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Roger, BMW Master Tech
Category: BMW
Satisfied Customers: 5889
Experience:  10 yrs. experience as BMW Master Technician, shop foreman and dealer.
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523i: Hi. Having transmission problem with my 2005 523i. Have

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Having transmission problem with my 2005 523i.
Have a fault code 4F92 Ratio monitoring, clutch A-C. I wonder what it means.
May I know which component to change?



Have you noticed any harsh shifting and does the transmission failsafe program come on when the fault sets? Do you know if the fluid was very dark and if it had a slight burnt smell to it? Can I please have the last seven digits of the VIN number? Thank you.

Customer: replied 4 years ago.
VIN is CM72108There is no harsh shifting but if I jam brake to slow down, I'll hear a knock coming from the front but I'm not sure whether its from the gearbox.The transmission oil was blackish but not sure about the smell cause the mechanic did the oil change and I just watched.There was a transmission fail safe warning light on the dashboard popping up every now and then. When that happened, I seems to lose usages of the lower gears.Car moved off using a higher gear from stop.The warning light will go away after I restart the car and the car will feel normal again.

Thank you for the additional information. When the transmission clutch packs do not operate correctly due to either slippage or loss of pressure this fault will set. With dark fluid and a slight burnt smell it means the transmission needs replacement or an over haul. The problem with getting an over haul is that not all shops know how to check each component in the transmission correctly and you can end up with the same problem.

I hunted down some additional information on that fault and it appears a common problem with bushing for the input shaft wears out and can cause this fault. The entire shaft would have to be replaced as well during the over haul of the transmission. Also the seal in the torque converter should be checked to see if there is any damage in the seal for the torque converter.

Unfortunately this is not an inexpensive repair either way. The problem will persist until the transmission is fixed or replaced but often you can extend the life of the vehicle by doing the key cycling to temporarily clear the code to allow normal driving at time.


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Customer: replied 4 years ago.
Sorry for my ignorant. If I just decide to change just the bushing, do I still need to overhaul the gearbox?
After replacing the input shaft, do I need to do software settings?
Thanks by the way, you've been very detailed.

Glad to help further.

The gearbox will need to be disassembled to replace the input shaft assembly so it would be highly recommended to have the clutch packs replaced at the same time and an internal check of the other components for any problems. Make sure to way the cost of an over haul VS. a replacement transmission. Many replacement transmissions will have a warranty. BMW will give a two year unlimited mile warranty if purchased from the dealer but the cost may be high compared to an over haul.

I hope this helps :)

Customer: replied 4 years ago.
Thanks for your help!
But could it the speed sensors?
Just an ignorant question.
No problem. Glad to help clarify. P0720 is usually the code for output speed sensors and the fault code description should have related to the speed sensor with that type of problem.
Please let me know if I can help further. Having the fluid changed was a really good idea by the way. Sometimes it can help prolong the transmission life and correct the problem but unfortunately I have often seen the transmission still needing replacement.
Best wishes to you and I am sorry that your BMW has had this type of problem.