AC experts ? High Side Pressure Too High - Vintage Mustang Forums

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post #1 of 23 (permalink) Old 09-12-2008, 08:26 PM Thread Starter
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Took the son's 68 to have the AC charged for the first time since the resto.
Systems Specs are:
-302 4v(rebuilt +.030 with about 5k miles)
-C4 auto
-Orig recored radiator
-Spal 16" curved high performance electric fan,puller, 2000+ cfm, on 165 degree sensor in bottom hose.
-New 6 circut condenser form Classic Auto Air(CAA)
Orig Drier rebuilt by CAA(since ALL the repo units do not have the front fitting in the right location on the top of the drier).
-New hoses from CAA
-Orig Expansion valve, cleaned out(not yet wrapped in cork tape so might get a little heat from the shorty headders, Heddman ceramic coated.
-Sanden 134a compressor filled with freash correct oil, was on my dad's '34 cabriolet until he decided to remove the AC about 7 years ago(it was stored with hose connections capped in a safe place).
-Orig evaporator, cleaned out with brake cleaner & air.


Went like this:
-It was about 80 degrees outside.
-Called Classic Auto Air to get specs on amount of 134A the system holds.
-Evacuacted system and charged system with 22oz per specs.
-Started car and after about 20-30 seconds the high pressure gauge went up to 400psi.
-Low side was about 40psi.
-Mechanic said sut it down.
-Called CAA they said high side should be about 300psi or a little less.
-Evac system again and charged with only 16oz.
-Same problem.
-Evacuacted it all back out.

-Went back by this week and talked to another guy in the shop that years ago did a lot of AC retrofits into all kinds of cars. He had helped me retro fit a system into '71 Volvo a looooong time ago. Old high school friend too, he was on vacation the first time.
-He asked if the radiator fan(elec.) was running when the AC system was turned on?
-No it wasn't it is set to come on based on the temp switch in the lower hose.
-He said that is porbably it, that the fan needs to run all the time when the AC is on to cool the condenser or it will get hot that fast and make the pressure too high.

Is this right?
Seems like a waste to have the fan running all the time when the AC is on. Especially at highway speeds when it should not be needed.

If so that means I need to worry about the daughter's 66 that runs a factory AC with Sanden 134A conversion and the same electric fan setup as her brother.

TIA

'68 J code coupe back in H.S.1st car. Now 65 GTFB
Daughter & I built her a '66 coupe feature car in 8/04 MM & '05 Meguiar's winner see it @ Meguiars.com
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post #2 of 23 (permalink) Old 09-12-2008, 08:59 PM
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Sounds like a restriction to me. Expansion valve?
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post #3 of 23 (permalink) Old 09-12-2008, 10:25 PM
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orfice tube

make sure it is present and clean

does the system have a high pressure switch

chris




[colorrange][/color]modded 68 fastback daily driver
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post #4 of 23 (permalink) Old 09-12-2008, 11:54 PM Thread Starter
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No high pressure switch on the original factory system.

Is the orfice tube the section between the site-glass hose and the drier?

How do I check the expension valve? Blow air through it while making it cold and warm(or not so cold)?

Thanks

'68 J code coupe back in H.S.1st car. Now 65 GTFB
Daughter & I built her a '66 coupe feature car in 8/04 MM & '05 Meguiar's winner see it @ Meguiars.com
Just finished on oldest son's '68 j code coupe
'67 FB project is next with/for youngest son
'92 5.0 LX vert 68k org miles
'74 TR6 Project for Wife
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post #5 of 23 (permalink) Old 09-13-2008, 06:21 AM
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Yes. The extremely high pressure on the high side and normal pressure on the suction side indicates that the evaporator is overheating. The fan needs to be turned on when the A/C compressor is on. Alternately you can wire-in a bi-pass to the radiator's temp sensor with an evaporator tempt sensor.

Now that I think about it, first check if there is good airflow between the fan and the evaporator. If the evaporator is too far forward air will not pass through it... it will simply go around the evaporator.

