traxtar928

I'm guessing the constant hiss is refrigerant going through the expansion valve. However, the air conditioning temperature is very inconsistent. Is the hiss indicative of a pressure or refrigerant issue?

Also (and this might sound silly), but after the car's been sitting outside in the hot sun all day (at work), and at startup, the air will blow HOT for a few minutes. Enough time that I start to sweat while driving. The recirculation flap is actuated so it's pulling cabin air, which is not nearly as hot as the air coming out of the vents.

While driving, it's reasonably cold (haven't stuck a thermometer to check the delta from outside temps). However, at idle it's just cool. In my mind, this points to the fact that a moving car has a better ability to exchange heat, but I'd think that an idling car should still have cold AC. Do others experience 'cool, not cold' temps for air con at idle as normal functionality?

Do these symptoms point to a specific failure mode? What other components can I look at to check or replace?

I've checked the freeze switch, low pressure switch, and the 14 pin connector for proper voltages and open/close functionality, and all seemed normal and operate as intended.

After driving the car for ~10 minutes, the air conditioning is consistently/adequately cold (except at idle, it's not quite as cold). In my opinion, this is too long to get the HVAC down to temp. Maybe a heater valve is taking too long to close, or never fully closing?

http://jenniskens.livedsl.nl/Technic...4/MyTip431.htm

That link makes me think the heater valve needs replacement.... just want to be sure before I start buying components dealing with HVAC trying to chase the issue down.

hb253

My experience in every car I've ever owned is that if you hear hiss from the expansion valve and the system is not consistently cold, you're low on refrigerant.

Hugo

traxtar928

I just went out to the car to go to lunch, and noticed that the AC is cycling. I can hear the 'hissing noise' and the RPM's will dip, indicating that the compressor is working, and it will suddenly go away. The air from the vents is HOT and did not cool off in the 3-4 minutes I was out there. When the AC turns off by itself, pushing the switch has no effect. I cannot turn it back on, though I hear the relay engaging.

So, some sensor is cutting voltage to the compressor. Any ideas?

griffiths

When was the last time you have the system, evacuated, charged and tested?

traxtar928

Did some diagnostics, and it looks like adding more refrigerant should solve the issue. Looking through the low pressure sightglass, it's a white frothy mess while the compressor is running, and you can see bubbles when the low side wiring harness is disconnected and the compressor stops. I think that the car needs some time for the pressure to increase and the low pressure switch stops cutting power to the compressor.

griffiths

Some R134a conversions make it difficult to see in a site glass (milky).

Pull a vacuum on the system and recharge it properly.

traxtar928

It's milky through the site glass. I had Sean R pull a vacuum on the system last summer and do a leak check, and he said it was operating fine. His recommendation is to top it up with R-134A from the parts store.

WallyP

Parts store R-134a = $12-15.
Walmart R-134a = $4.50.

Same stuff.

Don't use any with "leak-stop" or anything else - just pure R-134a.

Speedtoys

My system held GREAT under vacuum.

Did not hold under pressure.

If you're topping off, you're leaking.

Nicole

I would also check the vacuum system. There is a 4-way connector in the engine compartment, drivers side, below the air box near the brake booster. That's a good starting point.

All the rubber connectors are probably hard and many leak by now, given the age of your 928. Don't forget there is a rubber piece under the driver side fender as well, where the line goes into the reservoir.

You may find that you'll have to replace some of the flap actuators under the dash or console. I also had to replace the rubber manifold behind the solenoids, as it had become too hard to seal properly.

hb253

Same here, the trusted local shop I use for my other cars pulled a vacuum for 45 minutes with no leaks. Filled it with R12 and it leaked out within two weeks. I'm saving up for a Griffiths R134 conversiin, hope they can install it for me.

Hugo

traxtar928

I've already gone through all the diaphragms (besides the one behind the pod) and vacuum system, tracking down an issue where the recirculation flap wasn't closing when AC was on.

I did notice that the low pressure switch was opening (checked all the components for continuity when the clutch turned off by iteself) every few minutes while the engine was cold, cutting power to the clutch. So that explains why vent temps were so high while the car was sitting in the hot sun all day. After about 3-4 minutes of the engine running, the LP switch would stay closed.

I have an appointment with Sean in September and will probably have him evacuate the refrigerant, drain the oil, re-seal the system with new o-rings, expansion valve, and start fresh. I topped off the system this past weekend with 1 small can, although the dummy gauge on the bottle said the low side was at 35psi while the compressor was running. I did get the froth in the sightglass to go away and stopped filling as soon as I saw that happen. Might need a low-pressure switch if it still cuts out after topping off.

Mrmerlin

how old is the heater control valve?
if they are older than 5 years I would replace the HCV and the short hosem Otherwise remove yours and see if the internal seal has come loose and is letting coolant leak by the flap

Bill Ball

The other clue that the heater valve is failing internally is that the vent air will be warmer than ambient even though the valve appears to close properly (arm moves with vacuum). The valve takes ~15 seconds to close after the car is started, so warm air initially is expected, but once the car is under way the vent temp should soon be the same as ambient if you have the temp set to full cold and no AC on, of course.

traxtar928

That's an easy fix. I haven't replaced the valve in my 5 years of ownership, so I'll add it to the list of things to pick up.