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Discussion Starter #1
I have a 2008 850d that is new to me and that I’m restoring. I am stuck on an issue that I cant resolve on my own. The 4x4 switch is not activating the 4x4. Disconnected the plug where wires enter the front diff and tried putting 12 v directly and nothing. Am I missing something here ? Should I be doing something else to allow the device to show signs of life ? I noticed that my front diff differs from what is shown online at JD parts. I probably have an earlier version or something. I found my exact model front diff fitted on the 2008 620i. Thought I give you a try before I tear into this diff to see if I can diagnose the issue. Where is the fuse or relay located for the 4x4 actuator. What do these 2 wires that lead into the front diff connect to. My front diff does not have the actuator attached to the front diff housing like most that I see.
 

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The solenoid should become energized when supplying 12 volts to it. Perhaps use your voltmeter to verify coil resistance and to check for any shorts to the case.

You really need to get the workshop manual for the amount of effort you're putting into this machine. Yes it is somewhat expensive, but it can be as handy as your socket set.
 

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Discussion Starter #3
I broke down and downloaded the manual. I was able to determine and correct me if I’m wrong, this is an electromagnetic engagement that only activates when there is a need or slippage detected from the rear drive. There may not be a actual engagement like sound when 12v is applied directly to the front diff wires.
 

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You might be able to feel something if holding the coil, but it is not a loud clunk or other noise like you favorit 4x4 truck.

The manual contains a solenoid test procedure using an ohm meter. I'd do it just to know the solenoid coil is good.
 

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Discussion Starter #5
I checked a few things with no luck.key on 4x4 switch on, I have 12v going to the connector on the front diff when in gear and I lose it llike I should when I put back it back to neutral. I have 25.7 ohms at the connector which is within specs according to the manual. II took the front diff appart and find no damage. While the cover was off, I put 12 v to the wires that lead to the coil or electromagnetic device and nothing happens, should something happen. Nothing gets magnetized when power is put to it, should it .. I’m stumped please help
 

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The solenoid should become magnetized when electric is supplied and something should move or be allowed to move.
 

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Discussion Starter #7
Would it make sense to you that the electro device ohms are within specs and the coil is burnt just the same
 

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You claimed 12 volts were supplied to coil and there was no magnetic field created. If the coil is good there would be a magnetic field.

Remember back to grade school science class. Wrap some insulated wire around a nail, supply voltage and the nail becomes magnetized.

When supplying power to the solenoid, I'm also assuming you are providing a grounding point.

I believe the the computer provides a ground path when the 4x4 switch is energized if all parameters are met. So it could have 12 volt supply and not work because there is no ground.

Suggest you double check your results.

Did you ever buy the manual? Typically there is a section describing the theory of operation, that might help your cause.
 

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Discussion Starter #9
Thanks 200mph. I’ll double check again in the morning. The way I checked for magnetization was + to the tan wire and - to the black wire. I also grounded the case cover from same 12 v source simulating the case being bolted to the bike’s ground. Good chance that I didn’t verify for magnetization in the right spot. I was concentrating on the thrust plate being the energized material. I’ve learned since this afternoon that the actual magnetized location is inside the cover around the center hole. I did get the manual and it tries to explain the function of the wiring and how it all works but it falls short in the repair area if your wiring checks out
 

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Good luck and let us know the results.

Sorry for not mentioning where to check for magnetism earlier.

If you have 12V to coil, the coil is not functioning (becomes magnetic) and you feel confident the coil windings are good. Suggest you check ground path with 4x4 switch on and trans in gear. On the 855D these two conditions are needed for the VCU to switch the ground. Your 850D might be different on what conditions are needed to be met for 4x4 engagement.
 

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Discussion Starter #11
thanks. Double checking just now and it has magnetization as it should. Digging deeper I think I located the problem. One of the springs holding a roller was broke. There’s a good chance it lodged the cage preventing it from engaging the 4x4. See pics .. if anyone out there has a spare roller spring. I’d gladly discuss shipping and cost. Dealer is closed today but something tells me I’ll have to buy the whole cage just to get the spring.
0895F39F-6936-4F69-9B6A-260B9355C6BC.jpeg
2D08D088-F61E-464F-86E3-17691102A291.jpeg
 

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Glad you found the problem!!!
 
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