AC Adjustment question (Possibly stupid)

LarryE9E10

LarryE9E10

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Guten Tag all!

Driving home from the Vintage - going 75+ MPH in 85+ degree heat - the AC on my '73 3.0CS just couldn't keep up. Post-trip analysis shows system full of freon, compressor working and cold air blowing. But I noticed that the compressor keeps cycling on and off very frequently. I know this action is controlled by a de-icing valve. Can that valve be adjusted in any way to let the compressor run longer before it cycles off?

Or am I just expecting too much from a 40 year old system on car with a big greenhouse? I gave up on AC in my 2002tii decades ago because the system wasn't robust enough to provide adequate cooling on a hot day, even when new. Should I expect more from my coupe?

All advice and opinions will be appreciated. Thanks!
 
Unless he has a modern dryer/compressor with a pressure switch installed there or somewhere else it will run till it blows up.

HB Chris said:
An A/C that cycles on and off usually indicates a low freon level, it shuts itself down to avoid damage.
Click to expand...
 
FWIW,
The E9 system switches the compressor on and off based on the evaporator temperature. Usually off at ~28 deg f and on at ~34 deg F. The goal is to keep the evaporator just above 32 deg F as much as possible to keep it from being blocked by frozen water condensed by the colds metal.
The evap temperature is directly related to the low side pressure. (~ 1 degF per psi for R-12)
When the system is charged correctly the low side pressure will sit around 32 to 34psi (which results in ~ 34 deg F in the Evap.) with the fan on full. Hence the compressor does not cycle.......
When the system is undercharged, the evap pressure will be lower which makes the evap get colder causing the compressor to cycle..... It's a little counter intuitive until you realize that the cooling CAPACITY goes down as the low side pressure goes down. So the system can not cool the car as well because the AVERAGE evap temperature is actually higher.....
I was taught to charge the system until the low side comes up to 32 psi and then add 1/4 to 1/2lb more refridgerant.
DaveG
 
The flip side of the low side

is the high side presure. You need to buy/ borrow a set of cheap guages to see both pressures at a given temperature. There is a relationship.

It's true that the system could be low while the same symtoms could be overfilled

If your high side pressure is reaching say 400psi; you're over filled.

What compresor? What's the discharge temp of the vent?
 
DaveG: Excellent explanation! Which sensor senses evap temperature and where does it connect to?
 
I'm a fan of the old school method- keep filling until the temp doesn't drop any more and the pressures start jumping. Overfilled brings higher temps and higher pressures. Don't waste your time staring at the sight glass.
 
Absolutely! You HAVE to do this with gauges! The relationship between High and low side pressures is very important. Overfilling leads to burst hoses and a real mess.....
DaveG
 
Similar Questions for R134a System

DaveG and sfdon, this is one of the best explanations I've read. I have a similar question to Larry's.

I have an R134a a/c system in my E9. I did a from-scratch retrofit about 15 years ago. It's got a Seltec rotary-style compressor, the largest parallel flow condenser I could stuff into the nose, a three-row evaporator core I had Bob Poggi at I.C.E. cut for me in the same space as the standard two-row core, and an expansion valve supposedly selected for R134a for that core. Again, this all goes back a ways.

When I evac and recharge, I use gauges to make sure I'm not getting stupidly high pressures, but I don't have the feel for the gauges the way pros like you do. I learned the trick of charging until the line from the compressor to the evaporator starts to sweat with cold, and that seems to work pretty well.

My system has no pressure cut-off switch, only the temperature cut-off switch (the capillary tube going into the evaporator core connected to the temperature control that in turn cuts off the compressor). Like Larry's, mine cycles every so often, doing what I assumed was keeping the system from icing up.

The R134a system takes a little while to get the cabin cold, but once it's there, it maintains it very well. I've tinted the windows and installed a heater core bypass valve, both of which help. My next project is rebuilding the air box to block more of the outside "fresh air."

Like Larry, I was returning home from the Vintage and my a/c was keeping up nicely until I hit an hour of beastly hot humid weather in PA. Gradually it got warmer and warmer in the cabin and the air temperature out of the evap felt, obviously, warmer. Then I noticed I wasn't hearing the compressor cycling. When the slug of over-the-top heat and humidity passed, the compressor started cycling again, and the cabin cooled off nicely again.

--Do you think this was the compressor staying ON and not turning OFF, trying to maintain temperature?

--Or do you think this was the compressor staying OFF and not turning ON because things had iced up because of the high humidity?

Also, the temperature and pressure numbers you posted were for R12. What are the comparable seat-of-the-pants numbers for R134a?

Thanks for this fabulous contribution.

--Rob
 
Usually when the evap ices up you won't get any air flow with the fan running. It sounds like you just ran out of capacity and the compressor was doing all it could.
I'm in the midst of putting my system back together and have wondered if the Condensor fan runs all the time with the system running. My system was inop when I got the car so I have no baseline.....
Pressures should be 30psi low 200 high.... very similar to R12....just a little lower on the low side.
DaveG
 
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A few comments for you.. High humidity will turn your evaporator into an ice cube if your system doesn't cycle.
We're you draining water from the evaporator?
Was the compressor still running?
Ac pressures are very dependent on outside air temps- what pressures you see will vary on the temp.

http://www.ackits.com/aacf/ptchart.cfm

For normal temps think out side temp + 120% more to calculate your high pressure target.
 
BTW- I always install a middle heat shield unless the car has a ceramic exhaust.
 
Being a natural boneyard scrounger- I pay my 2 bucks and head over to see what is in stock.
Look for a late '80s BMW, they all have pretty much the right stuff after you snip and beat with a rubber mallet
 
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