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  #1  
Old 04-22-2009, 12:05 PM
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No voltage to AC compressor, what now? (AKA "I'm melting!!!")

When I bought my first MB a few months ago ('83 300SD) my wife dubbed her "The Phoenix", because she is gold with a tan interior, chrome wheels, and lots of chrome trim. She says we look like a retired couple from Phoenix driving around in it, and that my constant tinkering is like trying raise the proverbial bird from the ashes.

Anyhow... when I got it the AC didn't work and there was constant warm air coming out of the vents, but it wasn't a problem in February. Now that it's April but it feels like August and every shirt I wear has to be peeled off the MBtex like saran wrap, I think I better do something about it. The PO said that all it needed was the dryer unit to the left of the radiator, so that was the first thing I replaced. No go. Next up was installing a rebuilt main switch unit in the console and a new monovalve. This took care of the constant warm air issue, but still no AC.

Then I tried jumpering 12v to the compressor. This did the trick as a temporary fix, but I can't be telling valets to open the hood and disconnect the red wire running before parking my car. It would embarrass the hell out of the wife, and besides who wants to trust a valet making minimum wage plus tips with such an important task?

Yesterday I installed a new relay (the one with a "J" under it in the fuse box) hoping that would do the trick, but no such luck.

Where do I go from here? I stand waiting with VOM in hand and sweat beading on my brow...

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  #2  
Old 04-22-2009, 03:15 PM
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Do you know which AC system you have ? possibilities are ONE, TWO or THREE.
Ok... so when you have the compressor hot wired ( no pun intended ) you have cold air ?
Do you have R134a or R12 in the system ...if you know ....?
When it is running and cold is your aux fan on in front of the radiator ?
Do you have access to a legit vacuum for servicing the system ?
You do need to tell us how exactly you replaced the dryer.... everything you did before and after that installation.
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  #3  
Old 04-22-2009, 03:39 PM
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There is a "freeze switch" on the evaporator to prevent icing which can sometimes fail. It happened on my 300sd when the AC suddenly quit cooling. It's easy to test with a volt meter.
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  #4  
Old 04-22-2009, 04:14 PM
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Thanks for replying.

Quote:
Originally Posted by leathermang View Post
Do you know which AC system you have ? possibilities are ONE, TWO or THREE.
Not sure what you mean here. Do you mean there are three distinct types? If so, what are the differences?

Quote:
Originally Posted by leathermang View Post
Ok... so when you have the compressor hot wired ( no pun intended ) you have cold air ?
It's not ice cold, but definitely cooler than ambient air. I assumed that the system just needs a recharge to achieve full cooling.

Quote:
Originally Posted by leathermang View Post
Do you have R134a or R12 in the system ...if you know ....?
Not sure.

Quote:
Originally Posted by leathermang View Post
When it is running and cold is your aux fan on in front of the radiator ?
Not sure, but I can "hotwire" the compressor again and check.

Quote:
Originally Posted by leathermang View Post
Do you have access to a legit vacuum for servicing the system ?
I am sure I can borrow or buy one if they're not too expensive. Got any suggestions for where to find one and how to use it?

Quote:
Originally Posted by leathermang View Post
You do need to tell us how exactly you replaced the dryer.... everything you did before and after that installation.
I first disconnected the two lines going in and out. I then unplugged the wires and took the unit out. Installation was a reverse of steps 1 and 2. Did I do something wrong? It seemed pretty straightforward.
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  #5  
Old 04-22-2009, 04:15 PM
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Quote:
Originally Posted by PatricdeBoer View Post
There is a "freeze switch" on the evaporator to prevent icing which can sometimes fail. It happened on my 300sd when the AC suddenly quit cooling. It's easy to test with a volt meter.
Will this prevent voltage from reaching the compressor to engage it?
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  #6  
Old 04-22-2009, 04:55 PM
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Quote:
Originally Posted by ThePhoenix View Post
I first disconnected the two lines going in and out. I then unplugged the wires and took the unit out. Installation was a reverse of steps 1 and 2. Did I do something wrong? It seemed pretty straightforward.
Did you replace all the refrigerant that went "POOF" into the atmosphere when you opened the system?

There is a rigorous procedure for doing this, which includes thoroughly flushing the system (then flushing it again), introducing a measured amount of lubricating oil into the refrigerant circuit that is compatible with the refrigerant you plan to use, pulling a complete vacuum in the refrigerant circuit, and introducing a properly measured amount of refrigerant.

Some details omitted.....
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  #7  
Old 04-22-2009, 05:04 PM
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Quote:
Originally Posted by techguy512 View Post
Did you replace all the refrigerant that went "POOF" into the atmosphere when you opened the system?

There is a rigorous procedure for doing this, which includes thoroughly flushing the system (then flushing it again), introducing a measured amount of lubricating oil into the refrigerant circuit that is compatible with the refrigerant you plan to use, pulling a complete vacuum in the refrigerant circuit, and introducing a properly measured amount of refrigerant.

