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  #1  
Old 06-03-2002, 12:31 AM
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Location: Denver, Colorado
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AC Problems on 300D 78

well summers around the corner and i want to see if i can get my AC to work just by doing the work myself....thats if i'm smart enough!

anyways got problems with my AC.. it wont give me any cool air whatsoever. I'm kinda confused about the AC system in the first place...

1. When i turn on the AC with dial turn all the way down to the blue and let it sit for about 5 minutes...i get extremely hot air....just like if the heater was on.

2. When the system is in this position, The compressor switch is on "off". When i turn that on while its blowing hot air, the engine doesn't lose RPM's like it should...but instead, it makes the blower motor run a little faster (you can hear the difference in air blowing into the car)

3. I cant see anything through the little glass peep hole in the dryer/reciever canister. My service manual told me to look in there to see if it shows signs of bubbles in which case i'm running low on freon....I dont see anything at all.

anyone know what might be the problem here? where should i check first?

any help is gladly appreciated

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  #2  
Old 06-03-2002, 12:37 AM
Holson Adi's Avatar
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Well, look for the receiver drier or something like that. It's behind the passenger's side headlight. It's that tube that you looked into to see if you had enough freon or not.

There should be a switch next to it. If I'm not wrong, the cables are blue two cables into a 'switch'. Connect both cables and see if the compressor engages. It's pretty obvious when it does..
I think the purpose of it is to make sure that the compressor doesn't break when there isn't enough freon in the system.
That's how my mechanic told me that I should be happy..

There are two of them... one of them just starts the aux. fan. That switch is the freon pressure switch. If you jumped the cables and your aux. fan just starts spinning then try the other one.

sorry it's a lil vague... but hope that helps.
if you still need help I'll go and take a picture and tell you what I mean..tomorrow that is..

BTW, check the aux. fan fuse to make sure that it's not blown... the aux. fan is quite important in summer!

good luck,

*also the problem with the hot air might have something to do with the valves. I am not sure since I don't have a '78 (it has a different ACC system). It might be an expensive fix.. but let's hope not.

cool... my 555'th post.
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  #3  
Old 06-03-2002, 01:12 AM
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Re: AC Problems on 300D 78

Quote:
Originally posted by muybweno
well summers around the corner and i want to see if i can get my AC to work just by doing the work myself....thats if i'm smart enough!

anyways got problems with my AC.. it wont give me any cool air whatsoever. I'm kinda confused about the AC system in the first place...

1. When i turn on the AC with dial turn all the way down to the blue and let it sit for about 5 minutes...i get extremely hot air....just like if the heater was on.

2. When the system is in this position, The compressor switch is on "off". When i turn that on while its blowing hot air, the engine doesn't lose RPM's like it should...but instead, it makes the blower motor run a little faster (you can hear the difference in air blowing into the car)

3. I cant see anything through the little glass peep hole in the dryer/reciever canister. My service manual told me to look in there to see if it shows signs of bubbles in which case i'm running low on freon....I dont see anything at all.

anyone know what might be the problem here? where should i check first?

any help is gladly appreciated
Ok, I'm going to try to remember how this all works..

1. The older 123's have a servo system. There is a sensor in the cabin, the dial, a potentiometer in the servo, and an ambient sensor that is accessed through the fire wal near the wiper motor. You set the dial and it attempts to balance the voltage by changing the cabin temp accordingly. I believe, and I'd check this to make sure, but if the sensor voltage drops, that means the car is cold to the AC system. The result is max heat to "remedy" the situation. I would do a search on this, I found a lot of useful info on the older servo systems on the this board. There, you can find test procedures, etc.. to see if the switch is working right.

2. To really check if the compressor is coming on, try the defrost setting, otherwise, if you for example, have an open in the temp sensor, the car thinks it's way cold and will try like hell to warm it up.

