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Remote locking losing sync

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Apologies I know this has been covered many times before, but I just can’t see any threads on it now, can you search just this E39 section of the forum?


We have been using the keys in the doors for good few years now so this is hardly a pressing issue, but it’s one of the last few things not working & would be nice to get fixed. 


I changed the key batteries & have done that key re-sync process a few times, before & after changing the key batteries. It works for a little while but loses sync again before long. Should have realised it wasn’t working before cutting open the second key really. I found just after cutting open the second key you can actually get a read out of the key battery voltage somewhere, I think actually on the car dash, though it may have been in INPA I can’t remember. 

I tested the second key battery with a multi meter after cutting it open to find it was a healthy 3v, but changed the battery anyway as it was in bits. Now have two keys held together with tape for no good reason! (I have tried all kinds of different glue but one drop on our tiled floor at home & the glue comes apart, any how that’s not my main issue.)


I found the place to test the output of the receiver in the boot area & it shows the window antenna is receiving the signal from the key fine. So I assume it’s the next box in the chain that’s at fault?

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Sorry I probably should have started my last post saying I have the diamond keys, the RF ones with the battery sealed inside, not the infrared ones. I think they have both easily replaceable batteries & a red light somewhere that lights up possibility on the cars rear view mirror, mine doesn’t. I had to cut my keys apart to get at the battery. 


It seems Google is much better at searching this E39 section of the forum than its own search, but I still couldn’t find anything over what I’d already tried. (My fuel filler cap had already locked fine) 


So I thought I’d start again. However this time the key re-sync button press procedure didn’t work. 


So I pulled the cover off the rear N/S pillar (in the touring) to get to the diversity receiver. There are a few connections on it mostly larger ones for the radio but the radio aerial in the window is also the aerial for the key so that big long white box (the diversity receiver) also has a smaller black rectangular plug, that has two wires on it. You can get a multi meter probe into the connections fine. The ground is from the screws that hold it to the car. One of the wires is the 12v feed, the other is the signal back. The signal wire is always at about 11v, with the voltage dropping to about 9v when you press any of the buttons on the key - if everything is working ok. 

However since I last tried this I found I was only getting a voltage change from one button on the old key & nothing from the newer key. 


It turned out the pin from the battery through the PCB inside the new key had snapped & wasn’t connecting. A bit of re-soldering & it’s now working again. The older key has very dodgy switches on the PCB, so I didn’t bother trying to sync that one this time. 


So hopefully the issue was actually two dodgy keys, time will tell! 

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