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So, I Have owned my 1985 300d turbo for about a month now. About 2 weeks ago the drivers side rear window would not roll down but then days later it did and seemed fine. Just yesterday, I opened my trunk and the horn started beeping like crazy. Once it stopped I went about my business until I had to open it again upon my return home. It started beeping again so I locked and unlocked the car which seemed to make it stop. Today I opened it again and there was the beeping. I went to try the locking/unlocking thing again and the passenger side front lock was slow to respond and the two back locks hardly responded. Is this an indication that my vacuum pump is going?
 

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E320/E250 Bluetec Ford F350 6.7l
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You have several elements that might age on 35 yo vehicle.
Vacuum pump has rubber diaphragm that should be replaced every 10-20 years, but first to go are the elements inside the door that pop the pegs.
Listen for a hiss, or observe which door open last.
 

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1982 240D M/T 209,200 miles
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Nate77,
The windows I believe are electric, not vacuum driven. As for the horn beeping when you open the trunk, was a burglar alarm installed on you car? My 1982 240D had an alarm installed by the previous owner. I have a key fob, do you?
The slow lock/unlock response may be due to vacuum leaks, which would deplete the vacuum reservoir as the car remains idle. If you car was running at the time then it could be something with the vacuum supply to the locks, or lock system leaks.
If your car is shutting down fine, I would think you have adequate vacuum from the pump system to actuate the vacuum shut off valve, but something is wrong with the locking system.
Your car being a 1985 300d Turbo, I’m guessing you have a piston style vacuum pump, not the diaphragm type used in the older W123’s. Rebuild kits are hard to come by if the prognosis is the vacuum pump. Best choice would be to purchase a new pump. I just replace mine to resolve my vacuum issues and all is well. Would have like to repair the original, but it was more cost effective to just replace.
Welcome to the Mercedes Benz W123 owner world. Get ready to be your own mechanic as a reliable W123 mechanic is almost non-existent.
Here are some links to troubleshoot your vacuum system and lock system. I hope this will be of help to you.

EnjoY240D


 

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"Is my vacuum pump faulty?"
No way I could know without testing it. If it passed testing as operating properly and one was still interested in the vehicle's vacuum system, then one could methodically test it to find a fault(s).
 
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