how does that work then??

snickers · Post by **snickers** » Sun Sep 17, 2006 9:21 am

Does anyone know what exactly happens when the gearbox goes into "top gear"?
Its when the rev counter stops whizzing up and down and just increases gradually with road speed!

We have a noise that only happens when it goes into this mode and I thought if I understood a bit more about it I could diagnose the fault a bit easier!!

Any ideas?

Colin Lambert · Post by **Colin Lambert** » Sun Sep 17, 2006 10:20 am

When the box changes into top, it does, at a certain speed (don't know what that speed is though) 'lock-up'. An auto gear box is a fluid drive. Think of it as a boat propellor. i:e: the prop can be going round but if the obstruction is strong enough the boat will not move and so you get 'slippage'.
An auto box is similar in that you get slippage. This obviously wastes fuel.
(That's why outboard engines are so thirsty). To stop this occuring in an auto box a dog clutch engages at the aforementioned speed and locks the gearbox up so you have direct, positive drive instead of the fluid drive.
You will see from the attached that a dog clutch(this is from an outboard engine) is basically two castellated units sliding on splines they engage male with female to give a positive drive.
When the speed decreases or you 'kickdown' a solenoid disengages the dog clutch and returns the box to fluid drive to anable the box to change gear.
For full explanation of auto gear box function go to
http://auto.howstuffworks.com/automatic ... ission.htm

Hope this helps.

Dave up north · Post by **Dave up north** » Sun Sep 17, 2006 10:27 am

Whats the symptoms of the noise Snicksey?

Now that Big Col has posted probably the best definition of the dog clutch thing I have seen, perhaps he, or other members could help diagnose the problem with you.
All t best,

Big Dave

snickers · Post by **snickers** » Mon Sep 18, 2006 7:17 am

Dave up north wrote:Whats the symptoms of the noise Snicksey?

Now that Big Col has posted probably the best definition of the dog clutch thing I have seen, perhaps he, or other members could help diagnose the problem with you.
All t best,

Big Dave

1st of all thanks for your replies!!!!

the noise that we get sounds like a faulty bearing.(droning/thrumming)

It only happens when the box is locked up and it can be heard from around 40 mph right up to around 70-80mph where by the road noise and wind drown it out!

if when travelling at around 50-60mph we slip it in to neutral the noise goes away!!!

Also the noise is more pronounced when on the motorways etc when we are going around a right hand curve,ie, loading the left hand wheels ,but only when the box is locked and we are driving the wheels ie not in neutral.

the guy we got it from has checked all of the wheel bearings and said they are all ok!

any ideas?????????

moonshine · Post by **moonshine** » Mon Sep 18, 2006 8:12 am

Without hearing the noise it is virtually impossible to say for sure, but I would suspect a differential. It would certainly be worth getting them checked out. You don't say whether you are 2 or 4WD, but if the latter I would be inclined towards the front diff bearings.

If I am right, the noise could simply be more pronounced when in lock-up because the drive is now solid rather than fluid.

snickers · Post by **snickers** » Mon Sep 18, 2006 5:19 pm

hello moonshine!

Funny you should mention the diff because we have just had to have the rear diff, planet and star ?? wheels changed as they were worn!!

It is a 4x 4 so do you think it is worth getting them (front diff bearings)checked out and if so how much should I be paying???

Snickers!!

Colin Lambert · Post by **Colin Lambert** » Mon Sep 18, 2006 10:42 pm

It is a 4x 4 so do you think it is worth getting them (front diff bearings)checked out and if so how much should I be paying???

Yes and sorry I haven't a clue.
Moonshine beat me to it, I concur with his diagnosis. Either that or shaft bearing in the gear box but that would not account for more pronounced noise on turning. So I will go with front diff, specially since the rear diff is s*****d. How many miles has it reputedly covered?

dandywarhol · Post by **dandywarhol** » Mon Sep 18, 2006 11:02 pm

Colin Lambert wrote:When the box changes into top, it does, at a certain speed (don't know what that speed is though) 'lock-up'. An auto gear box is a fluid drive. Think of it as a boat propellor. i:e: the prop can be going round but if the obstruction is strong enough the boat will not move and so you get 'slippage'.
An auto box is similar in that you get slippage. This obviously wastes fuel.
(That's why outboard engines are so thirsty). To stop this occuring in an auto box a dog clutch engages at the aforementioned speed and locks the gearbox up so you have direct, positive drive instead of the fluid drive.
You will see from the attached that a dog clutch(this is from an outboard engine) is basically two castellated units sliding on splines they engage male with female to give a positive drive.
When the speed decreases or you 'kickdown' a solenoid disengages the dog clutch and returns the box to fluid drive to anable the box to change gear.

http://www.bankspower.com/Tech_understa ... conver.cfm
For full explanation of auto gear box function go to
http://auto.howstuffworks.com/automatic ... ission.htm

Hope this helps.

That isn't how the "Lock up" works on the auto box system on the Bongo (and many others) Colin

The "lockup" is a friction brake device in the torque convertor which prevents the slippage between the impeller and turbine.

snickers · Post by **snickers** » Tue Sep 19, 2006 7:04 am

Colin Lambert wrote:
It is a 4x 4 so do you think it is worth getting them (front diff bearings)checked out and if so how much should I be paying???

Yes and sorry I haven't a clue.
Moonshine beat me to it, I concur with his diagnosis. Either that or shaft bearing in the gear box but that would not account for more pronounced noise on turning. So I will go with front diff, specially since the rear diff is s*****d. How many miles has it reputedly covered?

The van has reputedley covered around 45,000 miles bought from a reputable? bongo dealer!!!

I'm starting to get concerned that repairing this diff, if that is the problem,will cost mega money!!

Do any of you know of a good bongo repairer in the Devon area near Newton Abbot as I really need a specialist I think!

snickers · Post by **snickers** » Tue Sep 19, 2006 7:07 am

.[/quote]
That isn't how the "Lock up" works on the auto box system on the Bongo (and many others) Colin

The "lockup" is a friction brake device in the torque convertor which prevents the slippage between the impeller and turbine.[/quote]

If this is the case and the gear box works in this way and not as previousley described, would the diagnosis of the fault be different or am I still looking at the front diff bearings???