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kweimer 04-25-2003 11:21 PM

1981 300sd
Up until this past Saturday I would not even consider buying a Mercedes later than 1976; however, at an impound auction I started the bidding process on a 1981 300SD and ended up with "the buy of a lifetime". I am in the process of repairing obvious things and need to fix the glow-plug system before I can start it---related to the glow plug system is fuse #15 which blows as soon as I connect the battery---I will have to track this down ; however, one item that immediately grabbed my attention was this: As soon as I connect the battery, the locking system begins a lock-unlock cycle and will continue this cycle until I disconnect the battery. I thought the locking system was vacuum operated---and this car has not run in a long time, so there should be no vacuum built up in the system. Can any 300SD owners clue me in as to what is happening here?

billrok 04-25-2003 11:53 PM

The 83 SD has an electric auxillary vacuum pump located in the spare tire well wedged in next to the spare. It's sole purpose in life is to come to life when you actuate either the front doors or the trunk which will lock or unlock all the doors. Disconnect the power lead to the pump and good luck tracking down what sounds like at least 2 electrical problems. Hopefully niether are related.

sixto 04-26-2003 02:09 AM

Covered this one recently:

The original poster never followed up so I don't know what became of the problem.

91 300SE
87 300SDL
83 300SD

kweimer 04-26-2003 07:00 AM

1981 300SD
Thanks---that also explains the "noise" I hear coming from the trunk area.

kweimer 04-26-2003 07:10 AM

1981 300SD
The thread that you give at least gives me a better idea how the locking system works---just about impossible to fix a problem if you don't know how a system is supposed to work.

engatwork 04-26-2003 07:51 AM

KW in a 1983 SD - I would have never dreamed it:). It had to been a buy of the century. Congratulations on your new "find".

Thomaspin 04-26-2003 10:27 AM

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!

kweimer 04-26-2003 01:22 PM

Thanks for an excellent rundown on the locking system---finding the location of the vacuum pump was my next concern. I will be going to your website to learn more about the 126 chassis. I believe I could disassemble and reassemble the 114-115 chassis MB's blindfolded---just take a little longer---but the 126 chassis is a whole new ballgame for me. I'm learning many new things every day and will probably learn most from this ShopForum site.

kweimer 04-26-2003 01:28 PM

The auctioneer couldn't believe I bid on anything later than a 1976 either---my comment to him was "I hope this is not the biggest mistake of my life". All the impound yards and auctioneers know that I say "anything later than 1976 is too new". By the way, make that a 1981 300SD (198,050 miles), although 1983 is essentially the same car.

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