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I thought the same. I thought it was P400 vs P500 related, I thought it was worse in 5th and 7th gears, I thought starting the car and having to reverse while cold may cause it. Fact is, that wiring probably caused all of it. Yours may move when shifting hard on P500, causing you to think they're related.
I replaced every mechanical component including the whole actuator tray, pump, all of it. It was the wiring the whole time.
Not sure if there's a way to check the wires while the transmission is in but worth checking. My bet is they're shot.
Good afternoon,

Son of krypton - Your responses were appreciated. I still have the M5. Sat in the driveway for a few months and then took it to an indie who is a master mechanic because this car is man vs computer and computer is winning. He ran codes and 5506 came up again. Along with ibs. He asked to bleed the unit again to eliminate that being the issue. He bled it and retaught clutch and it ran for 5 days before red cog and stuck in 1st gear again. He tried running the sequence and it timed out in fifth gear numerous times. He believes like you said it is the wiring so he wants to drop the transmission. He said he ordered the wiring from Germany it will be here in four weeks. He is only charging me 2g to do the job. He seems legit but I have two questions. I don't have ista and my foxwell nt530 sucks. Do you think I should replace 1 or both battery cables? Should I buy a used smg ecu off ebay that has a "working" history? I don't have a lot of money and am thinking if the ibs needs replaced might as well do it anyway. Thanks for your time.
 

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Hi mate,

Short answer is that the gearbox should come out. I'd hold off on ordering any new wiring until you get it out either way. I do suggest that you replace the SMG ECU as that simple (and easy) step allowed me to verify that the old one was faulty and that it also allowed us to get past the original code. The second issue that we found having done so was the actuator wiring connecting the shift position sensors to the actuators was pure and utter rubbish from factory. A little digging found that this is a common fault in certain build runs. The shielding for the wires inside the loom there are rubbish and fail completely, leaving a wrapped bundle of exposed trigger / signal wires touching each other (see pic). You can't buy a replacement for this as the whole actuator tray comes as a one piece and the wires and hall effect sensors are matched to the whole assembly (thanks BMW!) The only way to rectify this screw up is to cut and re-sleeve (heat wrap) every individual wire. There are a few really good threads on here about how and why to do this but if yours is like mine was, this is absolutely necessary.

Let us know what you find and how you get on!
940416
 
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