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when the engine is running i am unable to engage reverse gear without crashing the gears.
Edited by Pugugly {P} on 12/01/2008 at 20:55
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Is the clutch cable adjusted correctly?
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Try waiting a couple of seconds after engaging the clutch before going for reverse, or alternately select first or second gear beforehand.
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my '93 Astra 1.4 does the same - unless you're at a dead stop and pause first it crunches every time and I renewed the cable last year.
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The reason it crunches is because firstly there is no syncromesh on reverse combined with a clutch on its last legs assuming it is correctly ajusted.
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So how does that explain that brand new ones also do it then?
It's because the reverse gear is a straight cut gear and not hellically cut like the forward gears are.
Or to go further and split hairs, see RichardW's comment in the following thread:-
www.honestjohn.co.uk/forum/post/index.htm?t=36661&...e
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DD I missed the word "or" out I agree with you.
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>>It's because the reverse gear is a straight cut gear and not hellically cut like the forward gears are.
I'd suggest that the gear being straight cut isn't the cause. It is entirely possible to have straight cut constant mesh gear with synchromesh.
However, reverse gear on older cars isn't constant mesh - it's sliding mesh. This means that when you select reverse on an older car, you are actually sliding the gear along the shaft until it meshes with another gear. (When you change gears going forwards, you aren't actually moving any gears around, you are simply locking/releasing them from the shaft running through their centres)
Because a sliding mesh reverse gear can slide along its shaft, it has to be straight cut - for two reasons i) to ease the sliding action ii) to prevent the axial component of the tooth seperating force simply pushing the gear back along the shaft when you take up drive. Helical gears create axial thrust because of the helix angle, straight cut gears don't.
So, I'd say that reverse is a sliding mesh gear is the reason why you can get grating during engagement, and that reverse gear is straight cut is because it is sliding mesh.
Sorry for the pedantry!
Number_Cruncher
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My pre-GM SAAB has a sliding mesh reverse gear which is helically cut. This is done so that the reverse idler pinion can mesh with the gear on the layshaft otherwise used for first gear together with the dedicated reverse driven gear. Reverse is silent on this vehicle as a result. The axial thrust generated is no problem if the pinion bearing is designed to suit (all the other bearings for the various helical gears elsewhere in the gearbox have to be similarly designed, although these gears don't slide axially).
The problem with the Astra is undoubtedly clutch drag, for which there are several causes such as a dry spigot bearing, stuck driven plate splines, wear or distortion in the pressure plate or else problems with or poor adjustment of the clutch actuation system.
As a palliative, always ensure that you come to rest in a forward gear, or engage one at rest. Then put the gearbox straight into reverse.
659.
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>>My pre-GM SAAB ...
And, I suppose, being an idler which slides, the axial thrust from one mesh will cancel with the axial thrust from the other mesh, so the sliding gear doesn't try to slide out of mesh when you take up drive.
Number_Cruncher
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Absolutely spot on. This is a clever design; the only shortcoming is that you can't use a stepped idler, otherwise the thrusts don't cancel. It also means that in in order to achieve this you need a dedicated reverse driven wheel on the output shaft, which this gearbox has.
Pity I can't now buy a SAAB as well designed, or anything like as well made as the one I have!
659.
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The problem with the Astra is undoubtedly clutch drag....
I've yet to come across a Vauxhall that doesn't crunch reverse gear when it's selected - especially one with a few miles on it.
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>> The problem with the Astra is undoubtedly clutch drag.... I've yet to come across a Vauxhall that doesn't crunch reverse gear when it's selected - especially one with a few miles on it.
I tend to agree DD - if this is the worst problem you have with a 16 year old Astra, then you aren't doing badly!
IIRC, Vauxhall deleted the spigot bearing before 1990, so your Astra *shouldn't* have one. The other problems which 659 mentioned which cause clutch drag are all possible.
