BMW 730d with Transmission problems ?

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BMW 730d with Transmission problems ?

Postby Andy-Cape » Wed Oct 31, 2012 6:33 pm

we solved a electrical problem of a 2006 BMW E65 730d with the help of pico scope in a very convenient way. The car shut down intermittend with the following fault "Transmission Failure-please visit workshop". The car could be started after a short cool down period until same event happend.
The diagnostics scan brought up DTC for over voltage of the EGS (transmission control unit ) supply and system voltage to high or too low. That was already confusing - too high or too low? We checked the battery status and found undervoltage of 11.6 V and it was still the first battery since 2006.
After replacing the battery the customer did not come back for a week but now the fault became more frequently. We were afraid to guess what it can be - like a expensive control unit or lowering the gearbox to inspect the wiring plug? Because the gearbox got recently an overhaul. we decided to connect the scope to the fuse of the transmission control unit and also a 2nd Channel to the battery. We also connected a 20A current clamp to the transmission supply to monitor the module demand for power.
We wanted to know if we get voltage spikes or what ever we can find during drive.
After a short drive the voltage started to be rocky and above 17 Volts the transmission failure showed up on the dash.
I told the driver to accelerate / decellarate and at any sudden acceleration we got the over voltage. We thought it must be the voltage regulator of the alternator because the transmission unit did not demand any current change - and it was the regulator.
we did a straight forward repair and could proof success later also when we drove the car again with picoscope. The voltage stayed in its design parameters not under 13 V and not over 15 V.
See scope data attached.



andy from Cape Auto Diagnostics
before repair.psdata
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Re: BMW 730d with Transmission problems ?

Postby Alan » Wed Oct 31, 2012 7:26 pm

Great example of what we call "reflected codes". Sometimes a fault code points you at one component when the fault really lies elsewhere.

I hope you dont mind, but I added graphics of your waveform to your post.
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Re: BMW 730d with Transmission problems ?

Postby Robski » Thu Nov 01, 2012 7:39 am


what scan tool did you use & did any other modules tell tails of voltage issues ?

Did you run any other tests on the battery/charging system ?
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Re: BMW 730d with Transmission problems ?

Postby AuthorMike » Thu Nov 01, 2012 9:16 am

Nice one Andy! I hope the new premises is going well and you're all settled in....

....keep them Case Studies coming ;) :D
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