dr bob

Bill--

The freeeze switch in my car is adjustable, sort of. There's a coil spring, and a cute little bellcrank that transfers the action of the cap tube and bellows assy to actuate the snap-action microswitch. There's a screw "adjustment", accessible from outside the switch, that adjusts the tension from that coil spring, and therefore the amount of pressure from the temp sensor assy needed to get the switch to open/actuate. Counterclockwise on that screw = wamer. The snap-action swicth inside is a very common microswitch. A used spare I bought was perfect except the cap tube and bellows had leaked. The bellows is copper or brass, and it's likely that it has a limited fatigue life, and fails in the 'cold' position as a result. Both the one original on the car and that used spare have the same failure.

JHowell37

And let's not forget that POE (ester) is what should be used in cars that have been retrofitted. There's still great debate over the issue of ester compatibility with R-12. I've got a few spare compressors and I think and extra condenser coming my way. There's only one way to see if this is an urban legend or not...

blown 87

ester will work with R12, it just may not be the best choice.

worf928

Not very. I have yet to pull one out of any 928 that worked. The one in my '91 is about 8 years old now. I'll know - maybe - this winter if it still works, but I'm not betting on it as I've been cycling the compressor manually for about 3 years now.

blown 87

did you just bridge it and then push the "snowflake" button to cycle it?

SQLGuy

This thread seems to be in-line with my ongoing A/C project, so I'll throw my experiences and questions in as well:

When I got the car, the A/C didn't work. Over the last few months I've:

1. Flushed all lines, the rear evaporator, and the condensor
2. Replaced pretty much all the O-rings, the drier and both expansion valves
3. Dumped out the old oil from the compressor
4. Soldered leaking connections in the rear evaporator
5. Evacuated, converted to 134a and added POE oil
6. Replaced the bad A/C head relay
7. Replaced a leaking comb flap actuator bladder
8. Replaced the HCV
9. Adjusted the mixer flaps
10. Checked for leaks with freon "sniffer"

Currently, the A/C works, but it still doesn't seem to work as well as I'd like it to. All the components work correctly (clutch is staying engaged, recirc flap is opening, comb flap is opening, etc). I can't tell if the issue is a weak compressor or just the amount of heat soak a black on black 928 gets. The A/C shop that I had evacuate and recharge the system, to double-check it and ensure that a good vacuum had been applied, said the pressures looked fine.

When I went to lunch it was about 85F outside. I have an indoor/outdoor digital thermometer I was using to make some A/C measurements. The temp in the footwell was 112F, and inside the A/C center vent, on top of the evaporator, it was 124F. After running for a few minutes, on the road and moving at 50MPH or so, the front evaporator was down to about 60F. After 15 minutes it was down to about 51F.

I ran a similar test on my 2002 4Runner, taking it back to the office after lunch, it got down to about 45F inside the center vent, but I don't think the probe was as close to the evaporator as it was in the Porsche.

Am I asking too much? I see these postings from people getting 30F out of the center console in 100F days, and it makes me think there are still things wrong with my A/C. I previously had a '93 GTS that was converted to 134a, and it cooled quite well back when we were living in Orlando, which is plenty hotter than Colorado Springs; but that car used a different compressor.

Thoughts? Should I maybe be looking at one of these "6E171 replacement" compressors?

Thanks,
Paul

Bill Ball

Paul: I think all of our incredibly low vent temps are on a car that is not heat soaked. Still, my experience is that the 928 system gets chillier than any of my other cars, so I can understand your consternation if your 4Runner does better.

The initial 124F vent temp probably reflects the heater valve having popped open while the car was stopped, if the coolant was still hot, although heat-soaking of the dash components contributes. The valve takes about 15 seconds to close after start-up, at least mine does. I think 51F in 15 minutes is not bad. Eventually, the temps should be into the mid-30s, but that might take a while even at 85F if the car is heat-soaked. If I leave my garage on an 85F day, I have low 30s in the vents quickly and for the whole ride after that. But if my car is heat-soaked from sitting out in the sun for a few hours, it's a different story.

