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1998 750iL: gearbox..sport..45mph 2,500 rpm to force it to change..3rd…

Customer Question
My 1998 750iL gearbox downshitfs...
My 1998 750iL gearbox downshitfs when hot, no matter if in auto, sport or manual mode. For example, you have to be above 45mph 2,500 rpm to force it to change from 2nd. to third in manual mode, or above 75mph (approx 2,500 rpm) to force it to change from 3rd. to 4th. gear, and very fast to force it to change from 4th. to 5th. In auto mode it will never change above 3rd.

It's fine when cold, but a combination of hard driving and outside temperature elicit the problem. For example about 30miles in hot weather or 100miles in cold weather, it's definitley temperature related. If you drive really gently it will eventually revert to normal if the outside temperature isn't too hot. It's had a rebuilt gearbox which didn't cure the problem, a new gearbox computer, and BMW Chandler's have had it numerous times and can't figure it out. Any ideas?
Submitted: 8 years ago.Category: UK Car
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Answered in 7 minutes by:
6/26/2010
UK Auto Mechanic: Mike, UK Car Mechanic replied 8 years ago
Mike
Mike, UK Car Mechanic
Category: UK Car
Satisfied Customers: 39,198
Experience: Hi, I am a Car/ vehicle technician of 34 years. 29of -running my own repair/diagnostic business.
Verified
Hello and Welcome to justanswer..

Can I just ask you what the dealers have said now ? What they think the fault is....

Have you had it too a specialist at all ?

Who has done all this work ?

Kind Rgds - MIKE..........................................


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Customer reply replied 8 years ago

Hi Mike,

 

Gearbox was reconditioned by Auto Gearboxes Hove, long established I have used them several times in the past. They have no idea (ask BMW). They have asked around to no avail.

 

BMW (Chandlers Ferring & Hove) have done several scans but can't figure it out. There solution is "buy a new gearbox from BMW then it's guaranteed". Having already spent £2.5K on a rebuilt gearbox spending another £4K is not an option.

 

(Whoever designed the bloody gearbox must have some ideas, BMW don't seem to know who he is!)

 

Thanks,

Barry

 

Customer reply replied 8 years ago

Hi Mike,

 

See below scan sent to Auto Gearboxes

 

Thanks,

Barry

 

FAX: 722 320

 

TO: XXXXX XXXXX Gearboxes, Hove

P1/2

 

BMW 750

Diagnosis of fault gearbox changing down when hot in all modes (e.g., Brighton to Heathrow) presumably a speed sensor fault:

 

 

ENGINE

139 BMW 94 NOT KNOWN

OUTPUT STAGE MAP COOLING

 

ABS

89 NOT KNOWN

58 CAN INTERRUPT TO EAGS

 

EML

159 THROTTLE VALVE 1

STEPPING MOTOR OUTPUT STAGE

OBD 11 CODE P1558

113 ROAD SPEED SIGNAL

UK Auto Mechanic: Mike, UK Car Mechanic replied 8 years ago
Hi again - thank you for your reply and excellent info...

Ok... I have come across such problems before - on a 520 actually but same thing..

The scan as above indicates troubles with the engine sensors as you may well have figured..or transmission - but it has to be borne in mind that if the engine is not running properly as it should , it can and will affect transmission performance, range change etc..

However - Im not so sure this is relevant here as you don't mention any engine problems at all...for one.. for two, your outline of the problems do seem to id transmission troubles and three.. the codes as recorded above could well be historic and have no value at all...

The only thing that does stand out is the road speed signal fault code.. I would suggest having the codes cleared / deleted and re run and then re run the fcr test to see if the same codes come back...then you know if they are historic or not.

Specifically thinking about the transmission.. it still could be the transmission itself ...I have known this before on occasion even with a recon box... However, I have seen several with wiring / loom problems from the TCM - trans control module - out... and that is in fact what was wrong with the extremely troublesome 5 series one I mentioned at the outset - after spending absolutely ages , time and money trying to rectify it ..thats what it was..

Given your description of the faults - they are not at all dissimilar to the one I had a lot to do with personally ... so I do think that this is your next move on this one - Im not sure whether you want to just renew the loom - that isn't cheap...or, take it apart in stages to trace / look for faults... It can be anything from hardened / chaffed , split wires to poor connections etc... Its easy enough to check the connections all everywhere , well , relatively - but to check the wiring through takes time and patience etc...

I actually do suspect this to be the problem...Its obviously hard to know for sure - I have also known oil work its way up through the loom and causing problems therein.. thats another factor..

So at this stage - those are the thoughts I have on this one Barry - Obviously not an easy one as you have gone to a lot of trouble with it already...but those are the areas of suspicion I have that need checking to verify one way or the other or locate the fault..and then go from there...

It has to be said that if all the above checks out ... strange as it may seem, it has to then be a problem with the transmission itself ..

I Sincerely XXXXX XXXXX have helped you - Please ask me if you need more help...


Kind Rgds, MIKE

 

  • >>>>>>>>>Please don't forget ....Please press the ACCEPT button if you are happy with my answer/advice given -as that's how I get paid for my time ... Thank you...

 

Feedback and any Bonuses are always appreciated too. Thank you.

