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I have a VW Golf 5 from 2004. I have bought it about 4 years a go. I have 2 pairs of keys. With the first one, I could never open the car remotely. The second one worked quite well but its range decreased over time. At the moment, I need to stand right next to the car doors in order to open them remotely. I tried replacing the batteries, that never helped. I would like to understand what is the case of such behaviour and fix those keys. This is the PCB from the car key:

1st PCB side

2nd PCB side

battery holder

If I understand correctly, the LED is only an indicator of the button presses and serves only for the user feedback. Please correct me if I am wrong. The U2741B chip on the 2nd picture is an RF transmitter. The big black rectangle next to it its the crystal osc. The M44C890 is the MCU. The top copper trace (looking at the pictures) is the antenna. The bottom copper trace is the positive terminal of the battery.

How can I investigate this? What could be the reason for range loss over time? I would appreciate all help.

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  • \$\begingroup\$ use your watery head to focus the signal for greater range. \$\endgroup\$ – dandavis Sep 17 '17 at 19:45
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There is really nothing you should expect to wear out over time to reduce the range as you reported except for the battery.

Make sure the battery contacts are clean. The ones in the picture look clean, but we're missing half the battery holder.

Measure the battery voltage when the unit is just sitting there and when you make it transmit. You may need a scope to see the battery voltage drop during transmission. If the quiescent voltage is too low, or the dip during transmitting too low, replace the battery again. Maybe you got a bad one.

Also make sure the replacement battery is of the correct type. Simple mechanical fit isn't good enough. Most likely this fob uses a 3 V battery in CR2032 form factor, but that's just a guess on my part. Those batteries are widely available, but also widely counterfeited. Don't buy batteries on Ebay, or anyplace worse, like Alibaba. Batteries are things you need to buy from reputable sources.

Try connecting a 3 V (or whatever your battery voltage is supposed to be) bench supply in place of the battery. If the fob works fine then, the battery or maybe its contacts are the problem.

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  • \$\begingroup\$ I have edited my question- I added battery and the holder picture. I am quite sure the contacts are clean. The battery should be good, it was bought on Digikey I think (or simmilar). I dont have the tools on me now so I will measure the voltage drops tommorow and report back. Thank you for your answer. \$\endgroup\$ – Bremen Sep 17 '17 at 18:05
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I would check the receiver inside the car. The two keys may be a bit different in their transmit power and the receiver was never able to get a signal from the weaker key. Now the receiver could be so corroded it doesn't get a signal even from the better key.

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  • \$\begingroup\$ Thank you for answer. Any idea how to locate the receiver in the car? \$\endgroup\$ – Bremen Sep 17 '17 at 18:14
  • \$\begingroup\$ According to German language forums on the topic, the VW Golf V has the receiver inside the "Komfortsteuergerät" under the dashboard left of the steering wheel shaft. Don't know if there is an external antenna. \$\endgroup\$ – Janka Sep 17 '17 at 19:41
  • \$\begingroup\$ Thank you for answer. I need to find some VW disassembly guide. Do you suggest that corroded antenna inside the car is the case? \$\endgroup\$ – Bremen Sep 18 '17 at 5:35
  • \$\begingroup\$ I expect some wire broken from corrosion and/or vibration. If the receiver has an external antenna I would check that first. \$\endgroup\$ – Janka Sep 18 '17 at 12:04

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