Shift into 1st very late

[Mohnish]

Have searched the forums, but don't find anything similar. So help me out here.

When coming to a standstill, the trans shifts into first gear really late ie. at a speed that is almost near or at stop, which results in a slight surge just before stopping. Similar to if you would mildly jab the accelerator before stopping.

I tried in W mode and this does not happen. I figure cause in W it does not shift down to 1st.

Also, manually shifted (through the touch shift) into first at a slightly higher speed (15kmph) and this surge did not happen.

Have recently (3000km ago) changed the trans ATF and trans filter alongwith the engine oil and filter.

Where do I start?

Thanks in advance.

 

[Mohnish]

No help?

No one with any suggestion? Help me out here please.

Forgot to mention, its a 722.6 tranny.

 

[alexander patie]

hello again mohnish, not sure about this one at all. as for where do you start, getting the fault codes checked in all drivetrain modules and analysing live data etc would be a good place. otherwise i could only guess, keep searching though as i am sure someone will know - perhaps there is a tramsmission forum out ther somewhere?

good luck!

 

[Mohnish]

hello again mohnish, not sure about this one at all. as for where do you start, getting the fault codes checked in all drivetrain modules and analysing live data etc would be a good place. otherwise i could only guess, keep searching though as i am sure someone will know - perhaps there is a tramsmission forum out ther somewhere?

good luck!

Thanks Alexander,

Have checked the fault codes on diagnostics. They are clear. However, live data has not been analysed. Will give a go with that. Incidentally, there is a reset transmission (722.6) procedure doing the rounds of another forum which relies on learning (teaching) the shift points to the tranny. Will give that a go too. Am sending the related post to you via pm.

Thanks again.

 

[Bobby Dazzler]

Thanks Alexander,

Have checked the fault codes on diagnostics. They are clear. However, live data has not been analysed. Will give a go with that. Incidentally, there is a reset transmission (722.6) procedure doing the rounds of another forum which relies on learning (teaching) the shift points to the tranny. Will give that a go too. Am sending the related post to you via pm.

Thanks again.

It's not the one where you hold your foot to the floor with the engine off is it?

If so then it doesn't reset anything - it just fools the car into thinking that you're driving like a loon, and so it prepares itself for another thrashing, and is ready and waiting.

 

[Mohnish]

It's not the one where you hold your foot to the floor with the engine off is it?

If so then it doesn't reset anything - it just fools the car into thinking that you're driving like a loon, and so it prepares itself for another thrashing, and is ready and waiting.

Yes it is exactly that one!!! And I tried it and found no change whatsoever!!! However, there is another procedure also listed which is apparently supposed to "teach" the trans the shift points wherein you manually change from 1 through 5, going step by step ie 1 to 2, then 2 to 3 and so forth about 5 times each step at the rpm you prefer the changes to take place and then back down through the gears again.

Will send you the link for this procedure by pm.

 

[Buggerlugs]

Have searched the forums, but don't find anything similar. So help me out here.

When coming to a standstill, the trans shifts into first gear really late ie. at a speed that is almost near or at stop, which results in a slight surge just before stopping. Similar to if you would mildly jab the accelerator before stopping.

I tried in W mode and this does not happen. I figure cause in W it does not shift down to 1st.

Hi Mohnish,

Ok, the gearbox can't actually cause a surge in engine power at any time, this leads me to suspect that either the engine is actually surging during this time or the gearbox has a poor 2-1 downshift overlap, where the gearbox could be neutralising during the 2-1 resulting in a brief coasting period before 1st gear finally engages, this I suppose could give the impression of a slight surge?

In this case the culprit could be one of several things, the most likely being wear and tear (yes, even at relatively low mileage) in the valve body or an internal hydraulic issue. The fact that it does'nt do it at higher revs also suggests some kind of hydraulic pressure regulation error.

To cut to the chase, for this particular fault, I don't think you're gonna get a magic bullet, the symptoms just are'nt a known concern.