Calgary Ole

I also completly overhauld my AC as stated above including new bladders in the vacuum pods. There are hundreds of great posts on R-list that helped me along...One thing that I overlooked at first was my check valve at the brake booster which was an aftermarket unit. Since replacing with a new proper valve it seems my AC works a little better now.
Maybe some of our HVAC gurus can confirm that a proper working valve makes a difference or just my imagination?

dr bob

The HVAC vacuum system doesn't directly contribute to how cold the refrigeratrion side gets, but waek vacuum, leaky actuators and crumbing damper/flap seals will allow heat to migrate back in with the cold air in the airbox. For diagnosing the refrigeration side, and ASSuming that the system was ecavuated reasonably well before charging, the low-side pressure gauge is a very accurate indication of how cold it is inside the evaporator. Look on the gauge face, and locate a ring of numbers scaled as temps for your refrigerant. The temp on the outside metal surface of the evaporator will be close to the internal gas temp, warmed by air flowing on the outside surfaces, so on a day with higher cabin AC load, the difference will be larger than on a cooler or lower-load day.

To allow the system work well, the evaporator surfaces need to be really clean. Pre-GTS cars have no pre-filtering of air passing into the evaporator, so there's no reason to believe that the evaporator isn't enjoying a coating of caked- and baked-on dust turned to mud pack. Moisture accumulates on the surface of the evaporator when it's in service, so any airborne dust risks being captured on the evaporator. It could flow out with the condensed moisture, but it seems like a lot still hangs on to the fins and builds a caked-on layer. That protective mud coating is an insulator that keeps heat from transfering to the freon in the evaporator. It also slows airflow through the evaporator, again reducing the amount of heat that can be extracted from cabin air.

You can inspect the condition of the evaporator surfaces through the boot that connects the blower motor to the airbox. Under the hood, remove the plastic rain shield at the base of the windshield, behind the engine under the hood. The rubber acordian bellows on the passenger side can be drawn back, and with a mirror, a flashlight and some head twisting, you can see some of the surface of the evaporator. It may be more practical to unplug and at least partially remove the blower resistor pack where it sits in the front of the airbox, so you can look directly at the front face of the evaporator.

Got a dirty or caked-up evaporator? Of course you do! There are a few cleaning methods, of course. First, spray some water from a squirt bottle in there and make sure it comes out the drain hose in the middle of the car, over the exhaust ppipes near the front of the catalysts. Once you are sure that the water has a good way to get through the drain, you can start spraying some more and get at least the big dirt washed down through that drain. Don't plug the drain by doing to much dirt with not enough flushing. Plus, check the footwells inside for water from your spray bottle, especially over the CE panel. Put a towel over that area just to be safe.

For the actual cleaning, there are pro AC products, or you can use stuff that you are comfortable with and will probably have at home already. I mix a dilute solution of a non-ammoniated window-wash solution, for instance, and load it into the garden sprayer. Spray it all well up in there, then rinse gently. Don't overload the drains! Keep at it until the evaporator looks like new shiny raw aluminum again. You can help agitate the crud a little with a parts brush, so long as you don't bend the fins much. I have a few old soft giant toothbrush style brushes that help some. You may end up with your brush taoed to an extension of some kind; Get creative with an extension of some kind so you can extend your brush's reach as needed.

Rinse, repeat, until all the dust and caked dirt are completely flushed through and out.

blown 87

I have cleaned many home AC evaps, and my dumb A$$ never even thought about cleaning a automotive core, but most i take out are not what i would call dirty.

I have just the car to check tomorrow, a 928 that we did a R12 reconversion on that i do not think is getting cold enough.
I will get some temp and pressure measurements tomorrow on it.
The pressures look good, about 30 on the low side and 225 on the high at about 1500 RPM, all the pods are good, the heater valve seems to be working, it will have a new one in the AM and I have a new freeze switch coming for it.

That will only leave the foam, the electrical controlled flap and cleaning the core.
It also has a semi plugged drain, it will get pulled out tomorrow, how much does a wet evap core cut down on cooling?
It was spitting water at me today, but still draining on the floor so i know something is plugged up.

Tails

To get a fully operational system to the best extent possible all the components must be operating as close as possible to the designed parameters, that is the compressor, the condenser, the thermostatic expansion valve, and the evaporator.

drbob has described how to clean the evaporator and the condenser in previous posts. A bug, leaf, dirt and cigarette free condenser and evaporator will enhance the operation of the system, especially if the fins and tubes are nice and bright thereby giving good heat transfer by convection.

The internals of the system also need to be clean, by flushing and drying and then reinstated with the correct amount of oil, after all themoisture is removed and new receiver dryer fitted to take care of any moisture that is introduced into the system.

In my first post I mentionsed drying the system by adding a small charge of R12 to absorb any remaining moisture, this will work with R134a as well and re-evacuate the system.

As the air conditioning unit is a heat pump via the latent heat of vapourisation when the liquid freon is expanded to a gas at the outlet of the expansion valve into the evaporator which removes the heat from the air as it passes over the finned tubes, by convection.