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Customer reply replied 8 years ago

Hi Mike,

 

The fault codes were cleared before taking a test drive and eliciting the problem (otherwise there would have been hundreds of fault codes)

 

What I didn't mention was that this happened 5 years ago (when the loom was only 8 years old). Over 5 years the symptons have not changed, not what you would expect if due to degradation of insulation, or similar. As with all intermittent faults you hope that they will eventually become permament and easier to diagnose; but this hasn't happened! I suspect a more systematic or intelligent part of the system, but which part?

 

What do you think?

 

Thnks,

Barry

UK Auto Mechanic: Mike, UK Car Mechanic replied 8 years ago
Hello again Barry - thank you for your reply...

Ok, doing that with codes is good / correct ...so many people don't do it so I have to mention it...

This is interesting with the loom - it sounds almost as if the loom they have fitted may have had some problem within itself - maybe a manufacturing issue from day one...its a consideration not to be dismissed.. I have to say also though that I have known insulation degradation with looms not as old as that .... on some 3-4 years old and of course outside influcences don't help - oil , heat etc..even poor fitting practice of a replacement... again, something to be considered ..being its been replaced once. Often you can dismiss an item whatever it may be because its new - or not that old.... and the problems lies there all the time ! So don't dismiss the idea of that loom Barry..

The same applies of course to electronic ( and any other! ) components... Im thinking the computer ( Tcm) that you have replaced ...and indeed ...I have the transmission in the back of my mind on this one all the time..

As the transmission is a major part of the system - it makes sense for them to work through the other items first...

I would suggest start at the beginning on this one Barry and work through..so nothing is missed.. This may involve them substituting parts - such as the Tcm unit - loom etc...with others as the best way forward - -for testing purposes.. and then see what that reveals.

Im suggesting this because from what you have said - everything "normal" has been done.... So this has to be the sensible way forward now... doing one piece of the system at a time and test drives in between - you can see what I mean. Sometimes it has to come down to this process.

If when all has gone been gone through as above....nothing sorts it... then it has to be down to the transmission...

That as above - all things considered, has to be the way forward on this one Barry...

I hope that helps you further - but do feel free to ask me if you need anything / discuss it more..


Best Rgds, MIKE

 

 

 

  • >>>>>>>>>Please don't forget ....Please press the ACCEPT button if you are happy with my answer/advice given -as that's how I get paid for my time ... Thank you...

 

Feedback and any Bonuses are always appreciated too. Thank you.

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Customer reply replied 8 years ago
Relist: Incomplete answer.
UK Auto Mechanic: Mike, UK Car Mechanic replied 8 years ago
Hello again Barry...

I must confess Im somewhat surprised to see you have re listed the question as "Incomplete answer" - given the amount of time and effort and help I have given you....

Can you perhaps explain to me what else I can possibly do for with an on line advice service?

Kind Rgds, MIKE.....................................
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Customer reply replied 8 years ago

Hi Mike,

 

I appreciate your efforts, but I really want to find someone who has come across this exact problem before and knows the answer without having to go throught he whole diagnosis again, how do I find them? At the moment I think the only use for the car is banger racing

 

Thanks,

Barry

 

 

 

 

UK Auto Mechanic: Mike, UK Car Mechanic replied 8 years ago
Hello Barry..

The info I have offered you is based on experience I personally have of these motors , their systems and faults, experience of others and known faults ( within the motor trade ) (of these models ...re the exact faults you have )

I appreciate you don't want to go through the whole process again - the only reason I suggest that -( though Im not saying fully ) but, enough to be sure nothing has been missed ..because, something has for sure ... or, something has not been done right - such as the fitting of a faulty part s( Tcm) or a poor build on the transmission....neither of which are unknown..

If you think about it - you have basically done everything in that system - so, it has to be as above - ie, something missed - or something not built right , trans - or faulty part ..as just explained... These are the reasons Im saying you need to have a "new" look at the situation.

With any situation its very often not a silver bullet - its nice when someone can describe a situation and you can say - ah yes - that will be that... and while that does happen quite often... not so with such a fault as yours where you have done all that you have...

You mentioned wanting / hoping to find someone who had this problem before..No -one has come to me and said , tell him to try this or do that ( we have that facility internally )... but too, I did mention to you that I personally had a situation such as this with a 5 series... which is why I know you are struggling..

Its not a straightforward situation Barry , you know that for sure - so I have offered you all the benefit of my experience on this by way of all that I have said... whereas I know some would have just said to you ... well, not a lot....or, they didn't take the question simply because they can't help you at all...

So thats where we are up to Barry , I hope you can see that I have been doing all the very best I can for you ..( as I always do).. Plus you have had BMW diagnostics and techs on it...and the results of their tests / investigations etc are looking transmission ( faulty rebuild ) ... As I said to you - I tend to think the same...but , as I mentioned, rather than just go for that ...its much easier and makes lot of sense to go through the other items in the system again..to be sure nothing has been missed... Thats "good " diagnostics for you ...the way to go for sure... Then if all checks out - its down to the transmission and you would need to impress the rebuilders back into the job...

I hope that helps / clarifies for you ... by all means get back to me if you wish...

Kind Rgds.. MIKE




 

  • >>>>>>>>>Please don't forget ....Please press the ACCEPT button if you are happy with my answer/advice given -as that's how I get paid for my time ... Thank you...

 

Feedback and any Bonuses are always appreciated too. Thank you.


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