Like many Discovery 3 owners, I’ve experienced the intermittent failure of the car to lock or unlock using the key fob. This issue had been driving me mad for a couple of years. One minute the key would work, and two minutes later it wouldn't. Often, tapping the key fob on something would get it working again.

I had previously renewed the key fob cases and, like some others, put some packing behind the battery. That seemed to work for a while, but not forever. Both key fobs had become increasingly unreliable. Finally, after trying several suggestions from different forums, I found the problem!

I thought there might be some dry joints on the circuit boards inside the key fobs. Fortunately, the fob cases I have now snap together and use a locking screw, so I was able to take them apart quite easily. I re-soldered a couple of joints that looked suspect on the surface-mounted micro switches, but that didn't help. I also made sure the batteries in both key fobs were at the correct voltage using a multimeter.

I was examining the battery terminal tabs that come through the circuit board because someone mentioned re-flowing the solder on these had fixed the problem for them. The joints looked good, but then I noticed I could move the battery slightly away from the circuit board. On further inspection with a magnifier, I discovered that the negative battery tab was broken BETWEEN the battery and the circuit board – it was the same on BOTH key fobs!

This explains why putting packing behind the battery in the key fob can sometimes be a temporary fix. The same goes for tapping the fob on something; the vibration moves the battery enough for the broken halves of the battery tab to make contact.

I’m guessing the metal negative tab (which is much narrower than the positive tab) probably suffers a stress fracture due to rough handling of the keys over a long period.

Although re-flowing the solder with the battery in situ may have worked, I knew there would be no way of ensuring a really solid solder joint between the two halves of the battery tab. It would also be a rigid joint prone to a stress fracture again in the future.

After de-soldering the positive battery tab, I was able to remove the battery in its plastic holder from the circuit board on both key fobs. My solution was to hard-wire the negative connection using flexible multi-strand wire soldered to the broken tab on the battery and the negative terminal on the circuit board. I felt this would be more resilient than a rigid connection.

Both key fobs now work faultlessly. See the attached photos.

If you are doing this job, I suggest you use a very fine soldering iron and fine solder to minimize the risk of damaging the circuit board or the battery. I used a miniature soldering iron with a 1.25mm tip and 0.5mm solder. You'll also need a solder sucker, some suitable multi-strand wire, and new key fob cases if you haven't already replaced the original ones with fob cases that can be re-opened.

In case anyone isn’t aware, the battery in the key fob is rechargeable and is charged wirelessly via an induction coil behind the ignition lock. Judging by my own experience, the batteries last a long time. Mine are 17 years old and still hold a charge for ages - my Disco hasn’t been driven for two years, so the key fob batteries haven’t been charged in that time, and they still work.

One other thing I should mention. My original key fobs could not be prised open - they appeared to be a one piece sealed moulding. I had to cut all around each fob case with a small hacksaw blade to get them open. If your key fob can be prised open because it is a two part case which has been glued, then it's likely that the fob case has already been replaced at least once.

I hope this helps someone!
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Replacement (non original) key fob case.
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broken battery terminal/tab.
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Tools I used (pencil to show size)

 

Thanks for the pictures, helped me confirm the only key fob that came with this LR3 is actually a genuine board. Battery terminal solder joints look awful and show 3.2volts but maybe replacing the battery is necessary if it really is 19 years old (it's a Panasonic rechargeable)

 

Mine aren't much younger. 17+ years. If it's supplying 3.2v it's fine. If the battery terminal solder joints look bad, perhaps someone has had a go at re-flowing the joints, or the battery has been replaced previously.

Did you have to cut the fob case to open it up? That's the sign of an original fob case, because they were a molded plastic case. There's no way of opening the original cases without effectively destroying the case.

Is the remote central locking not working?

 

The circuit board and key had been transferred to a generic case already.
Turned out being corroded solder joint in wire loom at passenger door (SJ50 I believe). Re spliced the wires feeding the cars remote receiver and it has been working flawlessly