3. Mine was all messed up with gunk etc.. could tell squat through it.

The fact that you have a ton of heat says that the amp is at leat reading something and trying to correct for it. Can't really tell if you have a good compressor or not until you try the defrost where it should come on hot or cold.

Just and aside, but when my 79 300D starting blowing compressors, it required a complete replacement of all the hoses, seals and cores due to the deterioration over time. The resulting debris was clogging the expansion valve and taking out compressors. Went through about 3 of 'em before breaking down and gutting it. In the end though, I had a whole new AC system that worked awesome.

Frank.
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  #4  
Old 06-03-2002, 01:32 AM
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The first thing I'd suspect is a bad SERVO.

Late 70s/early 80s cars like yours (and mine) have the "Chrysler-type", servo-controlled Automatic Climate Control system, and they were notoriously bad. It was Mercedes' first attempt at automatic temperature control, and it was not a good one. The system actually works great when everything is in order, but they fail easily and often.

The servo on your car, I think, is in the front corner of the passenger side of the engine compartment...a black squarish thing with several small coolant hoses, tons of vacuum lines, a little "motor-looking" thing next to it, and assorted other things coming in and out of it.

Check it for signs of cracks and/or coolant leaks...But even if you don't see leaks or cracks, it can still be bad. Since you have hot air coming out, even at the lowest temp. setting, I'd bet yours has likely gone bad. (Often when they go bad, they will take the ACC amp (black plastic square thing, in your dash behind the glove box), and the pushbutton control unit with them, due to electrical overload. You may have other problems too, but that's where I'd start.

My 79 300SD had a bad one when I got the car...I got lucky and found a good used one. The best prices I've found on new ones are around $330-350. OUCH. Luckily my ACC amp and pushbutton unit were OK, even though both show signs of having been pretty hot at one time...so now I have heat, and all the vents work when and how they're supposed to...I still don't have air conditioning, though...I have a leak somewhere.

I know this post sounds very "bad news"/"worst case", but unfortunately the ACC system that our cars have are probably the worst MB has ever offered. Good thing that the rest of the car is so solidly built to make up for it!

Hope this helps,
Mike
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  #5  
Old 06-03-2002, 10:05 PM
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I can't believe I missed the most obvious option, Mike is right, first check the servo. My 79 went south as well. The servo fails in a normally open position which equals full heat.

This AC system is a pain, I don't miss it one bit. Like I said, if you do a search, you'll find plenty of info on where to test with a multi meter to see if various things like the temp sensors are reading right. It doesn't take long at all to check this. If they are working, then the servo or amp are likely bad.

Frank.
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  #6  
Old 06-04-2002, 02:24 AM
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well thanks for the help guys

I went through the papers from my fathers repairs on the car since he is the original owner of the car.....just about 4 years ago he had the AC servo replace (the black squarish trapazoidal looking thing with vacuum and coolant hoses coming out of it) by some dealership here in Colorado...the job cost him $950 including parts and labor so i'm assuming that the AC servo was new not rebuilt or used and is still in good condition since the car was out of commission for about 3 years.

I've also tried to turn on the defroster too as well and both the left and right vents (passenger and driver) come on at full blast with hot air as well as the vents on the dash board, i dont notice any loss in RPM's which signifies the AC Compressor that it turned on. I will look into the glove compartment to find the AC Amp and see if i can test if the Compressor is still in top shape by jumping.

come to think of it, since there are vacuum lines running out of that AC servo...i might just have a vacuum leak somewhere around my car...but not sure where if this may affect the AC system. However, when i turn off the car when the AC is blowing hot A$$ air (heheh sorry) i hear a small but audible whining or buzzing sound comming from the AC push button controller... When i switch the buttons to bi-level it get a little but quieter and when i switch it to defroster it get hella loud whining or buzzing sound. When i switch it to off, i get no more sound......SOUNDS CRAZY!!!! Well anyways i think this may be affecting my door locks too as they take considerable amount of time to lock and unlock when the car has not been runned for 1 week......somewhere i'm loosing my vacuum pressure slowly but dont know where.
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  #7  
Old 06-04-2002, 07:18 AM
jcd jcd is offline
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Here's the cheap fix that was done in my car

