desminton Member

    Messages:
    61
    Location:
    Melbourne
    Mine was jumping out of 4th, I got Aaron from Aaron’s Auto Speed Shop to sort it out. My first big drive in it this weekend and absolute dream to drive. He’s based on the GC so should be ideal for you.
     
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  5. nils

    nils Well-Known Member

    Messages:
    1,444
    Location:
    newcastle
    By the sounds most likely the 3/4 slider, synchros or dog teeth on 3rd and 4th gears. (Or a combo of all)
    Worst case would be the main bearing bashing your gear carrier into submission.
     
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  6. Syncro27

    Syncro27 Well-Known Member

    Messages:
    6,076
    Location:
    Terrigal, NSW Central Coast
    Here is some light reading from Michael Ghia in the UK - a very experienced T3 gearbox person.

    Information Post:

    Jumping out of 4th gear? (5th gear on a 5 speed 2WD)... then this is probably why.

    The issue of jumping out of top gear in the 091 and 094 gearbox is a common issue and usually, (but not always) it's down to the mainshaft moving forwards, and effectively as a result, the selector fork (for top gear) pulls the selector ring out of 4th gear. It's the same action as if you nudge the gear lever out of gear when under the most strain. Please don't do this!! You'll ruin your gears almost immediately

    So why does the mainshaft move at all? Well... it shouldn't!

    The reason that there is force there to start with is the Helical cut on the gears. The Helical cut makes the gears quiet (just try driving a car with straight cut gears!!) but it creates a side load on the gears as well. This side loading is transmitted through into the mainshaft and so on...

    The mainshaft is supported by two bearings. One at the diff end (small diameter roller bearing) and another much larger ball bearing located in the bearing housing.

    This larger bearing is designed to take the side load transmitted through from the gears, into the mainshaft.

    The bearing (when new) is a light press fit into the bearing housing and the next housing which bolts up against it (end casing on a 2WD and R&G housing on a Syncro) has less than 180° of metal surface holding that bearing in position... what's more when you fit a gasket between the end housing and the bearing housing you end up with that gasket's worth of clearance between the bearing outer race and the housing! Now... as the gearbox heats up, clearances change and that 'nice tight fit' that was there when it was cold becomes more of a 'push fit'. The bearing housing is made from either a magnesium alloy or aluminium alloy (depending which you've ended up with) and the bearing has a steel outer race. Obviously the alloy expands more than the steel does.

    So now we have a mainshaft support bearing, sitting in a nice warm housing, able to float around a bit (the thickness of the gasket) as you drive on and off the throttle. Not great really. What happens with time is that the bearing beats (literally hammers) itself into that 'less than 180°' surface as you go on and off the throttle... slowly bit by bit allowing the whole mainshaft to move further and further forwards (that's the direction it moves when under power in top gear) until it moves enough that the selector fork (that is staying stationary while all of the moving if going on) holds the selector ring and effectively pulls top gear... out of gear!

    The first that you know about this is that your gearbox jumps out of gear. 'Huh! I must've not put it in properly' ... you try again. 5 minutes later it does the same. This time you put it back in and 30 seconds later it pops out again. By this time... your top gear and selector ring are dead and the only way to keep it in gear is to hold it with your hands or wrap a bungee cord around it! Not great.

    I hope all of that makes sense because it's not easy to put into words [​IMG];)

    One last bit of explanation...

    'But what holds it in gear to start with?' I hear you cry!?

    Well, you need to start looking in detail at how your gearbox selects gears and works...

    Your selector ring and change gear have a set of dog teeth. These are not the large gear teeth that you can see easily. These are smaller.

    The selector ring turns with the mainshaft (for the top two gears) and the change gear is supported on roller bearings so that it can spin freely). When the selector ring engages with the change gear, it locks that gear onto the mainshaft. The teeth on the selector ring and the change gear sit together... the shape of these teeth is what we're really interested in.

    The shape of the dog teeth - when you first look at them - looks like a childs drawing of a house. Two vertical walls and an apex roof. Well... look again! The 'walls' aren't perfectly vertical! They lean out at an angle going up to the apex roof. It's the angle of the walls on both the selector ring and change gear that hold themselves together when you're in gear.

    When your gearbox jumps out of gear (for whatever reason) you start to round off the nice sharp edges of the dog teeth and the walls that used to lean out, end up becoming vertical or actually leaning in towards the roof.

    This is why, once it's jumped out of gear more than a few times.. it will have rounded off the dog teeth and the parts are dead. No amount of replacing other parts will fix the issue.
     
  7. Mr Beckstar

    Mr Beckstar Well-Known Member

    Messages:
    933
    Location:
    Bateau Bay, NSW
    No matter how hard I try I can’t get mine to shift into fourth.
     
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  8. cbus

    cbus Well-Known Member

    Messages:
    11,651
    Location:
    sunshine coast
    Auto.? :D
     
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  9. Mr Beckstar

    Mr Beckstar Well-Known Member

    Messages:
    933
    Location:
    Bateau Bay, NSW
    Yep ;)
     
  10. Grantus

    Grantus Well-Known Member

    Messages:
    15,554
    Location:
    Southern ACT
    Can’t find 5th gear also, I’m guessing? :rolleyes:
     
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  11. cbus

    cbus Well-Known Member

    Messages:
    11,651
    Location:
    sunshine coast
    Just go into" P" for "passing " o_O
     
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  12. Mr Beckstar

    Mr Beckstar Well-Known Member

    Messages:
    933
    Location:
    Bateau Bay, NSW
    Or “R” for Race :)
     
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  13. Wattie

    Wattie Well-Known Member

    Messages:
    912
    Location:
    Mildura. Victoria. Australia.
    Ha, I once had an auto full of neutrals, and, that was 3500 kms from home :( even worse it happened in the company of 3 Bays :oops:
     
  14. Dusty Waves

    Dusty Waves Member

    Messages:
    70
    Location:
    Qld
    Thanks so much for this! Very clear and helpful.
    Cheers
    Dusty
     
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