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  #1  
Old 06-14-2010, 02:09 PM
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AC Troubleshooting

I've had about enough of the sweaty car rides so I'm going to tackle the AC system next week. I don't know where to start however because the system never worked in the first place, and I'm not positive the rest of the climate control works correctly. I replaced the CCU and blower motor with used ones. I don't believe I have any temperature control though. How can I tell what needs to be fixed/replaced? I'm guessing I have to replace the compressor, drier, and expansion valve. Is there anything I should test before replacing these parts?

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  #2  
Old 06-14-2010, 02:52 PM
rrgrassi's Avatar
mmmmmm Diesel...
 
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Well, for starters, check all the fuses and the relays as well.

What on the system does work? Does the heater? How about when moving the wheel from heat to cold, does the monovalve close?

While you are replacing the compressor, drier and expansion valve, have the system pressure tested for leaks, say from all the hoses, condensor and evaporater core. Check the wiring to the compressor, aux fan, and also replace the temp switch on the drier and the pressure switch.

Others will chime in. This is just off the top of my head.

Also search the forum. Lots of good HVAC info.
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  #3  
Old 06-14-2010, 03:07 PM
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May get lucky and find out that you just need some freon...compressor won't switch on if the pressure is too low (unless you jumper the pressure switch)

Thorough diagnostics can save you GOBS of time and $$$.....ask me how I know. Looks like I bought all the parts you just mentioned only to find out that my problem is either a $40 evaporator switch or a $30 relay (or both).

You will definitely find lots of help here, but you have to be prepared to do lots of diagnostics to guide your guides. First thing one of the gurus (Larry Bible is my guess) will ask is what the high and low side refrigerant pressures are....of course to supply this info you will need a manifold capable of measuring the pressures.

Good luck - there are few things I have done on my car that have been as satisfying as fixing the AC!
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  #4  
Old 06-14-2010, 03:12 PM
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Be prepared to replace all the orings.
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  #5  
Old 06-14-2010, 03:28 PM
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I too have to go through this. The A/C failed on my W123, it is inop in the W124, and was working when I purchased the W126.

I am hoping to get our 2 wagons 'chilling' again.
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  #6  
Old 06-14-2010, 04:10 PM
showme's Avatar
Mama's 300D
 
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ME TOO! I just posted pretty much the same question before I scrolled down and saw yours. I'd like to know if there's a step-by-step guide, book or DIY thread that I can print off and take out there with me. I've always wanted to learn how to not pay somebody else to do this. Now that it's getting near 100* here in Missouri, I want to know even more! Thanks for any replies to saludayakers question. (I just ordered and received the control switch for the dash panel and was contemplating ordering a new compressor. But there's got to be a good tutorial somewhere on the subject.) I'd say this is the place to find out.
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  #7  
Old 06-14-2010, 04:37 PM
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A/C is not as difficult as you may think

ac still not working after ac conversion 87 300SDL

Post #70. Excellent write-up.

Search the Internet and there is a wealth of info on the fundamentals of A/C. There is nothing to it as far as the theory goes. The issue is the MBZ Control unit is difficult to understand, but it is like any high tech stuffs. Once you introduce computer or electronics into the system, the complexity goes up exponentially.
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  #8  
Old 06-14-2010, 05:07 PM
92 300D 2.5L OBK #59
 
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Here's the mother of links on Climate Control: Climate Control:
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  #9  
Old 06-15-2010, 01:30 PM
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Quote:
Originally Posted by LarryBible View Post
Why are you replacing all these items? You need to first properly troubleshoot the system. Is the clutch engaging? If so, what are the pressures at what temperature.

It would be very foolish to start doing such major work on the system without first properly troubleshooting. Help us to help you and borrow some gauges and do this right.

Told ya


Listen to the man - get some gages....not too expensive, cheapo ones from HF or AZ will do the trick for your needs.
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Current Mercedes
1979 maple yellow 240D 4-speed


Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT
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  #10  
Old 06-15-2010, 04:16 PM
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Thanks for all the good beta. To be honest I'm not sure if my compressor is kicking on or not. I never heard it so I just assumed it wasn't engaging but from what I've read they can be very quiet. Also it's interesting to note that the compressor will not engage if it's too low on refridgerant. When I have some gauges I will check the levels of the refridgerant and go from there.
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  #11  
Old 06-15-2010, 04:18 PM
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I should also note that I don't believe my monovalve works properly b/c I wouldn't really get cold air this winter. It's a little hard to tell now but it seemed like the only warm air I got was the air that was sitting in the vents for a while and after the initial 5 second blast of air after turning the blower on the air would go back to just a little warm.
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  #12  
Old 06-17-2010, 08:53 AM
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Ok, so here's a basic question that I hope I don't get flamed for: How is the AC supposed to work? Which buttons control what? If I have the temp dial on cold will the AC automatically come on?
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  #13  
Old 06-18-2010, 12:44 AM
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Get the Service Manual on CD. There's a section on the Climate Control. The schematics are pretty crummy quality, but the section on testing all the parts has better diagrams, and walks you through all the tests. The special tool isn't necessary if you read the schematics - all it really does is switch the test points so you don't have to move your meter. Most of the test points are through a connector on one of the control units. You can check all the sensors and inputs to the system this way and you will begin to get a good understanding of your system. Once you rule out the electrical items, then you can go into the engine compartment and start ruling out the mechanical portions.

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