Thankfully, it isn't a big job to do the clutch in these (the fastest I've done one is 15 minutes including driving it in, and a quick test drive round the block after!)
When worn, these gearboxes have very low friction, so 659's suggestion of stopping the whirring input shaft by selecting a forward gear first (relying on the synchromesh to brake the input shaft to a halt) is an excellent one.
Number_Cruncher
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read the handbook it says clutch fully down wait 3 seconds then engage reverse.
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Thanks for all the advice,sorry for the delay in replying I have to come to a public library to get on line.
To change the clutch and release bearing without removing the gearbox, will I need any special tools? Do you have any tips?
What exactly is clutch drag? Thanks a lot.
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dave.w,
I think you're unnecessarily worrying.
Before going to the trouble of changing the clutch, does the reverse gear still crunch when either holding the clutch down for a few seconds or selecting 1st or 2nd gear beforehand?
If not, then there is nothing to worry about.
As for changing the clutch, you'll need a spline socket (can't remember which size without looking) and some clutch plate clamps, which are easily made - there should be instructions in the Haynes manual of their dimensions.
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Dynamic Dave,
I have held the clutch down for ten seconds and been in to 1st and 2nd but it still crunches. I now turn off the engine select reverse and restart the engine.
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You asked "To change the clutch and release bearing without removing the gearbox, will I need any special tools? Do you have any tips?" No you can do this without removing the gear box and just asking this question tells us you should not attemping this. If sat in first engine ticking ever at what point is the clutch pedal at when the clutch starts to bite, i.e. the car starts to creep or the rpm drops slightly. What % of travel is this point. Regards Peter
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Peter D,
I ask about special tools and tips because I have the Haynes manual, and they sometimes seem over cautious. You can often get the job done without special tools.
The clutch bites at about 50%, I Can hear the release bearing taking up.The clutch pedal is a little higher than the brake pedal.Thankes for the advice. Dave W.
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Dave,
There are some things you *do* need.
You do need the three clips which clamp the pressure plate closed - these are really cheap, and easily obtainable.
If you have a 5 speed gearbox you do need some circlip pliers which can deal with internal circlips in a deep hole. If four speed, the hole isn't so deep!
You do need the spline socket to remove the little capping bolt in the inner end of the input shaft.
You do need something to pull with in order to withdraw the inside of the input shaft. Annoyingly, this is M7, which isn't a standard size. However, if you have an old clutch cable lying about, the adjusting thread is, happily, - M7! You can chop off the cable, screw the old cable end into the shaft, and pull using mole grips or similar. In some cases, these shafts are very tight - I made an adaptor to allow me to use the clutch cable end with a slide hammer, which makes the job really quick and easy.
If you get a genuine, or good quality clutch, there should be a cut out in the periphery of the cover plate, which should be lined up with an 'X' stamped in the flywheel for balance.
Number_Cruncher
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dave.w,
How hard is it to select other gears when stationary?
At rest, does the clutch pedal sit at the same height or slightly higher than the brake pedal?
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On most VXs of this period the state of the clutch adjustment can be checked by operating the clutch using a piece of 2 by 1 passed through the steering wheel & noting the distance the pedal travels through.
The correct distance should be in the Haynes manual IIRC - certainly this worked on Mk2 & 3 Cavaliers.
having had the 'crunch' sound on upwards of a dozen Vauxhalls I just regard it as a 'characteristic'.
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Just for the info,clutches for these should have the hold-down clips supplied with them.However,if it isn't slipping,how can it need replacing?Gear crunching indicates the opposite,not releasing enough,or drag.
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There are a number of driven plate faults which can cause this. Also there is the possibility of the driven plate not sliding along the splines of the input shaft correctly.
Pressure plates can fail so that they don't disengage fully all the way round - for example, if one or two of the fingers of the diaphragm spring break.
Slipping isn't the *only* reason ever to change a clutch.
Number_Cruncher
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