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  #1  
Old 07-27-2006, 07:51 AM
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Question to A/C experts (or any one else)

I work on A/C systems every day but mostly on Domestic cars. Ford and GM both place the expansion tube (they don't use expansion valves like MB) at or close to the output of the condensor, rather than at the input to the evaborator. This puts several feet of cold tubing under the hood. It works fine that way, but now the question. WHY would they do this? What are the advantages? I have ask several A/C people about this and none can give any help. They just get this blank look on their face and say nothing.
Got any ideas?

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Old 07-27-2006, 09:43 AM
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Really? wow, that is strange...

I understand the "blank look" I get that often from the questions I ask...
it is surprising that a fixed orifice would work in an automotive a/c environment due to the various loads and compressor speeds. also, having the orifice drop the pressures before passing through the engine compartment sounds ludicris. why add all that heat to the cooling system?
when did the domestic manufacturers start using orifices? are you sure you are not looking at the strainer that is usually on the output of the condenser? if it was partially plugged up, it would react like an orifice and drop the temp on the liquid line.
I really cannot fathom a reason for them to do this, R 134a is ineffeciant enough, why add to the problem?
John
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Old 07-27-2006, 10:16 AM
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good question

The blank look might mean you've surpassed their knowledge with your question. The less cold line after the pressure drop under the hood, the better, I would think. Remember, Mercedes is about effeciency.
Yes, those are orifices that get clogged under the hood. Somewhat easy to change.
I once had a Mark V Lincoln with the suction throttling valve setup that was supposed to keep the evap pressure at something like 21 psi. It would often clog & freeze up. Paul.
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Old 07-27-2006, 03:42 PM
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They really did it

John and Paul,
I just looked up a sampling of some cars and/or trucks that have the orifice close to the condenser.
2001 Cavalier at the condenser but 2002 Grand Prix has orifice at Evap
1998 GM P/U at condenser
1996 Ford Tarus is at the condenser
2000 F150 is about half way between cond and evap.

One thing for sure, it saved the manufacturer some money. I just can't understand how.
Maybe they could use a little less freon or something. Just think of how much money they could save if they could cut the freon by 1 oz. Seems to me it would take more freon but what do I know.
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Old 07-27-2006, 03:49 PM
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The domestics all switched to fixed orifice systems from suction throttling valves, TXVs, etc. about 30 years ago. I think the reason is obvious - it's simple, it's cheap, and it works. How many complaints do you hear about the a/c performance in domestic cars? It's perhaps the one area where they really excel.

Same deal with the location of the orifice. It likely has more to do with manufacturing costs than anything else. That one line of tubing through the engine compartment only has so much surface area, hence can absorb only so much heat. While it certainly doesn't help the performance, I doubt it hurts it all that much either. If I owned a car like that I'd probably go all OCD on it and insulate the line from the orifice to the firewall.

- JimY
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Old 07-27-2006, 10:48 PM
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A little more

I assume the cold line in reference goes across the top (84 300d) of my engine. It has no insulation, but appears to have once had it. Would it be benificial to put a wrap on it? Something like a header wrap might help seperate the cold tube from the hot engine.
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Old 07-28-2006, 05:55 AM
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Quote:
Originally Posted by Mercury
I assume the cold line in reference goes across the top (84 300d) of my engine. It has no insulation, but appears to have once had it. Would it be benificial to put a wrap on it? Something like a header wrap might help seperate the cold tube from the hot engine.
I insulated mine with 1/2 inch pipe insulation from Home Depot (dark grey hard foam stuff) and attached it with tie wraps. It seems to help a little and it looks fairly stock. All the material costs less than $5.
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Old 07-28-2006, 10:36 PM
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My '91 4matic had the cold tubing under the hood, and a heat exchanger to cool the gasoline via air conditioning system.

Pretty, um, "cool".

- Jeff Miller
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