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post #6 of 23 (permalink) Old 09-13-2008, 07:29 AM
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You may or may not have the "quick dis-connect" couplings in your line, but they are infamous for causing a restriction in your system. When they are replaced one should always purchase both halves. Most/many were thrown away over the years, but if yours are still in place that may be your obstruction. Just a thought, and most AC techs today are not aware that these were used on a 68 Mustang.

murf
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post #7 of 23 (permalink) Old 09-13-2008, 10:23 AM
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The tech is right on, the fan needs to be running at all times with the A/C on. If this is not done the high side will be off the charts. Even factory vehicles with electric fans run the fan all the time with the A/C on. Add a relay to the fan circuit using the a/c compressor circuit to turn it on and the system should cool fine.
Phil

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post #8 of 23 (permalink) Old 09-13-2008, 12:36 PM
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Quote:
Originally Posted by mustangfactory
The tech is right on, the fan needs to be running at all times with the A/C on. If this is not done the high side will be off the charts. Even factory vehicles with electric fans run the fan all the time with the A/C on. Add a relay to the fan circuit using the a/c compressor circuit to turn it on and the system should cool fine.
Phil
+1

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post #9 of 23 (permalink) Old 09-13-2008, 09:24 PM
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Fan has to run, the 134 gets hot as hell when compressed (as do all things). If the pressure went up to 400PSI as soon as the compressor kicked on then you have a restriction. If it came up to 400 after 20 or so seconds, the lack of fan is getting you.

To test it, you can get a strong shop fan and blast it into the condensor and retest the system. If it is not a total blockage restriction, start at the orifice tube or the expansion valve and work your way towards the condensor by feeling the hoses/components. Be careful as they can be HOT (and likely will be hot).
You will find the spot of blockage when you find the temp differential point. If you have a infared temp gun, use it instead of your hands.

I'd lean more on the lack of fan running.
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post #10 of 23 (permalink) Old 09-14-2008, 01:12 AM
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With the pressure gauge still on the AC pour some water on the condenser and see if the pressure drops... if it does, then there's your problem!

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post #11 of 23 (permalink) Old 09-14-2008, 01:14 AM
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Yup, fan must run. To test for yourself, just jumper the relay to have the fans run and then restart your test. You will see a world of difference.

When I charge a system, I put a big box fan in front of the car to help with airflow as much as possible.

John
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post #12 of 23 (permalink) Old 09-14-2008, 08:17 AM
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"-Orig Expansion valve, cleaned out(not yet wrapped in cork tape so might get a little heat from the shorty headders, Heddman ceramic coated."

If using R134a then the factory expansion valve (R12) cannot be used. Expansion valves (TXV) in simple terms are not a 1 size fits all, each valve is designed with a particular reference charge in the bulb to control the opening of the expansion valve orifice based upon load in the evaporator, basically to control the amount of superheat desired in the evaporator by the system designer. The type of gas used in the TXV , usually is the same refrigerent as the system; however based on application other gases may be used. In addition R134a systems use a larger compressor than a comparibly sized R12 system since R134a must be compressed to higher pressures in order to get the desired saturation properties for the system to work correctly. In other words you will not get the system to work correctly with R134a unless all components are matched.

I currently do not work in the automotive air conditioning field any longer; however in the past I worked for a "Big 3" climate control group, a Fortune 100 manufacturer of automotive a/c TXVs, driers and hose assemblies, and finally for a manufacturer of custom aftermarket Mobil A/C systems. The last place where I met fellow vmfer "Jim", I am currently a little rusty on the topic, therefore if you have any further questions I would contact him since he works with Mobil A/C every day.
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post #13 of 23 (permalink) Old 09-14-2008, 09:03 AM
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Put AC in a 65 GT cv that I owned. It was original except for a Sanden adapted to fit and be part of the OEM package. The pressures were extremely high. AC person said that was part of using 134 and it was part of the deal. When fully charged it blew cold and still does 6-7 years later.

A friend gave me 4 cans of R-12 and oil for compressor so am going to use it for my current car. It has all original parts under hood except the Sanden and the Clasic Air underdash evaporator. Condenser and dryer are new and still sealed.
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post #14 of 23 (permalink) Old 09-14-2008, 09:32 AM
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I agree with having the fan on before going anywhere.

In order to manage head pressure on my car, I had to add 2 x small electric fans on the condenser. I did it between the radiator and the condensor so you couldnt see them.

It dropped pressure dramatically.

I do agree the expansion valves are matched to the system, but in my experience, I havent had any problems using the stocker.

Most thermal switches also have an a/c input, if yours doesnt, get a different one, all you do is trigger the a/c input from the clutch trigger and whenever the a/c is on, the fan is on.


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post #15 of 23 (permalink) Old 09-14-2008, 11:33 AM
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Not to hijack the thread but am I reading correctly that the stock compressor can not be used if upgrading to R143a? On my 68 coupe I want to get the A/C working, everything is there and original but I wanted to use 134 instead of R12.

Doug
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