1982 300TD, recently hada new AC compressor put in and recharged (134a, had already been converted, etc..). First thing in the AM, works fine. As day gets hot (when I NEED AC) a loud humming sound comes through the center control panel when i put on the AC. what could this be?
Then found that the car starting overheating while using AC. now i thin kithe AC was on all the time, causing overheating, so I unplugged it until i can get it checked out. weird thing is that even with compressor unplugged, still get loud hum in center dash when i use any climate control buttons.
any ideas?
thanks so much

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1982 300TD, 265k
1984 300D, 282k

Is your fan blowing fine inside?
Does it hum the same no matter which of the AC buttons you push ?
Is your temperature ' wheel ' set to max ?
Does it sound like a relay or a fan hum?
Turn on your defrost and see if that changes the sound.

leathermang, thanks for responding.
answers:
the blower fan (under glove box) seems to work fine.
it does seem to hum the same regardless of which button i push
yes, both with testing heat and air, i had temp wheel to max
i don't know if it's a relay or fan sound. interesting question. i recently had a new glow plug relay put in. i wish i could better describe the sound. it sounds very much like a fan, but i don't think there's a fan anywhere near that location??
it did make the sound with the defrost as well, with temp wheel set all the way to hot.
still unplugged, engine temp still goes up to 105 up any small hill or after driving around for a while. seems related to ambient temp (its in the 90's here in LA).
thanks for any thoughts

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1982 300TD, 265k
1984 300D, 282k

Why did you have the compressor replaced ?
Was anything else done to the car at that time...AC related or not ?
Did this sound appear immediately after that work was done ?

I had taken it to a shop to get the AC checked out (car still new to me) and they pointed out the loud rattle in the compressor and said first off replace compressor because whatever's banging around in there would evenutally break the clutch etc.
the sound in the dash didn't start until a few weeks after the work. in fact, after the relay replacement and new thermostat (done by a different shop).
they must have hooked the AC up wrong too though because when i turned on the heat the other day (trying to cool the overheating engine) it blew cold. that's what made me think the AC was running full-time, taxing the engine and why i disconnected it.

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1982 300TD, 265k
1984 300D, 282k

Reconnect the compressor wire and turn the AC on max cooling and max interior fan ( you would use low fan to check for vent temperature but that is not this test ).
Go and look at the CENTER part of the front of the compressor while engine is running and see if that is turning... then disconnect the wire again and see if that changes.
When they replaced the compressor what else did they do ?

ok, i'll go check that.
they did nothing else when they replaced the compressor besides recharge the freon. i had already had it checked for leaks with a vaccum.
i hadn't found this forum yet and had no idea the system should have been flushed, and the expansion valve and dryer should have been replaced (right?)
oh yeah, i had them prop open the center vents because they weren't working.
while i trouble shott the sound, i am also sitting here wishing i had retrofitted the system back to use R12, since the 134 seems so weak temp wise.

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1982 300TD, 265k
1984 300D, 282k

"they did nothing else when they replaced the compressor besides recharge the freon."

OK,, are you saying that it was vacuumed, then they opened it up (which lost that moisture removing vacuum ) and replaced the compressor ... but they did not vacuum it again before installing the refrigerant ?

"i had already had it checked for leaks with a vaccum."

"i hadn't found this forum yet and had no idea the system should have been flushed, and the expansion valve and dryer should have been replaced (right?)"

That is the normal and usual course of action for those shooting for long life from the repair.

"oh yeah, i had them prop open the center vents because they weren't working."

LOL, Oh yeah, you need to see if how they did that is causing the noise.


"while i trouble shot the sound, i am also sitting here wishing i had retrofitted the system back to use R12, since the 134 seems so weak temp wise."

Even though it will cost some to redo some of the things you have already done you really might be better off in the long run to do that... AND install a filter in the suction line before the compressor.... there are lots of good threads thrashing out all this type of stuff.... and also check out the knowledge section of Aircondition. com

while i wait for the holiday to end so i can talk to the shop about what they did and didn't do when they replaced the compressor, i did the test you recommended.
compressor center front spins when plugged in, doesn't spin when not plugged.
another thought about sound (and yours about propped vents good one too, could be connected) is the blower fan? hard to tell, but maybe its repsonsible for the loud hum. could be something stuck in the blades? i'll open that up next and have a look, even though it mainly comes from center dash.

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1982 300TD, 265k
1984 300D, 282k

Can you find the fuse for the inside blower ?
If you can then pull it when the noise is being produced...

leathermang, if you're still there, sorry i didn't get back. had to take a break in hunting for hum source as i have had major overheating problems to tackle.
can you direct me toward the location of the fuse for the blower fan? thanks. i don't have a manual yet.
i'm also trying to get a minute to take the blower fan out and see if there's something stuck in it.
thanks

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1982 300TD, 265k
1984 300D, 282k

well, weeks later, with my son finally enrolled in daycare, I finally took out the blower fan and found leaves stuck between the blades. cleaning this out and then vacumming out the leaves that I found under the battery tray (i took this all out too), the hum in the dash is gone.
glad it was so simple. my first real DIY job on my first MB. thanks leathermang for the ideas.

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1982 300TD, 265k
1984 300D, 282k