Is key charger working?

[jac]

Hi,

Is there a way to check if the charger for the key's battery is working?

My key is working sporadically, sometimes the car will start but sometimes it won't.

I have already replaced the battery in it. When I first replaced the battery it worked without issue for a couple of months, but it's now having the same symptoms that drove me to replace the battery, which is that the car won't start at times.

Thx

[Qsilver7]

The key inside the battery has nothing to do with its ability to START the car. EWS is the immobilizing system...and it is self powered thru induction. A signal from the EWS control module is sent to the Ring antenna that powers the EWS transponder inside the key.


The battery inside the key is utilized to power the two other features/systems that the remote key has...and that is FZV (remote central locking) and DWA (anti-theft alarm siren system).


The battery inside the key powers the transmission of a 315 MHz radio frequency (in North America) or 433 MHz in Europe) that is picked up by an aerial embedded in the rear left cargo window (on SAVs & Tourings) or the rear window on sedans. That signal is then sent to amps in the C or D pillars (on sedans & Tourings)...or top section of the tailgate on SAVs...then onto the GM where commands are sent to lock/unlock & arm/disarm the vehicle. Don't confuse lock/unlock & arm/disarm (FZV & DWA) functions with EWS. All 3 systems are independent of each other but do work in conjunction with each other.

So if you are having issues starting the vehicle...it has NOTHING to do with the battery inside the key. You may either have an EWS issue or signs of a failing electronic ignition switch.

[andrewwynn]

I've worked on a few of these keys and though one battery was a little weak after 17 years every battery was still miraculously working. 100% of the keys I've taken apart to determine if the battery was ok had bad solder joints. Reflowing the joints got the remote functions working again.

If you are having intermittent start issues that usually ends up being the receiving antenna in the steering column or the start switch. That said the initial isolation test is to use a different key and determine if only one key has the problem.

If any key has the problem it's in the steering column if only one key has the problem it's almost certainly a bad solder joint in the internal key antenna circuit as mentioned above it's an inductive circuit that is self powered from the antenna in the steering column. The same circuit that also charges the battery.

I noticed last night that when I used my NT510 to force my wife's X5 to sleep mode I put the key near the key slot and It woke up the car. Had to do within the first minute because when put to sleep the car goes to sleep in stages apparently. You might be able to get a sense of where the problem is if you can do the same thing

[jac]

Thanks @QSilver7 & @Andrewwynn!

The other key works just fine so far. So I will check the solder joints throughout the board in the failing key.

Based on the above then, it is a coincidence that replacing the battery in the failing key made it work for some time. :-)

[andrewwynn]

What I discovered add I worked on my keys was that manipulation of the PC board would twist things just enough to make the bad solder joints make connection and get the key to work a few times. Once I reflowed she joints both keys came back to life. I didn't have a start problem just remote didn't work.

[oldskewel]

The fact that the second key works just fine definitely rules out a lot of stuff.

I agree with andewwynn that it's not just coincidence - most likely the opening up, etc. restored whatever flaky connections are in there, and now they've opened up again.

I would measure the battery voltage when you first open it up. If it's good, you know it is not the battery that is the problem. If it's low, then you need to keep that as an option, although it would point to the charging circuit in the key fob as where the problem is.

Also, I don't have this yet (one day ...), but if you just want to take care of the key fob charging with a different method, this looks good to me:
https://xoutpost.com/bmw-sav-forums/...w-key-fob.html

[andrewwynn]

When I repaired my keys I took photos.

https://www.icloud.com/sharedalbum/#B0S5kuVsbYSr6d

Take note of the pictures with the safety pins. If you poke through the rubber joint exactly where shown you will hit the positive and negative contacts and can measure the current voltage on the battery.

The battery circuit is separate from the ews circuit something BMW did well