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Diagnosing manual Gearbox problems.

Discussion in 'Diagnosis/Help' started by Stig, Apr 17, 2009.

  1. Stig

    Stig ST162 Guru Donated!

    These boxes are becoming older and the clearances inside are no longer what they should be. Gearboxes and clutches are expensive items and not an easy job to do, a little care will make them last a lot longer.

    Always change the release bearing, when they go, they sound terrible and will eat through the "fingers" on the clutch basket in no time.
    Extra insurance is to change the flywheel and input shaft seals while it's open, nobody likes seeing oil stains in the driveway! A leaky seal will destroy your pressure plate.
    Brake/clutch fluid is anhydrous (hygroscopic)- it absorbs water from the air and rusts your cylinders and pipes from the inside, change it once a year or when it becomes yellow.
    Change the oil annually - The oil may still be clean but will be contaminated by tiny metal filings which are busy munching away at the bearings, syncros etc.
    It is wiser to use thicker oil in all boxes older than 10yrs, 80W/90 is a safe bet.
    ATF in manual boxes is an old racers trick, it cuts the friction but does your syncros, bearings etc no favours.
    Burnouts are fun but take a heavy toll on clutches, gears, diffs and driveshafts - be warned & start saving for expensive repairs.
    Spend the extra bucks and fit an upgraded clutch, it will last a lot longer, esp if you don't abuse it.
    Always change the plate/basket & bearing as a unit, prevents compatibility issues and you know the age of all the components.
    Light flywheels may make your engine blip quicker and may help acceleration, but the tradeoff is less torque (more fuel consumption) and a drop in top speed. The lighter the wheel, the worse it gets.


    Burning smell on acceleration - Usually accompanied by the rev counter "moving around". Will be most noticeable in 1st & 2nd and when climbing steep hills in a low gear. It will rapidly spread to other gears and all drive will be lost - Start saving for a new clutch.

    Clattering at idle - Thrust bearing (goes away when pushing in clutch), Gear bushes/bearings worn or incorrect secondary shaft/diff spacing. Also the torque springs in the friction disc itself can be loose, the only harm is the noise.

    Noisy while driving - Bearings worn, clutch fingers broken, thrust bearing loose, NO OIL.

    Grinding into gear - Air in hydralic system, old or wrong oil, insufficient oil, selector fork bent, syncros worn out, syncro teeth rounded, Cables stretched, push rod too short.

    Becoming more difficult to select gears - Air in the clutch master/slave cylinder, or one may have a stuffed seal. Stand in front (mr2 = rear) and get someone to push the pedal - you can then suss out the fork movement & bleed if needed. If it's a cable clutch - it may just need adjusting (tightening)

    Fitted a new clutch and now it won't work - Sometimes a clutch bearing has a different height from the flywheel (clutch basket is bigger/smaller?) - we then put in a longer or shorter actuating rod in the slave cylinder to get it operating right again.

    Fitted a new thicker Pressure plate, fork is moving OK but can't select gears - The new Pressure plate is too thick for the clutch basket and there isn't enough travel to release the pressure plate. Can also happen when a flywheel has been incorrectly skimmed. Remedy is to fit a bigger clutch basket or to shim the basket away from the flywheel with washers.

    Jumping out of gear - Worn/loose selector rings, forks, detents worn on selector shafts, cables stretched, Spiders & dogs completely worn out – 2nd gear usually goes first as it takes the most punishment

    Cannot select any gear - Broken selector cable or clutch or clutch master/slave cylinder. Broken gearbow internals.

    Pulls to one side or spins one wheel - Check diff clearances, LSD damage

    Judder on pulloff - Incorrect diff/planetary clearances (too tight in one direction)

    Stuck in gear - Broken/loose selector fork, broken or loose cable.

    Slamming into/out of gear - Faulty dog ring and/or syncromesh failing

    No drive from motor - Faulty clutch, broken pressure plate or broken input shaft or planetary gears

    Leaking oil - Worn seals on input shaft/selector shaft (check shaft for wear), Faulty driveshaft seals

    Gearbox jammed solid - 2 Gears engaged simultaneously, gearbox bearings seized or broken part stuck between gear teeth.

    Clutch operates too high on the pedal - Fit shorter actuator rod, cable too tight (cable type)

    Clutch operates too low on the pedal - Bleed clutch hydralics, fit longer actuating rod, no joy = worn cylinder seals. Tighten cable (cable type)

    Speedo not working - Nylon speedo drive worn out in gearbox, speedo cable broken
  2. Green Celica

    Green Celica Well-Known Member

    Thank you for diagnosing the gear problems! (jumping out of gear)
  3. elduderino

    elduderino Well-Known Member

    Disregard I read it wrong! Thanks for the info.
  4. phamon

    phamon Guest

    Cluutch pedal goes to the floor

    I was driving one day and with out worning to me, after I stoped the clutch pedal went to the floor.

    I bout the car used, I did not know that I had a clutch master cylinder, I found it and it had no brake flued in it

    I am thinking I may need to replace nthe clutch master cylinder and clutch slave cylinder

    Any coments or questions?


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