Central locking pump + motor

[Nime]

Hi all, I'm a fresh new user and got an A Class W168 which one had problem with the vacuum pump.

I examined the pump, the motor has gone (burnt)...
I just replaced it with a new similar one (choose the same ohm value,
mine was 1.2 ohm) and it worked as well. Then it has gone again.
I bought another motor for 5$, but this time I noticed the motor
produces heat. Why? Coz it runs for about 30 seconds and
in a closed case, it burns itself.

I think I got a mechanic/electronic problem with the vacuum pump' circuit.
5 seconds is quite enough time to un/lock the doors, however pump's
interior phenomatic switch doesn't sense?? the doors locked and continue
to pump air, until main unit cuts the electricity.

For the people who have problem with vacuum pump motor;
yes, you can replace the motor with an ordinary one, the only thing to do,
you should trim end point of the rod/axle to recap miniature plastic guide.

[lofty]

Sound as though you have a leak on the air vac system, the pump is continuing to run to achieve locking /unlocking if you leave the hoses off the pump and block the inlet out let manifold pipes does the pump still continue to run or stop, in other words try and create a test circuit,

my page 41 has a layout of the various connections on the locking system, but it isn't goint to be easy to trace the leak. http://www.aclassinfo.co.uk/mypage.41.htm

[Nime]

I tried to test pump at home with a 12V dc source, however I couldn't start it, there are 3 wires, positive, ground and say pulse the last one... I think pulse wire says the direction (suck/pump). I couldnt produce the pulse signal and it didn't run at home.

Next time in the car, I'll close the air output holes and see what will happen...

By the way nice to meet you Lofty, I follow your site and no doubt it's great! Last week I have had another problem, I lost the "heater". It blows at the max position but there is no heat. Went to a garage but it has worked again there. When came back, it has gone again. I couldnt wipe the mist away from windscreen last week then I bought a liquid thingy. What you think?

[lofty]

Take a look at mypage.61 it could well be the resister, but would have thought that would only have effected the actual blower motor speed, I'm not sure if it would sort the loss of heat! In an old banger where the heater didn't work effectivly we would have said check the thermostat, but on this car that's not straight forward.
The resister is easy to fit but difficult to get at. it's a fiddle more than difficult.

Read the story it's not so difficult as to require the complete dash to be removed as one MB branch told an owner, quoting hundreds of pounds to do so. he like you asked the question and then did it himself in minutes, disgusted at what MB had tried to extort from him.
Remember if the information helps or not we would really appreciate feed back.
Bert Rowe's-A-class info. Heater Blower/Fan Units.