Alfa Romeo Forums banner

Strange A/C Issue

1K views 11 replies 6 participants last post by  144 
#1 ·
Was driving the LS today with A/C on. Ambient temp around 87, humidity around 60-65 pct. After about a half hour I noticed A/c had stopped blowing cold. Took the car home and looked under hood. Saw compressor clutch wasn't engaged. Had to attend to several other chores,came back to car about 2 hours later. Started it and clutch was now engaged- A/C cold. Drove car for another half hour, A/C works fine again. Any ideas on what's about to fail here?
 
#2 ·
Since it came back on it's not a fuse or hard failure of the coil.

Was the radiator fan coming on? If that has failed, it's possible that the pressure in the AC system tripped the pressure switch. Or an intermittent failure of the AC control panel (Econ switch?).

The only thing that I can think of that would cut out the compressor is the hi/low pressure switch, i.e. pressure lower than 29 psi or higher than 391.5 psi. With it that hot, it's possible that the high side could easily get that high with the car not in forward motion and getting cooling air over the condenser.

Keep in mind that on these systems with a POA valve, the compressor should run constantly. If it doesn't, there's something amiss.
 
#3 ·
Also, if by some chance low side got cold enough to freeze up anti-frost switch will open groind to relay and disengage clutch.
 
#4 ·
I'm a real noob at this stuff so I soak up any discussion on it I can find and it's interesting to note what John pointed out that the compressor runs all the time if the system is ON and ECON switch is OFF. I hadn't latched on to that aspect before and since, in your case, it wasn't running and then it was that really narrows down the possibilities.

You could of course hook up a gauge set and look at the pressures but as it is with all intermittent problems, if they ain't happening at the time you aren't going to see anything. Might be some tell-tale borderline indications though.
 
#5 · (Edited)
Ran to dinner before finishing the thought.

Steve is, as always, correct. The frost switch could cause the compressor to disconnect. But, that said, the frost switch is also a fail-safe item. The POA valve should regulate the pressure so that the condenser doesn't freeze, with the frost switch as a backup. Given that the Alfa system is JBA (just barely adequate), not likely, unless the expansion valve was on the blink.

I'd recommend checking the fan comes on at the right pressure (217.5 psi), as a start.

The POA (pilot operated absolute) systems are rather old-fashioned technically and were pretty much abandoned in US car systems by the 70s in favor of compressor cycling systems regulated by a pressure switch. Not that they are bad, but do have another valve to go bad plus the added cost of another valve. Why Alfa went with one is a bit of a mystery to me. I believe systems sold by Alfa in Europe were not POA systems.
 
#6 ·
Mine is certainly JBA but it has been adequate for my purposes. When it comes to driving around town and coming out to a parked black on black car under the midday California sun however it can not cope. But even on days of at or near 100 degrees if I start out from the garage and drive to work it will keep me comfortable, though I may have to put the blower on 3 to do so.

With all of the alternative fuel and power vehicles being developed I came across this really odd little car made in India that runs on compressed air. One of the advantages of it is killer A/C as a side effect. Good in India I guess but it doesn't seem like it would work too well if you were stuck in an Indian traffic jam.
 
#7 ·
The usual cause of this is a faulty expansion valve seizing intermittently. Eventually it just seizes and the AC compressor overpressures every time it operates.

When the expansion valve restricts or blocks flow system pressure rises and triggers the hi pressure cut off.

I think normal operation cycles the hi pressure switch to turn the compressor on and off. The expansion valve controls the speed of circulation of refrigerant (it's a volume control device). 87F (30C) isn't particularly hot even for Alfa AC.
 
#8 · (Edited)
I think normal operation cycles the hi pressure switch to turn the compressor on and off.
In normal operation, the high pressure (217.5 psi) side of the pressure switch turns the radiator/condenser fan on and off. (217.5 on, 169 off). Here's the spec data for the various pressure break points:

COMPRESSOR
Low pressure cutout (<29 psi, compressor won't come on/cuts off)
Hi pressure safety cutout (>391.5 psi, compressor is cut off)

FAN
Hi pressure side cycling (217.5 psi, fan turns on // 159.5 psi fan turns off)

The absolute final safety is the over-pressure valve on the high side that will vent refrigerant at some value over 400 psi. A failed fan motor or fan switch and a failed hi-pressure cutout switch with no airflow over the condenser can easily cause this very exciting event, as Steve experienced a while back. And, of course, if the valve fails as well, then you blow a hose or the compressor head and vent all the refrigerant and compressor oil into a hot engine compartment. Memorable. Probably generate some choice language that your grandchildren have never hear from you before.

If you want see people to run away from your car, that would do it.
 
#9 ·
Johnalfa's problem could also be something simplier: a flaky spade connection at the compressor (the ca. 6" pigtail that where the compressor gets +). It's not the most sophisticated connector, and it's a terrible location for doses of road grim and engine oil. If the problem recurs it's worth the slight effort to reseat the connector.
 
#10 ·
Thanks everyone for the responses. I did note that the cooling fan was running when I opened the hood. Pinino thanks for the suggestion re the connector . Will check that when I get home tonight.
 
#12 · (Edited)
LOW SPEED FAN operates:

1. A/C "on" and hi press side at 217.5 psi or greater. Shuts off when pressure drops below 159.5 psi.

and/or

2. Engine coolant temp of 198F or greater (OEM thermal switch).

HIGH SPEED FAN operates:

1. Engine coolant temp of 206F or greater (OEM thermal switch).

If A/C is on and pressure is such that the fan should be on low, the fan will remain in HIGH speed for engine cooling.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top