Thread: 1981 300sd
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Old 04-26-2003, 10:27 AM
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Thomaspin Thomaspin is offline
Join Date: Jun 2001
Location: CA
Posts: 531
The driver's door has a mechanical switch.... vacuum, in your car. Operating the locking knob or key moves a rod in the actuator sending 12 volts down the yellow or blue wire to the pump in the trunk. The source wire in the door is red/black, 12v at all times. Thus the acutator has a three pole connector. 12v in at all times, 12v out on one wire or the other.

When the pump is activated it either suck or blows, closing/opening the other 3 doors, the gas flap and trunk lock.

To check the pump, remove the spare tire, undo the two 8mm bolts holding the metal cover (RR corner of trunk) and remove the pump from its protective rubber housing. It has a 3 wire electrical connector (the blue/yellow pair referred to above) and a brown ground. Unplug the connector (pull aside the semicircular plastic 'lock' to do this) and pull off the yellow vacuum hose. Apply 12v volts between the ground pin and the others in turn. As the pump sucks/blows, cover the air orifice with your finger. It should stop. It ceases to suck and blow at 450mBar - that's how it knows the doors are locked/unlocked. If it fails that test get a replacement from a wrecker. Mine ran $85 in Silicon Valley - anywhere else would be cheaper.

If it passes that test, place a vac gauge (Mityvac or similar) on the vac tube in the trunk and see if all the doors lock (driver's door s/b locked when you do this with the key) when you evacuate. If they do not you have a leaking vac diaphragm in one of the passenger doors, the fuel flap or trunk lock. Replace it. Trunk/gas flap diaphragms differ from those in the passenger doors.

If that test holds vac, take off the driver's door panel and pull the actuator. Measure resistance across the common pin (red/black wire) and the others in turn as you pull up/push down on the actuator shaft. It should go to 0 ohms at either setting in turn. if it fails that test, replace it witha new actuator ($25), p/n 002 820 55 10. A used one is a false economy. Note the p/n - later ones are different as they use vac to the driver's door. that's NOT the one you want.

Driver's door panel removal and actuator R&R are illustrated on my site.

Good luck!
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