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  #1  
Old 10-31-2003, 07:17 PM
lakelover
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Where is a/c control unit?

The a/c on my '88 420 SEL is "broke." I live in the boonies so I don't have easy access to a dealer. However, I do have a mechanic who is computerized and quite knowledgable. He says that the compressor clutch circuit has an open in it. He suspectes that there may be a fault with the air conditioning compressor control unit but he can't find it. The unit is not where his reference says it should be. Can anybody give me a hint as to where that unit might be (chassis: 126.035; engine 116).

He has quoted about $700 to replace the compressor and dryer, including labor ($90), compressor oil and refrigerant. This does not include a control unit. Of course if the problem is the control unit, then perhaps the compressor is okay. When he finds the control unit he says he can check it for proper performance. I'm checking parts costs, here, at Mercedes Shop.

Jerry
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  #2  
Old 10-31-2003, 11:22 PM
sixto's Avatar
smoke gets in your eyes
 
Join Date: Jun 2000
Location: SF Bay Area
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There are several interrupts along the circuit that enerizes the compressor clutch. The two easiest to check are the 105C/115C switch that cuts off the compressor when coolant temp gets to 115-120C, and the low pressure switch on the drier that cuts off the compressor if there isn't enough refrigerant pressure in the system. It's safe to temporarily bypass the 105C/115C switch when the engine's cold and probably safe to momentarily bypass the low pressure switch.

Beyond that there's a relay that compares compressor revs to engine revs. One input is compressor revs, the other is engine revs. Even if there's something wrong with either or both inputs the compressor should come on for a moment. If your tech is familiar with MBs, this is probably the device he's trying to locate.

Then there's the ACC temp control unit and push-button unit that consider inputs from you, the cabin temp sensor, the temp control wheel and the evaporator temp sensor to determine whether or not to engage the compressor.

What I'd do is disconnect the 105C/115C switch to see if that does it, then jump the low pressure switch to see if that does it, then check for an appropriate input to the AC relay to see if the problem is inside the cabin. The rest will depend on the results.

BTW, I'm not familiar with W126 V8s so some of this is guessing.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD
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  #3  
Old 10-31-2003, 11:38 PM
Gilly's Avatar
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Location: Evansville WI
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The control unit that controls the ac clutch is between the double-firewall and is marked on top "KLIMA". It's a black box slightly larger than a pack of cigarettes.
But sixto is correct, i'd suspect an input problem before I'd suspect the Klima relay, probably low pressure in the system.

Gilly
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  #4  
Old 10-31-2003, 11:49 PM
lakelover
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Thanks for your reply. My mechanic is not a Mercedes "specialist", just a well schooled, experienced generalist. I'll pass your info along. Question (pardon my ignorance): where do I find and tell the mechanic where 105c/115c and 115c/120c switches are or can I expect him to know. He gave me the diagram of the locator sheet he got off his computer and the illustration is very non-descriptive in terms of actual placement of the components, though he said he found all but the a/c control unit. I think that unit may actually be the ACC temp. control unit. I've read elsewhere on this forum that cold solder points in that unit are troublsome. I may try resoldering the unit if I can get it out. One thing for sure is that I don't want to replace the compressor if there's a simpler fix. Here's what happened with the a/c: sudden complete loss of cooling with hot air flowing out of the defrost. I understand that is a normal default when the a/c quites. I also lost the fan because a fuse blew. That fan operates when the fuse termals are shorted.
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  #5  
Old 11-01-2003, 04:53 AM
sixto's Avatar
smoke gets in your eyes
 
Join Date: Jun 2000
Location: SF Bay Area
Posts: 19,746
The first thing I'd do is replace all the fuses. Don't even check them, just toss them and fit all new fuses. If you get the fuses through the FastLane link above it'll be under $5 (plus shipping) for all the fuses you need.

The 105C/115C switch is probably the only 2- or 3-pin coolant temp sensor near where the upper radiator hose meets the engine. I doubt you'll have trouble finding it if you're the least bit familiar with automotive cooling systems. Removeing the connector (2-pin) or connectors (3-pin) will get the AC clutch engaged if the switch is bad.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD
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  #6  
Old 11-01-2003, 11:05 AM
lakelover
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I'll check the 105c/115c swith this morning. And I'll change all the fuses. BTW, I got a variety pack of fuses at AutoZone for $1.99 per pack of 6 fuses.
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