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Low-side too low?, High-side too high?, Compressor cutting out?

jamyers on Fri August 07, 2009 6:16 PM User is offlineView users profile

Year: 1962
Make: Ford
Model: Ranchero
Engine Size: 200
Refrigerant Type: R134a
Ambient Temp: 95
Pressure Low: 10-15
Pressure High: 225-240
Country of Origin: United States

I've had an aftermarket add-on underdash system in my '62 Ranchero for 4-5 years now, and I just recently added an parallel-flow condenser (which fits and works GREAT, btw) to replace the smaller tube-n-fin one. The vendor I got the system 4 years ago said to fill it with something like 20 ounces of refrigerant, but I've often thought that was too much (and the condenser too small) as the high-side pressures were upwards of 300psi and the high-pressure switch would cut it off frequently in stop-n-go driving, and the system blew cold air but wouldn't really cool the cab on really hot days.

So this time around (after pulling/holding a vacuum overnight, with 95* ambient air, engine at 1200-1300 rpm and a fan blowing at the condenser) I filled it with R134a until the sight glass stopped showing bubbles (right at 14 oz), which got me 10psi low-side, 225psi high-side, and 37* air at the vents.

I added a total of 6 of ounces more (a couple at a time), and the low-side pressure went to 15, but the high-side climbs up to 250-260. I took the system back down to a total of 16 oz, and the low side dropped to 10psi and the high-side came back down to 230psi. Throughout all of this, the vent air temps (doors open, car in the sun, blower on max) stayed below 40* - usually right at 37*.

I then turned off the shop fan off and let the engine idle down (worst-case scenario). After 10 minutes the pressures climbed to a max of 35 low and 360 high, and the inside air temp goes up to 45-50 degrees. (looks like I'll be adding a fan shround soon) I turned the fan back on and bumped the engine speed back up, and everything dropped right back down to where it was before.

Driving the vehicle so far, it's cooling very well with vent temperatures below 40, usually around 35 - and once I saw 25, but the thermometer may have been up against the evaporator tubes.


So I'm thrilled with the system performance (especially the new condenser - Thanks, guys!), but have a couple of questions:

1. Isn't 10 psi awfully low? Is that starving the compressor? (but if I add R134a to raise it, the high side goes up WAY faster than the low side)

2. I understand that high-side pressure should be roughly 2.2 times ambient air, which in 95* would be 209psi. Are my high-side pressures too high?

3. About once every 5 minutes or so the compressor cuts out like a pressure switch is kicking it off. But the high-side is way below the old high-side pressures it was seeing (iirc the high-pressure switch opens at 375psi?), and the low-side is pretty consistent at 10 psi. I've seen the low-side dip down to 7 psi and the compressor kept going. The system has a switch on the top of the receiver/drier, I know it's a high-pressure switch but I'm not sure it's both high- and low-. Looking at the compressor (Sanden), the clutch wire disappears between the compressor and clutch and I don't see any other switches unless they're hiding inside there. The evaporator has a TVX valve, and a "Temp" knob that switches the compressor on/off depending on a capillary tube stuck into the evaporator coils. I've always left this knob in the 'max' position, which used to have the compressor on all the time.

Any ideas?

TRB on Fri August 07, 2009 10:40 PM User is offlineView users profile

Before we address the pressure issue get or build a full fan shroud and if you don't have one get a 5 blade rigid fan.


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jamyers on Fri August 07, 2009 11:14 PM User is offlineView users profile

It has a 6-blade rigid fan. Fan shroud is on the to-do list.

I take it that in the meanwhile I'm not going to damage things by running the system. (?)

jamyers on Sun August 09, 2009 1:01 AM User is offlineView users profile

I made a fan shroud and got it installed this afternoon. The change is that at low idle, the pressures only go up to 20 low-side and 300-325 high-side, and the vent air temp goes up to 45 degrees indicated. Otherwise, the pressures, temperatures, and compressor cutting out are same as without the shroud.

The underdash evaporator is so compact that it's hard not to get the thermometer into the coils, if it matters.

The Compressor is still kicking out, and it seems to relate more to vent temp than to pressures. After sundown, the ambient temp dropped down to 80, and the system will blow cold air all the way down to 25 degrees, then kick out the compressor until the vent air goes up to 40-45 degrees, then the compressor kicks back on. Not that I'm complaining about 25-degree air ... but with the compressor kicking in and out, driving it during the heat of the day the system isn't really cooling the interior of the car like I'd think it ought to. For example, in 100 degree sunny weather, driving down the road at 60mph with the system on max the a/c system only pulls the interior down to just below 80 degrees - and it's a little bitty pickup cab. I thought that turning the "temp" knob to the max cold position over-rode the thermostat and kept the compressor on all the time.


edit: I got the a/c system from Classic Auto Air, if it matters.