 

[Mohnish]

Hi Mohnish,

Ok, the gearbox can't actually cause a surge in engine power at any time, this leads me to suspect that either the engine is actually surging during this time or the gearbox has a poor 2-1 downshift overlap, where the gearbox could be neutralising during the 2-1 resulting in a brief coasting period before 1st gear finally engages, this I suppose could give the impression of a slight surge?

In this case the culprit could be one of several things, the most likely being wear and tear (yes, even at relatively low mileage) in the valve body or an internal hydraulic issue. The fact that it does'nt do it at higher revs also suggests some kind of hydraulic pressure regulation error.

To cut to the chase, for this particular fault, I don't think you're gonna get a magic bullet, the symptoms just are'nt a known concern.

Yes, it is exactly as you described in bold above. However, the coasting is extremely brief but adequate to produce the lunge, more particularly as it tends to shift into 1 from 2 at or almost at stop. If the shift from 2 to 1 when deccelarating would happen at a higher speed (say 20kmph) then this would not be noticeable even if the brief coasting was present.

It feels like in a manual trans, if you would let out the clutch pedal with the engine revving a bit higher and let out the clutch suddenly producing a kind of lunge forward (surge).

Also, this is an intermittent problem (about once in fifty stops in a couple of hours of driving) and only present after the ATF is at operating temp (about half hour driving).

Incidentally, this problem manifest itself only after an ATF and filter change. I did not drain the torque converter when ATF was changed. Maybe, I should do a complete drain flush and refill?

Thanks for your thoughts.

UPDATE:

I also noticed today that everytime this happens, there is a puff of black smoke (soot/unburnt fuel) out the tail pipe (its a diesel). Wonder why that happens? Could this be as Buggerlugs suggested that the engine coasts and at which time there is unburnt fuel as the fuel delivery to the injectors is higher being based on the load at idle wth gear engaged and the box goes into partial neutral?

 

[AllanE]

By partial neutral, you mean that 1st is slow in engaging?
That surge you are feeling could be the engine changing from "fuel off" to "fuel on", if that makes sense...
Whilst slowing down approaching traffic lights, for instance, the car is slowing, your foot is off the accelerator, the fuel will be cut.
The car is driving the engine through the transmission and torque converter.
At some point first will be engaged - this will still have a braking effect until your car slows to a few mph.
Once the engine speed drops to a 1000rpm-or-so the engine ECU will start fuelling again to stop the engine stalling and I suspect that this is the surge you are feeling.
As you have noticed this after the ATF change, I suggest making sure the level is 100% correct before any resetting is done.
It could be that the phenomenon you are desribing is normal and the transmission has gradually masked it until the ATF has been 'refreshed'.
...Of course I could be wrong!

 

[Mohnish]

Hello Allan,

Thanks for your response:

By partial neutral, you mean that 1st is slow in engaging?

I mean that the tranny goes into first at a very low speed ie at almost stop, sometimes immediately after stop. It "feels" like the tranny was "late" in engaging first.

That surge you are feeling could be the engine changing from "fuel off" to "fuel on", if that makes sense...
Whilst slowing down approaching traffic lights, for instance, the car is slowing, your foot is off the accelerator, the fuel will be cut.
The car is driving the engine through the transmission and torque converter.

Yes, I do follow, however, if I drive in W mode (where 1st is never engaged) this does not happen. Should'nt the surge happen, if it is a "fuel off on" situation, even in W mode?

At some point first will be engaged - this will still have a braking effect until your car slows to a few mph.

Which is not happening as the first is engaged at a very low speed. If I engage 1st manually, the surge does not happen.

Also, it is an intermittent problem...If this is a "fuel off on" problem, could it be due to a defective idle speed regualator (or whatever its called in the diesel cdi). I ask this as there is a slight intermittent 50 rpm waver at idle, more pronounced when cold. But this manifests only very rarely.

 

[Buggerlugs]

Mohnish, have you checked the gearbox fluid level?

 

[RFdesigner]

Sorry to be the harbinger of bad news but it may be warning you that the transmission is about to fail.

Mine did similar things for a little bit before letting go.

Thankfully a complete new transmission isn't always necessary, a rebuild would be, at around £1000 for Labour and a full set of clutches and the TC clutch (which I understand has a habit of breaking up and sending swarf down into the box) (I had a chat with the technicians that rebuilt my transmission).