Some details omitted.....
Ok, thanks. I have not done any of what you mentioned, and it sounds like a job for my local mechanic. Is this a separate issue from voltage not reaching the compressor? Am I going to damage anything running the car without having this procedure done right away?
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Old 04-22-2009, 05:05 PM
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Quote:
Originally Posted by ThePhoenix View Post
Will this prevent voltage from reaching the compressor to engage it?
More specifically, it will prevent voltage from reaching the compressor relay, as will the low pressure cut-out switch on the receiver/drier. Checking for continuity across both of those switches would be a good idea.
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  #9  
Old 04-22-2009, 05:07 PM
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Quote:
Originally Posted by ThePhoenix View Post
Ok, thanks. I have not done any of what you mentioned...
Does that mean that you replaced the receiver/drier but you did not recharge the refrigerant?
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  #10  
Old 04-22-2009, 05:14 PM
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Quote:
Originally Posted by ThePhoenix View Post
Ok, thanks. I have not done any of what you mentioned, and it sounds like a job for my local mechanic. Is this a separate issue from voltage not reaching the compressor? Am I going to damage anything running the car without having this procedure done right away?
The receiver dryer has a pressure switch on it. It will prevent the compressor from running (cut voltage to it) if your refrigerant pressure is low.

If you accurately described your actions, and you did open the system and release the refrigerant, then your pressure will be by definition zero and this would be enough to keep the switch open and the compressor off.

When you opened the lines, did it hiss like a mother as if it was releasing gas? If not, your refrigerant was already gone - most likely via a leak.

By opening the system and not vacuuming it immediately, you introduce moisture. This reacts with oil to make corrosive acids that eat away at the system internals. The sooner you get it out by flushing the system, the better.
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  #11  
Old 04-22-2009, 05:18 PM
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Quote:
Originally Posted by tangofox007 View Post
Does that mean that you replaced the receiver/drier but you did not recharge the refrigerant?
Correct. Regarding your other post, under what conditions should I see continuity?
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  #12  
Old 04-22-2009, 05:28 PM
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Quote:
Originally Posted by ThePhoenix View Post
Correct. Regarding your other post, under what conditions should I see continuity?
Forget about checking continuity!!! Your a/c isn't going to work with no refrigerant in the system. If that concept isn't crystal clear to you, you need to study up on a/c systems or get some help from someone who has.

Unfortunately, it's possible that you damaged your compressor by jumping the clutch. No refrigerant equals no lubrication.
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  #13  
Old 04-22-2009, 05:44 PM
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Quote:
Originally Posted by tangofox007 View Post
Forget about checking continuity!!! Your a/c isn't going to work with no refrigerant in the system. If that concept isn't crystal clear to you, you need to study up on a/c systems or get some help from someone who has. Unfortunately, it's possible that you damaged your compressor by jumping the clutch. No refrigerant equals no lubrication.
Of course I realize that refrigerant is the key to making cool air. Is it possible that I lost all of the coolant just by replacing the drier? I assumed that air not being as cool as possible was due to some refrigerant being lost when I installed the drier, or just a lack of recharging having been done for who knows how long. I wasn't aware that losing a bit of coolant in gaseous form (and maybe a few drops in liquid form) could be catastrophic as far as lubrication goes. That said, I didn't run it for long with the wires jumpered, so I am hoping for the best. It will be off to the mechanic tomorrow for a recharge and testing.
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  #14  
Old 04-22-2009, 06:01 PM
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Quote:
Originally Posted by ThePhoenix View Post
Is it possible that I lost all of the coolant just by replacing the drier?
It's not just possible, it's 100% guaranteed. And if you didn't hear any refrigerant escaping when you cracked open the first fitting, there wasn't any there in the first place.
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  #15  
Old 04-22-2009, 06:28 PM
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A/C

" It will be off to the mechanic tomorrow for a recharge and testing."

Comprehension has Not Occurred!

1.You have released ALL your refrigerant(and most of your Lubricant) by opening the system.
2.You have CONTAMINATED your system by Closing it up
(After R+R-ing the Reciever/Dryer) WITHOUT Properly FLUSHING It.
AND then pressure testing with Nitrogen...Then Vacuuming for 24 Hrs.
Before installing Refrigerant(+New Lubricant).

All a "Recharge and Test" will do is further Destroy your Compressor!

Any time a system is "OPENED",It gets a new Receiver/Dryer and Vacuumed.
(Smart Money is on always Flushing, Also!)
(Compressors are "Hand Rotated" whilst installing New Lube to "Flush" old Lube)
[The "Hat Trick" to Success is a pressure test with Nitrogen]

The Really,Really Smart Money is on BG's FrigiQuiet Lubricant!
(No Acid Formation EVER)

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Last edited by compress ignite; 04-22-2009 at 06:37 PM.
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