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A smart entry system is an electronic lock that controls access to a building or vehicle without using a mechanical key.

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Repairing a damaged car remote

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Hey everyone.

So what I’ve got here is something a friend of mine handed to me to fix since it stopped working when she tried to replace the battery, and isn’t prepared to pay the outrageous price it costs to get a replacement one. I’m not sure exactly what the car is (probably Mitsubishi based on the logo). Nor do I even know the model of the remote (please check the pictures), but JX331BA seems like a half arsed guess based on a Google search. I also tried battery testing both battery cell coins however I believe they are both fine.

I think the problem is the battery connectors, but I don’t know exactly how the contacts should be made to fit so I’m first hoping if someone can track down a picture of it.

Also, advice on the smartest way to repair it.

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Is there anyone in the Wilmington, NC area that repairs such key fobs? Thank you.

Peter Basil



Is there anyone anywhere in or around the Wilmington, NC area that can repair my 1998 MB S class key fob? Batteries have been replaced twice with no results. Something has failed in the electronic key board.

Thank you for any help.




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Are there two batteries in the key fob?

Were they sitting on top of each other e.g, -ve face down on both to give 6V between the connectors?

If there is only one battery in the key fob as is usual with key fobs, perhaps the easiest way to repair it is to unsolder and remove the old battery connectors from the pcb and clean up the board terminals and then solder a CR2032 battery with leads - example only (as long as it fits in the case OK that is)

You can also try this option and connect it to the key fob to test if it still works or not, before getting a replacement CR2032 battery with leads(I don’t know if it will physically fit into the case but you can use it to test if the key still works ;-)

Unfortunately it is not shown on the board which are the +ve and -ve battery terminal connections on the board, but I’m thinking that there should be reverse battery protection built into the circuit to allow for when people re insert a new battery back in, in the incorrect way and it doesn’t work and they have to reverse the battery to make it work.

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I put the cell battery between both contact points to see if the thing worked, and it did when I pressed one of the buttons and the LED lit up.

I then went and tried reconnecting the old metal parts (as pictured, but soldered together), and the multi-meter shows that the 3V is active on the line. But when pressing any of the buttons the LED doesn't light up.

Perhaps a short that the multi-meter can't detect near the battery?


Hi @jay_jwlh ,

Just verifying that you connected the battery between the terminals on the pcb and it worked but when you connected the battery to the repaired connectors that are attached to the pcb it didn't work even though you measured 3V on the pcb terminals, is this correct?

Two things come to mind:

1. Was the battery connected the correct way around, i.e. +ve of battery going to the same terminal on the pcb as when you connected the battery directly to the pcb? Sorry but I have to ask ;-)

2. Have you measured the resistance of each of the repaired battery connectors between their connection point on the pcb and the end of the connector which contacts the battery?

It may be that you measured 3V OK on the pcb terminals but this may be just the "open circuit voltage" as there may be no circuit until a button is pressed, to help prolong the life of the battery

If there is a resistive connection in one of the repaired connectors then when a button is pressed and current flows there will be a voltage drop across it and the resultant voltage available to the circuit will be less and the circuit may not operate.

Just some thoughts.


Turns out my multi meter red/black was backwards, so fixed that. The +ve side of the battery used on red now results in a positive 3V.

As you can see from the two added pictures at the bottom of the original posting, I have soldered a new battery in place.

A few checks:

Battery voltage is 2.71V (when wired to the board)

Voltage is 2.88V from under the circuit board

Resistance is negligible

Voltage is -0.74V when battery is reversed


Hi @jay_jwlh ,

So it works when the battery is directly connected but not with the repaired connectors?

if this is the case you're still looking for a battery holder that the customer can use rather than them having to solder in a new "battery with leads" everytime, right?

If you can't find a suitable replacement battery holder, perhaps you can "scavenge" a holder from the 'option" above (remove from case if whole unit doesn't fit into key fob) and double side tape it into the key fob


Sorry I neglected to mention. Despite the connection being fine, the remote still doesn't work.


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If the central locking stopped working after she replaced the battery it is possible you may have to resynch the key to the car as the radio frequency may have reset. Follow the below steps or look up resynch my key in the owners handbook :-

1- Insert key into ignition

2- Turn key to position 2 (radio on engine off)

3- Press and hold any button for a slow 5 count

4- Remove key from ignition and try buttons

PS DONT go to a dealership if you do end up replacing the key.  Other businesses will charge upto 70% less.

Good Luck

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Jason sarà eternamente grato.
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