It's worth getting the bottom off the box and seeing if it's full of swarf... if the ATF was changed recently and the pan was removed but there was no swarf in there then you're probably in the clear.

Derek

 

[AllanE]

No answers, but some thoughts...
A 'puff of smoke' with a diseasel usually means overfuelling - coinciding with the 'surge'?
1st is a lower ratio than 2nd (obviously), but that also means there is more spiff at the wheels (that's why Winter mode starts in 2nd), so any engine torque is magnified more in 1st than 2nd.
The comment about the lock-up clutch then got me thinking...
As the lock-up clutch (which lives in the converter) should be disengaged in 1st and 2nd (I expect - may have to check), if it then engaged, it would be like engaging the clutch in a manual, rather than just relying on fluid slip, so may cause a bit of a surge.
It would be really nice to be able to see just when the lock-up is engaged, as if this is what's happening, the above 'harbinger' needs heeding, so that remedial action can be taken before the friction material is worn off the lock-up clutch and starts 'making swarf'.
Did you see the ATF filter that was removed?

 

[Bobby Dazzler]

The comment about the lock-up clutch then got me thinking...
As the lock-up clutch (which lives in the converter) should be disengaged in 1st and 2nd (I expect - may have to check), if it then engaged, it would be like engaging the clutch in a manual, rather than just relying on fluid slip, so may cause a bit of a surge.
It would be really nice to be able to see just when the lock-up is engaged, as if this is what's happening, the above 'harbinger' needs heeding, so that remedial action can be taken before the friction material is worn off the lock-up clutch and starts 'making swarf'.

 

[AllanE]

Bobby Dazzler - is this for all gears or just 1st, 2nd?
Do you know if the output shaft referred to is the transmission input shaft?
Anyway, it would be the lock-up engaging that is going to cause a surge rather than disengaging.

The lock-up issue that befalls some of our transmissions is due to it losing it's grip on the input shaft (spliney bit slips), so I wouldn't expect that to cause the problem and if it did, it would happen in manual mode, too.

None of which really helps you!

In one of my cars, when I'm approaching the end of our street, I'm usually in 3rd.
As the engine drops below 1800rpm-ish (engine cold), the ECU takes over and the car feels like it accelerates - it doesn't really, the ECU is maintaining the engine speed, which is maintaining the car speed, which 'feels' like it's a surge, because I was slowing down (hope that makes sense).
Could it be something similar? - It could be that 1st is engaged at the exact instant the ECU starts refuelling in 1 in 50 cases and the combined effect gives a surging feeling?

I'm intigued by that procedure that teaches the shift points...

 

[Bobby Dazzler]

Bobby Dazzler - is this for all gears or just 1st, 2nd?
Do you know if the output shaft referred to is the transmission input shaft?
Anyway, it would be the lock-up engaging that is going to cause a surge rather than disengaging.

AFAIK, all gears, output speed of the engine (ie as shown on the rev counter).

I'm intigued by that procedure that teaches the shift points...

IIRC the adaptive settings are based upon the most recent 40 shifts, but only in automatic mode, rather than manually selecting each gear. No source to back that up, just picked it up off some bloke on the interwebhighway.

I don't think that teaching transmission it's shift points or the foot-to-the-floor reset would do anything on this particular problem.

 

[Mohnish]

Mohnish, have you checked the gearbox fluid level?

Yes I have, immediately after the ATF change, however will do again (just to be "sure").

Sorry to be the harbinger of bad news but it may be warning you that the transmission is about to fail...

I certainly hope not. Here (India) they cost the earth to replace/rebuild.

...It's worth getting the bottom off the box and seeing if it's full of swarf... if the ATF was changed recently and the pan was removed but there was no swarf in there then you're probably in the clear.Derek

By "swarf" I presume you mean muck/metal/dirt etc. No it was clean, and so was the filter (which was also replaced).

