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Discussion Starter · #1 ·
Hi everyone!

The last couple of weeks I have noticed that my MGF's gearbox seems to be poorly sick. When starting off on a journey everything seems fine, but within just a few yards when braking for a junction the gearbox drops down into 1st gear to soon causing very sudden braking sensation throwing us forward!

Does this sound like I need to re-mortgage my house?

Thanks in advance!
 

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Hi Sarah. I drive a steptronic too. Are you setting off in Sports mode or Drive mode? There are no gears as such when in Drive mode but mine stalls after being left overnight if it has been chilly so I leave it running for a few minutes and then drive off and it is fine.

It is something to do with the balance between engine torque and gearbox torque that the MGOC said to me they could fix if it bothered me too much.

Check back on here frequently as there is a couple of guys who know this gearbox inside out from working at MG Rover. I will be interested in the diagnosis also.
 

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Discussion Starter · #3 ·
I dont use the Sports Mode part, so always in drive. I do find it judders sometimes when cold but like you say, easily fixable while hooking my ipod up!!

This is almost certainly something with it dropping into '1st' to quick when I brake.
 

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I don't think mine actually drops a gear when slowing down when cold but it certainly drops revs quickly. As I said, there are no gear changes in Drive mode. When was it last serviced? When was the gearbox oil last changed?
 

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Discussion Starter · #5 ·
The revs don't drop but actually rise to around 3500 when braking and preparing to stop. Although not physically dropping down a gear you get the same effect as if you were to go from 2nd to 1st too soon in a manual.

I had a pipe pop out from the inlet manifold recently which caused the revs to go berserk and the issue seems to have started after that was refitted although i don't remember if it had happened before that or not
 

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If ity was refitted by a garage, I would take the car back and ask them to sort out the problem they have created. Be adamant that it didn't happen before they fixed it!
 

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Just wanted to bump this back to the first page to see if anyone can help Sarah Jayne. I am not knowledgeable enough and bumped it in the hope that Shaf or Staffain might see it.
 

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On the whole the CVT box is pretty good. If something goes wrong then it tends to be electrical. When this happens is it possibly going into "limp home" mode. Sounds to me that something has been disturbed when the garage did the recent repair and as suggested in the first instance you should take it back to them. If you have no luck with them I would try and get it hooked up to a testbook to see if this throws up anything.

I've just noticed that you say that your problems start when you brake. This would also suggest that something is being disturbed by the sudden movement and again could mean that a wire or connector has been disturbed during the recent repair. Try holding your "gear lever" in the one place as you brake to see if it's something in that area that's causing the problem.
 

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Hellooo,

Firstly, sounds normal to me. How long have you had the car? If for a long time, then has it changed significantly?

The clutch and gear ratio variator both run off the same hydraulic supply (in the gearbox).

The hydraulic pressure is proportional to the engien speed, but is capped. So when stationary with enigne at about 800rpm idle the clutch has very little pressure applied. When you press the accelerator there is more engien torque than clutch torque so the revs rise. This causes a rise in hydraulic pressure and then more pressure on the clutch. So the cltuch transmits more torque, the car accelerates... Once over about 2,000 rpm the hydarulic pressure is so high the clutch will not longer slip.
So far I have not explained you problem but bare with me.

The gear ratio is controlled by hydraulic pressure applied to a big pair of cones. This moves the cones towards each other. In between the cones is a V shaved belt. the cones moving towards eachother forces the belt outwards. This then changes the relationship between the radius the belt runs on the input and output drums.
Still none the wiser eh?

Now for the problem.
When slowing down from high speed you are usually in a longer gear (Lower engine speed). Longer gear is achieved by having the ratio variator at higher pressure.
As you slow the ratio starts to shorten with the aim of being in low ratio before you reach a stop.
To shorten the ratio hydraulic pressure is bled out of the variator and back to the transmission pump.
In the meantime the pressure on the clutch is still quite high because you are over about 1,500 rpm.
As you get close to a halt the engine speed drops below 1,500 and the clutch pressure drops. The negative (Braking) torque of the engine causes the clutch to slip and allows the engine speed to drop slightly.
If at this point the ratio variator reaches low gear, then there is a short pressure surge in the hydraulic system as it suddenly releases the last of the pressure it has in it.
This pressure surge gets through to the clutch and temporarily causes it to close again. This causes the shock you notice.

Sorry but there is not a lot you can do about it. Bit of crap design by the transmission manufacturers, and poor management decision not to go with full electronic control.

You shoud find that it happens less in sport, and probably not at all if you select gear 1 in steptronic mode. Not much help, but there is not a lot you can do.


If the shock has changed significantly recently then get your engien mounts checked. They should damp out a bit of the shock.

Shaf
 

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My name is dsmiley, and i have just joined this forum, because i am having similar problems with our 02 reg mgtf.
It's an auto with paddle shift option.
The first time this occurred was when i was on a country road, and changed from auto to sport mode, when doing about 40mph - got the shock of my life when the revs went thru the roof and the car 'braked' then appeared to lose power.
I pulled over, left it for a while, then tried to move again in auto. Initially the warning light on the dash stayed 'on', so I waited a while and tried again and this time it moved off with no problem.
Y'day in town, driving in auto only, it did the same thing .
Question - is this easily fixable - should i be worried/ Obviously I will be if the warning light stays on today, and how is it fixed/cured?
Dave


Hi Sarah. I drive a steptronic too. Are you setting off in Sports mode or Drive mode? There are no gears as such when in Drive mode but mine stalls after being left overnight if it has been chilly so I leave it running for a few minutes and then drive off and it is fine.

It is something to do with the balance between engine torque and gearbox torque that the MGOC said to me they could fix if it bothered me too much.

Check back on here frequently as there is a couple of guys who know this gearbox inside out from working at MG Rover. I will be interested in the diagnosis also.
 

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Hi dsmiley, welcome to the forum.

I think what you are experiencing is normal!

The Auto/Sport change effectively changes the set point for the ratio 'holds' or 'changes' (from the above description you willl see it has no fixed gears/ratios) so moving from auto to sport will make the gearbox think you want to hold a 'ratio' longer to a higher engine rev limit. In effect it will 'change down' hence the higher revs and engine braking effect unless you apply a bit more throttle to counter balance this 'lower ratio'.

Can be used to your advantage by using the effect as if changing down a gear for overtaking.
 

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Thanks TipperMG - good points, but seems like a design flaw to me.
Maybe i should just stay in Auto, however y'day i was only in Auto
when the problem occurred.
Good to know that it's not more serious.
Will have dealer check it out this week.
 

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You need to be cautious of dealers, even xMG ones 'cos they don't know much about the CVT gearbox. This appears to be a worldwide problem as the MINIs with the same gearbox also have a lot of problems with their USA dealers being 'dumb'! Lots of changing gearboxes for loads of money because they don't know how they work. Auto 'box specialists also don't appear to know much either!

One essential is to ensure the oil level is correct and changed regularly and only use the recommended oil which is Esso EZL799 which appears to be only available from BMW MINI dealers now. Do not allow other oils to be put in the gearbox as the manufacturers, Punch Powertrain formerly ZF, claim damage will occur.

Search for other similar threads and you will find more info particularly from those that know more than me, ie Shaf and Stafainn. :URGOD:
 

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Gearbox oil

TipperMG,
Appreciate your comments - points noticed, and I will be wary of the Dealer.
Other thought that occurs is that this could be an 'electrical' problem, because today the battery appears to be ok, but the engine wont turn, so could be either the alternator or starter. Something is using the 'juice' however, as the car was starting no problem only a few days ago.
David
 
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