CENTRAL LOCKING PUMP - How it works & trouble shooting ?
VACUUM LOCKING PUMP: These semi-older model Mercedes use an electric vacuum pump to centrally lock the car. This pump is located under the rear seat on the passenger side in the w124 and w201 chassis, and in the spare wheel compartment on w126. The early models only had central locking from the drivers door, then from approx 1984 onwards, Mercedes included in the passenger door and trunk lock.
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The vacuum pump has internal electronics and sensors which rarely mal-function, but if there is a problem, you can test it with your MB key before you buy your Aussie keyless entry system.
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There are three locking channels that control the pump, those being drivers and passengers doors and the trunk. (The Aussie Keyless entry sends control signals to the vacuum pump via the trunk channel.)
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Sometimes older pumps will have electronic problems, and if it does, it will isolate the channel that is faulty.. This could cause one of the doors or trunk not to centrally lock the car, while the rest of the doors continue to work. Another possiblily is the central locking goes crazy on occasions, locking and un-locking the car like it has a mind of it's own. This is particuarly bad if your keys are inside the car! (If this happens, just wait a bit, as it will probably un-lock itself within the hour - no kidding!)
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If your vacuum pump is malfunctioning, you will encounter these problems using your MB key, so it is easy to test before you buy an Aussie. Usually dis-connecting and re-connecting the cars battery or fuse will reset the pump, and it may work normally for about half a day, but then it will resort back to it's bad behaviour.
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It is for these reasons that it's a good idea to lock and un-lock your car ONLY from the trunk for a day or two to test there are no problems with the trunk channel, as most owners have rarely used the trunk channel.
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Generally vacuum pumps do not malfunction, but if you are having these problems, you need to find a replacement vacuum pump. Used pumps are approx $25 at wrecking yards.