...As the engine drops below 1800rpm-ish (engine cold), the ECU takes over and the car feels like it accelerates - it doesn't really, the ECU is maintaining the engine speed, which is maintaining the car speed, which 'feels' like it's a surge, because I was slowing down (hope that makes sense).
Could it be something similar? - It could be that 1st is engaged at the exact instant the ECU starts refuelling in 1 in 50 cases and the combined effect gives a surging feeling?...

Very possibly, but then why is it an intermittent problem? Interestingly, it only happens when coming to a "normal" stop. If you stop suddenly or coast to a stop, it does not happen!!!

...I'm intigued by that procedure that teaches the shift points...

Didn't help at all in any way whatsoever (though at on epoint in time I did think it made a difference). Not in the current problem, nor did it in any way change the shift points. I personally feel it amounts to naught...But then, I could be wrong.

...I don't think that teaching transmission it's shift points or the foot-to-the-floor reset would do anything on this particular problem.

As said earlier, it did not.

Its the puff of smoke/unburnt diesel thats got me wondering if this is a transmission issue in the first place. Incidentally, there is an intermittent waver in idle speed (about 50rpm), more pronounced on a cold engine. My indy said this to be a faulty "Fuel Pressure Regulator" which is fitted at the back end of the fuel rail (Part No. A6110780149). Now, I was wondering if this part could be responsible. If it is resposible for a wavering idle speed (intermittently) it may also be responsile for "allowing" more fuel causing the surge and smoke?? Reference : AllanE.

Did not change it as its expensive and the indy had no way of verifying that the part is actually faulty.

(Off topic: Just got a set of Michelin Primacy LC 225/55/16)

 

[Aeromerc]

Lazy Autobox

This interests me as I was literally just about to post a similar question... sorry to be adding more questions and not solutions! I think I will cross-post this in engine/gearbox in case it is a management computer 'thing'.

The following observations all apply to speeds between 0 and 40-50mph. (High speeds are fine.)

I had an identical experience as Mohnish - slowing down for roundabout (etc), the g/b would wait until the engine was almost stalled to change down. I also agree with Mohnish it only happens when slowing down gently, not fast.

I get the feeling that my g/b is a bit slow to change generally - we were in the Lakes in the summer and the g/b never seemed to be in the right gear (e.g. to get up a hill, I really had to put my foot down to get the g/b to change down, so for a few seconds the car was slowing down - and then it would change and I would have to back off the throttle).

I have C-cruise and S-sport modes in my car and it doesn't seem to matter which I use - when accelerating it changes up too late, and then when decelerating it 'hangs on' to the higher gear for too long. Does anyone have a feeling as to how much difference this button should make? I drove an Audi A6 recently and that had a S-sport button which really meant it, the power was instant and the gear was always the right one (in the French Alps, lots of gear changes...).

Back in my car, at a roundabout the 'lazy gearbox' problem usually means I am in second just when I want to squirt away quick - and then when I put my foot down harder to compensate for the low revs it changes down and it all gets a bit undignified.

When it is cold (straight after starting) it hangs on to first gear until around 2000rpm (maybe to compensate for lower engine power when cold?). Then it changes up with a bit of a jolt.

I didn't know if the lock-up clutch is universal, but my previous car (a Mitsubishi!) felt as though it had what I would call a 'freewheel' whereas the Merc always seems to have a direct connection between engine and wheels (e.g. when I take my foot off the throttle, it really slows down with the drag of the idle engine, whereas the Mitsi would just coast along.) Should it do that?

A related topic - are these gearboxes are meant to 'learn' your driving style? is there anyway of turning this off? Maybe it is this feature that is causing the problem? How can the computer possibly know what you want now based on what you did 10 minutes ago?

As this is my first Merc, can anyone say whether any of this sounds normal/crazy/expensive?

Thanks.

 

[Nordboy]

Aeromerc, did you get to the bottom of your problem? If so, what was it if you don't mind me asking.

 

[Aeromerc]

Sorry, hopeless at keeping on top of my posts. I recently had a freebie 'at home' service by MB dealer and they put the STAR machine on and it didn't have any gearbox faults - I mentioned this issue and they didn't know what I was talking about. Having had the car a few months I think that it is just me. Now I have learned the RPM the gearbox likes to change when warm/cold it doesn't seem to be such a problem...