Driveline backlash ?

Rob, the term "backlash" refers to the very necessary torsional clearance between rotating parts of the drive train e.g. between the teeth of gears in the gearbox, transfer case and diferentials; in the needle roller bearings of universal joints; between surfaces in constant velocity (CV) joints; and in splines. Backlash is necessary otherwise the drive train would very quickly break due to interference between closely fitted parts. The amount of clearance reduces as parts warm up because the meshing parts expand as the temperature rises which reduces the clearance. 4WDs need more backlash than cars for two reasons (i) the drive line has more components - transfer case, centre diff if fitted, front diff (each component adds more backlash to the system), and (ii) 4WD components are larger and so need more clearance to allow for temperature effects. The expansion coefficient is a constant for any given material but the amount of expansion in mm is proportional to temperature and the original length or size of the component. So don't expect your 4WD to be like your car in terms of backlash. To check for the presence of backlash is simple. Drive at a constant speed (say 80 kph) then suddenly slip your foot off the accelerator so that the accelerator springs up off the floor .. you will hear (and feel) a clunk as the drive torque is reversed. One moment the engine was driving the wheels .. the next moment the wheels are driving the engine. However, this reversal of torque does not happen instantaneously, rather it is delayed while the backlash is taken up. This clunk is more noticable in a diesel engine with manual transmission because diesels have very effective engine braking so the enginge slows more suddenly. Automatic transmissions absorb some of the mechanical shock in the torque converter. Whether or not the backlash is excessive you will need an expert to determine - go to an independant mechanic. For your own comparison, try driving a new model vehicle of the same type as yours and check it for backlash. You can also get a feel for how much backlash is in your drive train by (i) jack one wheel ("Wheel A") off the ground, (ii) chock the other wheels (iii) release the handbrake, (iv) engage a gear(1,2,3 or 4), and (v) feel how much you can freely rotate "Wheel A". Sudden forces on the drive train are not good for your vehicle. Once you sort out your problem it would pay to learn the ways of driving that minimise the harmful effects of backlash. Good luck.

Rob , yes we once had the same problem you describe but with an older vehicle. I'm not sure how this relates to you, as you seem to indicate yours is newish? Still under warranty? Anyway, we had a '94 diesel troopy (HZJ75) we had happily driven around Australia once (9mth trip with trailer) it then developed the jerking you describe. This is different to "crunching" and wasn't a syncro mesh problem or broken teeth in gear cogs. After significant research we were led to believe that it was a cracked drive shaft (apparently a common problem with that vehicle but the cost to investigate this meant the labour cost of removing and then re-installing the entire gearbox plus the parts and could cost more than a brand new gearbox. It was actually cheaper for us to buy a complete gearbox (inc. drive shaft) from a wreck of a new vehicle - (the same gearbox is still used) and have it installed. It fixed the problem at $3000! Best of luck!

Guys,

Just an update, I have been back to Toyota and again they insist that the jerking is normal :-(.
I'm still determined to get to the bottom of the issue as I do not wan't to leave it and wait for something to break.
I will be taking up an offer by the service manager and will be test driving his vehicle and see if this is normal ..... or am I being too parnoid ?

Thanks for the detailed responses

My 14 year old troopy has travelled 372 000 kms and has only just started to display backlash and only then if I am a little premature with the upshift. Therefore (in my opinion) driveline backlash should not be considered normal, and you should be taking up the offer of the test drive...

dave