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Rear AC evap and function

 
Old 02-19-2019, 06:08 PM
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merchauser
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Default Rear AC evap and function

Debating if I should replace my rear AC evaporator. a previous owner crimped the lower tube from the evaporator at the expansion valve. (looks like a small tube has been crimped as well)


not going to pony up for a new one, but will search for a viable used one; (anyone got collecting dust?)

if the rear AC solenoid is unplugged, does that stop the flow of Freon to the rear unit?
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Old 02-19-2019, 06:11 PM
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"if the rear AC solenoid is unplugged, does that stop the flow of Freon to the rear unit?"

Yes.

I rather enjoy the rear AC, keeps back of my head cool.

Dunno why people have to destroy things, and not do it right...
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Old 02-19-2019, 07:23 PM
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Originally Posted by Speedtoys View Post
"if the rear AC solenoid is unplugged, does that stop the flow of Freon to the rear unit?"

Yes.
interesting: it does not look like the evaporator crimps were braised, so if I turn the rear AC fan switch on, would the unit
leak at those crimps?


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Old 02-19-2019, 07:31 PM
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Originally Posted by merchauser View Post
interesting: it does not look like the evaporator crimps were braised, so if I turn the rear AC fan switch on, would the unit
leak at those crimps?

Id say likely if not properly sealed...if the solenoid was also plugged in.
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Old 02-19-2019, 07:42 PM
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Originally Posted by Speedtoys View Post
Id say likely if not properly sealed...if the solenoid was also plugged in.
the solenoid was not unplugged, but not sure it matters since the valve is only activated when the rear air fan switch is turned on?

not sure if I understand the WSM regarding removal. do I HAVE to remove as a complete unit with the blower, and then disassemble?
or does the evap come out on its own, with housing for blower left in place?

also, if the solenoid is not engaged, and Freon does not circulate, can I remove the rear evap and not worry about loosing my front AC cooling?
I still have some Freon in the system and front is keeping me cool on recent warm days
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Old 02-19-2019, 07:58 PM
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Originally Posted by merchauser View Post
the solenoid was not unplugged, but not sure it matters since the valve is only activated when the rear air fan switch is turned on?

not sure if I understand the WSM regarding removal. do I HAVE to remove as a complete unit with the blower, and then disassemble?
or does the evap come out on its own, with housing for blower left in place?

also, if the solenoid is not engaged, and Freon does not circulate, can I remove the rear evap and not worry about loosing my front AC cooling?
I still have some Freon in the system and front is keeping me cool on recent warm days
True, if rear switch never comes on, solenoid cant ever come on.

Yes, you can remove it. Probly worthwhile for a GT to keep it whole...
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Old 02-19-2019, 08:02 PM
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^^^^thanks
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Old 02-19-2019, 09:32 PM
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Consider that there are TWO connections to the rear evaporator. The high-pressure connection, somewhat isolated by a not-tight-shutoff solenoid valve, and the low pressure connection back to the compressor suction. If you decide to remove or otherwise isolate the rear system, you'll want to isolate both connections where they connect to the main system. With the car elevated, look for the bulkhead pass-through under the passenger's seat where the AC lines pas up through the floor. Follow those forward to where they connect to the main system lines.

Your mission is evacuate the system, remove the T fittings on both circuits, and find a suitable threaded cap that maintains the integrity of the main system. Then you can remove as much or as little of the rear system as you like. Doing the work this way leaves a path to restoration in the future.

Crimping high-pressure vapor lines is seldom leak-proof. My slugging percentage when trying this is miserable. If you must cap off a line, plan on cleaning it well and brazing a cap on each of the lines feeding the rear system. If you want to try capping the copper in the rear, remember to use silver solder; regular plumbing solder will leak. Remember that you can't do just one.
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Old 02-20-2019, 08:49 AM
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dr. bob, thanks for the info, but my mission is to return the rear air to full function. many have told me that it makes a nice compliment to the front air in warm climates.

since there are two lines, and the solenoid is only on one line, it sounds like taking the evap out will deplete the Freon, and I will have no cooling? correct? if yes,
I will wait until I have my new evaporator; then I can do an evacuate and recharge at the same time.
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Old 02-20-2019, 12:05 PM
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Yes
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Old 02-23-2019, 10:32 AM
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what is the best way to test my new (used) evap? I could pressurize and dunk in a bucket of water, or maybe a mityvac would be better?
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Old 02-23-2019, 01:55 PM
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Pressure and the water dunk. With a functioning expansion valve, the max pressure you would normally see in the evap is maybe 100 PSI, so choose your test pressure accordingly. The rear evap is copper tubing, with joints that are silver-solder brazed. If you need to do some repairs, heat the evap for a while in the oven to make sure there is no residual moisture present at the repair location. Cleanliness is king while prep is almost king. Be sure to clean up after the fix, including removing any remaining flux.

In normal operation, it's common to have the evaporator mass at 150║+ soaking in the hot sun, then introduce liquid into one with a flash vapor temp of 20║. There's a bit of stress as temps move around. Besides the obvious need for freedom to expand and contract, there needs to be room for some twisting. As you arrange connections and support again, keep these things in mind, so you don't add extra stress at the connections.
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Old 02-23-2019, 02:22 PM
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Well, I am a little bit in your situation for a number of reasons too. Apparently, my rear AC evaporator has a leak. On the one hand, with the stock AC infrasturcture, I am not convinced that the rear AC really does much. (Maybe it would, if I upgraded to GB's super AC system.) On the other hand, I don't want to really hack up a car that came with rear AC and just remove it. (Even though the space would be nice for hi-fi audio equipment.) Another datum in this whole matrix is that the evaporators are rather pricey, and I suspect that materials used dictate a finite life time to these components. I bought two new evaporators a couple years ago for my private stock to fix the 90' with the slight leak. I just like things to be fixed how they came, which while doubting the effectiveness of the rear AC is why I bought the parts. Had the car been delivered without rear AC, I would be just as happy. The AC on these cars is a little bit of a quandry.
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Old 02-23-2019, 02:50 PM
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For automatic S4's and all GTS's a/c performance is constrained by the condenser so the rear a/c should be seen as a means of redistributing the a/c performance rather than adding to it once the system limits are reached. That being said the other qualifying piece of information is the ambient temps the car is being run in. It is ages since I bothered to run my rear a/c but as I recall it performs well up to about 30C ambient temps, it is still of use to about 35C but above that it is more or less useless as one clearly feels the temperature out of the front a/c drop off. Of course road speed is also another factor in the mix. For manual transmission S4's and the GT's they have larger surface area condensers so the system as a whole performs better and thus can utilise the rear a/c when fitted to better effect. .
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