Put a large ball bearing, to plug it up, in the hose that feeds coolant to the heater core. It should be the hose on the passenger side, on the firewall side of the servo if it is near the front of the car, or the hose closest to the firewall at the back of the servo is your servo is near the firewall. Obviously make sure the ball bearing is large enough so it won't move around. This will eliminate the coolant flow from the servo, probably stuck in the open/heat mode. Then give the AC a try. The tech that did it was trying to give the car a cheap "permanent" AC mode for my parents who owned the car and lived in FL at the time.

Even though I bought a used servo from MikeMover, which I have not put in, I probably going to put in a gate valve so I can manually switch from heat to cool and avoid the whole ballbearing approach that the tech utilized.

I'll also offer,........again, to fax some ACC troubleshooting documentation that Larry Bible gave that should help you in your troubleshooting efforts. It is very helpful. Just send me a private message with your fax number and I'll shoot it over. (no soft copies)

Good luck,

JCD
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  #8  
Old 06-04-2002, 10:34 AM
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Quote:
Originally posted by muybweno
well thanks for the help guys

I went through the papers from my fathers repairs on the car since he is the original owner of the car.....just about 4 years ago he had the AC servo replace (the black squarish trapazoidal looking thing with vacuum and coolant hoses coming out of it) by some dealership here in Colorado...the job cost him $950 including parts and labor so i'm assuming that the AC servo was new not rebuilt or used and is still in good condition since the car was out of commission for about 3 years.

I've also tried to turn on the defroster too as well and both the left and right vents (passenger and driver) come on at full blast with hot air as well as the vents on the dash board, i dont notice any loss in RPM's which signifies the AC Compressor that it turned on. I will look into the glove compartment to find the AC Amp and see if i can test if the Compressor is still in top shape by jumping.

come to think of it, since there are vacuum lines running out of that AC servo...i might just have a vacuum leak somewhere around my car...but not sure where if this may affect the AC system. However, when i turn off the car when the AC is blowing hot A$$ air (heheh sorry) i hear a small but audible whining or buzzing sound comming from the AC push button controller... When i switch the buttons to bi-level it get a little but quieter and when i switch it to defroster it get hella loud whining or buzzing sound. When i switch it to off, i get no more sound......SOUNDS CRAZY!!!! Well anyways i think this may be affecting my door locks too as they take considerable amount of time to lock and unlock when the car has not been runned for 1 week......somewhere i'm loosing my vacuum pressure slowly but dont know where.
If all your flaps are working your pneumatics are prolly ok. But.... under the battery they often completely deteriorate from the acids. Mine were totaly gone under there. It's a place to start!

Frank.
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  #9  
Old 06-04-2002, 01:40 PM
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JPL JPL is offline
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Mailbox Full

Hey JCD;

I would like a copy of that AC information too, if you are willing. But when I attempted to send you my fax# the mailbox was full. Let me know if you have another email address so I may send you my fax #.

Thanks,
James
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  #10  
Old 06-04-2002, 01:44 PM
jcd jcd is offline
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JPL

Try it again,,,,I just cleaned out my private message folder.

Let me know if it doesn't work.

JCD
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  #11  
Old 06-04-2002, 02:09 PM
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Did it

JCD;

you should get a message soon.

tx,
james

EDIT: Got the fax: Thanks
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85 300SD 285k
Charcoal Gray/Grey MB-Tex

79 300CD 142000mi "Rabenshwarz"
Black / Black MB-Tex, Burlwood Int. TOTALLED - 10/24/02 --

Last edited by JPL; 06-04-2002 at 03:31 PM.
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  #12  
Old 06-04-2002, 03:31 PM
jcd jcd is offline
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Just faxed it to you 3:30 PM EST

Good luck,

Jim

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