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Discussion Starter #1
Hi all !
I'm the owner of a nice T4 syncro 2.5 TDI (ACV) from 1999. Recently, I got stuck in some mud: both front wheels in the mud, both read wheels on the road. So I was expecting the viscous coupling system to do its job and transfer full engine power to the rear wheels, while driving backwards...
But, for some reason, using the rear gear, the read wheels did not rotate at all ! I was a bit surprised, got scared that my transmission had a problem...
After a bit of research, I found several videos on Youtube showing viscous coupling tests, and apparently, the viscous coupler does not transfer power in the gear rear...
So, to this day I still don't know if this is a normal behaviour or not ! What could explain that ? I thought that the viscous coupler would engage in both directions...
So question 1: Is this normal, or is something broken ? And question 2: if this is normal, can anybody explain me why ? Would that by because the disks in the viscous coupling system can only engage the rear differential in one direction ? Or maybe it comes from the differential itself ?

Thank you very much for any suggestions !
Best regards,
Julian.
 

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Hi does the rear drive OK going forward if it does there’s no problem with the viscous coupling, if you have ABS it disconnects the rear diff when you press the brake, if you don’t have rear diff lock there’s one actuator on the rear if you have ABS & diff lock there’s two actuators, unplugging this reverse light switch on the gearbox should stop it disconnecting in reverse, don’t mess with the ABS
 

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Discussion Starter #3
Hi ! Thank you for your answer ! So, I read a lot on the topic, and I have now a good understanding of that freewheel control. What happened in my case is that the freewheel was not locking when engaging the rear gear. I discovered that the freewheel relay was actually not present in the van ! Instead, there was an empty space for a relay ! So I ordered one on ebay, and well, now it works perfectly ! So finally I have 4x4 working in any direction !

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