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Some technical questions about my new 280SE: A/C belt tensioner and vacuum

Started by LazyLightning, 13 September 2023, 02:15 AM


Hi All,

I recently picked up a lovely (IMHO) 1977 280SE:

I have a couple of issues I want to sort out:

1) When I turn the A/C on, the compressor belt squeals, almost certainly due to the belt being too loose (it is obvious there is too much play).  But: the tension adjuster is maxed out (screw all the way in), and the tensioner even looks bent in a bit.  Can the tension adjuster mechanism alone be replaced, and would anyone have a part number?  I tried taking a few pics (attached).

2) There is a vacuum leak, based on the fact that the locks don't work after the car has been sitting for a couple of hours.  No other vacuum-related symptoms to report.

Does anyone have a recommendation for a shop in the Bay Area that works on old Benzes like 116's?  Preferably in Marin or SF?  I'm finding many reputable shops (e.g. Collie Autoworks in Marin) no longer work on pre-1980 Benzes!

Thanks for your input!
'74 Alfa Spider
'77 MB 280SE
'81 BMW 528i
'01 BMW 740i M Sport
'04 Land Rover Discovery
'05 Porsche 911 Carrera S


I'd check out Kent Burgsma on Mercedessource on YouTube for tips on the vacuum leaks.  My central locking lasts for about 20mins so enough time to lock the car but not enough to reopen her for all the passengers.  Ive done some chasing of the lines for obvious leaks without luck.  The vacuum tank is under the rear parcel shelf.  If you are getting 2 hours, its actually pretty good.  My tensioner also looks bent but otherwise no squeals.  I do have a noisy ac clutch and the system runs quieter with the ac on
Never stop loving
1980 280SEL


The primary suspects for vacuum leaks are the distributor cylinder in the driver's door and the rubber diaphragms of the actuator units in each door, trunk and fuel cap lock. The cylinder is still available from many sources, no idea about the rubbers. The german fan association VdH had repros done years ago, but I guess only for members, and sold out.

     Good luck     Bernd


1976 Merc 6.9   Magnetit blue


'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'21 E450 Cabrio


Nice looking car

AC belt. Been there.
Tensioner at max + belt still loose = belt size too long.
Undo nut behind to allow free displacement of the tensioner/pulley
Move assembly all the way down (towards compressor).
Remove belt.
Read length marking (960mm from memory, but I might be wrong).
Using a string simulate belt assembly.
Remove string. Measure length.
That gives you a ballpark of its length.
Buy belt of next length available (longer) to allow for installing over the pulley.
Pull tensioner back out.
Problem solved.

Good luck


Try replacing the black/blue check valve on the vac lines in the engine compartment; one of the only things I fixed on my vac system that actually made it better. I used to just have lock issues and that made it remarkably better.
1979 300 SD "Liesel", 282k Miles


Thanks everyone for your invaluable info - this is super helpful!  I think I have some good leads to take a crack at both problems.  I'll report back soon...
'74 Alfa Spider
'77 MB 280SE
'81 BMW 528i
'01 BMW 740i M Sport
'04 Land Rover Discovery
'05 Porsche 911 Carrera S


Welcome, and congratulations on your purchase.  It looks like a very nice car for a reasonable price.  Original overall, but with some practical updates: it appears the vacuum climate control regulator has been replaced with an electronic one and the AC has been converted to R-134a.

I've just managed to put a 1977 280SE that I picked up for parts car money a year ago back on the road after 10 years of neglect.  It's not nearly as nice as yours, but it's also a one-owner California car, complete with original manuals and blue license plates.  Happy to exchange notes and experiences any time.

I'm curious about the long snorkel attached the the air cleaner, which my car doesn't have.  Where is the far end attached to?

1977 280SE
1983 300D (sold)
1988 BMW 535is
1999 E320T (sold)
2009 E350T (wife's)