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  #1  
Old 06-20-2008, 12:45 PM
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Simple A/C issue. w123 300TDT. Half working...NEW ISSUE!!

1982 300TDT.

SO I recharged my a/c with some freeze 12 two days ago with no luck. I let the car sit since then.

I just went out and fired it up again to see if I could get to the bottom of the issue. Vents still blowing out warm. Not hot. Just very warm.

I popped the hood open, checked the pressure at the line in front of the valve cover (low side, right?) and pressure looked good. Then I saw something that gave me hope... a very sweaty, very cold line! The thing nearly felt freezing. So, where do I start? Climate control unit? Would the line still be getting cold enough to sweat if the ccu is out? I think the CCU is working just because when I turn it to "heat" it gets REALLY hot.

Dirty evaporator? I'm not sure where to look next.

Any help is greatly appreciated!
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'90 300SE --- 173k miles --- (Odometer just stopped working!)

'85 Cali 300D --- 193500 miles --- (Second one) Sold Nov '10

'82 300TDT --- 236,xxx miles --- (My first one) Sold July '09

Last edited by scoodidabop; 07-08-2008 at 10:02 AM.
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  #2  
Old 06-20-2008, 12:54 PM
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Check heater control valve

Open the heater control valve (near the firewall in the engine compartment, two electrical leads, - 4 screws on top). If the rubber diaphram is torn (common issue on 123s), hot water is going to the heater core and the AC and heater are competing. Same issue if the electrical leads are loose as the default mode is open.
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1983 300D, bought new, 215k+ miles, donated to Purple Hearts veterans charity but I have parts for sale: http://www.peachparts.com/shopforum/mercedes-benz-cars-sale/296386-fs-1-owner-83-mb-300d-turbo-rebuild-parts.html
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  #3  
Old 06-20-2008, 01:14 PM
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Quote:
Originally Posted by Cr from Texas View Post
Open the heater control valve (near the firewall in the engine compartment, two electrical leads, - 4 screws on top). If the rubber diaphram is torn (common issue on 123s), hot water is going to the heater core and the AC and heater are competing. Same issue if the electrical leads are loose as the default mode is open.
Hey thanks for the quick reply.

I replaced my monovalve about 6 months ago. I just pulled it out and checked it again though. All the rubber looks good on it. Should I maybe just try a new one? What you said makes perfect sense.

Well actually, I didn't test the electrical connection there yet. I'll make that my next course of action.

Thanks again.
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'90 300SE --- 173k miles --- (Odometer just stopped working!)

'85 Cali 300D --- 193500 miles --- (Second one) Sold Nov '10

'82 300TDT --- 236,xxx miles --- (My first one) Sold July '09
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Old 06-20-2008, 02:02 PM
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I got a defective mono valve repair kit about 6 months ago.
Check the connections and be sure you are getting 12V.
Open the valve and apply 12V to the terminals. Does the valve advance about 1/2 inch (closed) and hold that position? The defective one (actually two) I bought cheap would just stutter and not stay closed.
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1983 300D, bought new, 215k+ miles, donated to Purple Hearts veterans charity but I have parts for sale: http://www.peachparts.com/shopforum/showthread.php?t=296386
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  #5  
Old 06-21-2008, 01:09 PM
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Quote:
Originally Posted by Cr from Texas View Post
I got a defective mono valve repair kit about 6 months ago.
Check the connections and be sure you are getting 12V.
Open the valve and apply 12V to the terminals. Does the valve advance about 1/2 inch (closed) and hold that position? The defective one (actually two) I bought cheap would just stutter and not stay closed.
Once again, you guys are on point. Thank you so much.

I have 12v going to the connection there. I pulled the valve out to inspect the travel distance and you're right; it only travels about 1-2 mm when current is applied. I suppose the problem part more likely the valve itself than the inductive housing, right?
__________________
'90 300SE --- 173k miles --- (Odometer just stopped working!)

'85 Cali 300D --- 193500 miles --- (Second one) Sold Nov '10

'82 300TDT --- 236,xxx miles --- (My first one) Sold July '09
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Old 06-21-2008, 05:16 PM
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It is unlikely to be the inductive housing, more likely to be a moving part, something that hangs up because it is dirty, corroded, worn, or covered with scale from a time in the past when the coolant was years overdue for changing. Sometimes you can take the valve apart and clean up the moving parts.
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Old 07-08-2008, 10:06 AM
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Well thank you guys. That did the trick. I lucked out at the pick-a-part and found a very good valve and housing (my old housing was leaky).

I did create a new problem for myself though. I was trying to pull off the wire connection on top of the mono valve when the cover busted off and the two leads touched. Now I don't have any voltage going there anymore. The fuse for the valve looks good and tested fine with a meter. Where does that thing lead to, and more importantly, what did I short out??

Thanks again!
__________________
'90 300SE --- 173k miles --- (Odometer just stopped working!)

'85 Cali 300D --- 193500 miles --- (Second one) Sold Nov '10

'82 300TDT --- 236,xxx miles --- (My first one) Sold July '09
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