Edited: Sun August 09, 2009 at 1:07 AM by jamyers

TRB on Sun August 09, 2009 11:30 AM User is offlineView users profile

It's an expansion valve set up correct? Your low side pressure is in within range for this type of set up. Doors closed on medium blower what is the vent temp?


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jamyers on Mon August 10, 2009 5:09 PM User is offlineView users profile

Yes, expansion valve. Also has a thermostatic switch ("Temp" knob) that cycles the compressor on off based on evaporator (airflow?) temperature (cap tube in the evaporator).

Doors closed on medium blower, with "Temp" knob set to max, and lots of airflow through the condenser: the vent temp goes down to 30 degrees - and then the compressor cycles off and it goes up to 40 or so before coming back on.

Glad to know that the low-side pressures aren't too low, I'd hate to tear something up (and I'm so used to seeing 30-35 psi with R-12). How about the high-side psi and/or the 'spread'?

I guess my concern is that when this knob is set to the "max" position, I thought it was supposed to run the compressor continuously (I hate cycling compressors), and the system is cycling the compressor off/on - and I can't seem to figure out why. I've seen it cycle off when the vent temps get to 30 degrees, and the next day it will get down to 25 degrees. It's cycled the compressor off when it reached 10 psi, and then it's kept the compressor going down to 7 psi.
Is there an internal low-pressure switch inside Sanden compressors?

Thank You for your help!


Edited: Mon August 10, 2009 at 5:10 PM by jamyers

mk378 on Mon August 10, 2009 5:41 PM User is offline

The thermo switch is cutting it off. Even at "max", if the evaporator gets too cold it will freeze up, and the switch is there to prevent it. Pretty impressive that your setup can do that at idle. The only way to prevent icing other than cycling the compressor is to have a POA, STV etc. valve or a variable compressor.

jamyers on Tue August 11, 2009 10:40 AM User is offlineView users profile

OK, so I guess it's just working "too well", so I'll stop worrying about it and get used to having great a/c.
Thanks for the help!


jamyers on Tue August 25, 2009 2:30 PM User is offlineView users profile


The system was cooling so well it would pull down to 25F or so degree vent air, then hit the thermostat switch, cycle off until the vent air went back up to 45F or so, then kick back on, etc, etc, etc. Love the cold air, but the on/off cycling was bugging me, especially around town (small 6-cylinder engine, manual transmission and all).

So I waited for a 101-degree day, then evacuated the R134a and pulled another vacuum on it for just over 45 minutes. Turned off the pump and it held vacuum for an hour, so I figure no leaks. I hooked up a 12-oz can, vented the yellow line, and let the can and system equalize pressures with the engine off. Turned on the engine and a/c, and *sloooowwwwly* added R134a, watching the pressures and vent temps. The low side stayed right at 7 to 10 psi, and the high side climbed up to 190, with the vent temp dropping down to 30-35 degrees F (without cycling the compressor off at all). I stopped there, which was a grand total of 12 ounces R134a in the system (please don't tell me that's too little...) .

Driving the vehicle around town and on the highway in 80-95 F weather, the vent temps are now ranging from just below 30 to 45 degrees, most often right above 30F, and it's not cycling off/on at all that I can tell. Right now it's a sunny 95 degrees outside, and the a/c pulled the inside air temp (thermometer clipped to the sun visor) down from 110 to 75 in about 10 minutes - I'm loving it!

Count me as one happy AMA customer, I'm crediting your Parallel-Flow Condenser (and this forum) with making a WORLD of difference.

TRB on Tue August 25, 2009 3:12 PM User is offlineView users profile

That's nice to hear and thanks for supporting the forum and AMA!


When considering your next auto A/C purchase, please consider the site that supports you:

jamyers on Mon August 03, 2015 7:10 PM User is offlineView users profile

Update: 6 years later, the same system is still blowing ice cubes out the vents - WAY TO GO ACKITS.COM!!!

Dougflas on Mon August 03, 2015 9:00 PM User is offline

your low side is most likely on the sution of the compressor so you will see lower PSI ranges.

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