Any water in the air (relative humidity) or water on the fins will also give up heat to the evaporator, the air will transit through the ducts and the water will drain away, so you are loosing some of the cooling effect given to cooling the water which drains away and does nothing to cooling the cabin. The higher the relative humidity the less efficient the system is.

Dehumidifers (some times called a Thermal Mass) are only air conditioning units that remove the water from the air, so the system is only used to create dry air not cold air, so airconditioning systems are also dehumidifiers.

Some of you may have noted in aircraft that the air conditioning system can make the air extremely dry (good for drinking more beer), as they have quite a large amount of recirculated air and aircraft systems are also fitted with water spray after the evaporator to slightly re-humidify the air as the cabins are sealed and pressurised.

Our cars do not need this degree of sophistication, there is enough leakage into the cabin to keep the air with some moisture in it.

To get the maximum cooling effect on a hot day you need to have the recirculating vent open, however the extent of the cooling depends on whether the car is first driven out of a cool garage (as mentioned by Bill in his post #22 above) or it has been parked in the sun and become heat soaked by radiation.

Unfortunately if the car is parked on the sun after reaching full operating temperature we have another effect on the aircondition system that is the vacuum controlled heater valve that will open fully depending how long the vacuum system can hold a vacuum, as a leaking vacuum actuator in the system is one of the air conditioning systems worst enemy in this case, as it will allow the heater valve to open quickly and thermocycling will allow hot engine cooling water, around 194 degrees F, flow into the cabin heat exchanger that will heat up the air box even higher that the sun's heat that has heated up the cabin when parked.

Add to the mix is also the colour of the car, black being the worst, being the best absorber of heat and white being the best, the less absorber of heat.

In the case of pumping out the heat from the cabin on a hot day as fast as possible I would suggest the following procedure:

1. recirculating flap open,
2. temperature slider to maximum cold,
3. crack open a window, as the outside temperature will be cooler than the cabin temperatue,
4. air condition on as you start the car, as this will give both condenser fans operating at full speed, vent flaps fully open (if fitted) on S4s, GTs and GTSs up until the flaps were removed.
5. select cabin vent fan speed that is the most efficient in giving cold air. (if the system is not working at peak efficiency then at full cabin fan speed the velociy of the air through the evaporator will not be cooled to the maximum exttent possible, so a lower fan speed may be necessary).
6. Leave the aft a/c unit off to start as it will make the forward evaporator more efficient (maximum BTUs removed by the forward evaporator) as this unit has to do the most work due to the hot heat exchanger caused by the heater valve being open.
7.if there is a vacuum leak then it will take a little longer to shut the heater valve, so the cool air will first have to cool down the heat exchanger and overcome the heat being given off by the heat exchanger. If the valve is leaking, it could take longer still,
7. If you have a poor seal on the double flaps in front of the heat exchanger, or they are not adjusted correctly then there will be heat seep into the cooled air, and
8. if the mixing flap (comb flap) actuator rubber diaphragm is spit then there will be a mixture of hot and cold air coming from the centre vent, hot at the top and cold at the bottom as there is a slight thermo split as the air is not being mixed correctly.
9. When the cabin is down to it coolest then the after unit can be engaged and the cabin temperature will stabalise, mindful that the control circuit via the cabin sensor (its inlet located on the dash facia) and the outside air temperature is controlling the operation of the compressors clutch.

If the system is working correctly you can have the fan speed on maximum, however, to get the lowest cabin temperature it is best to gradually reduce the fan speed. I usually run with the fan speed on 2.

Fortunately on a hot day when I have the aircondition on I also get a lower engine cooling water temperature, especially in city stop-start traffic due to the full speed operation of the two cooling fans. I would suggest that if this happens it is a good sign that the condenser and the radiator fin and tubes are clean and doing their job correctly. I would also suggest that if the engine temperature increased then I would schedule a condenser and radiator fin and tube clean ASAP.

Tails 1990 928S4 Auto.

SQLGuy

There are a few theoretical points here I'm not sure I agree with. For instance, you talk about the HCV staying closed depending on how long the system can hold vacuum. However, the vacuum solenoid that controls the HCV does not hold anything... it either connects to vacuum (when engergized) or vents (when not energized). So, as soon as the car is shut off the solenoid will be deenergized, vent, and open the HCV.

Also,

"6. Leave the aft a/c unit off to start as it will make the forward evaporator more efficient (maximum BTUs removed by the forward evaporator) as this unit has to do the most work due to the hot heat exchanger caused by the heater valve being open.
7.if there is a vacuum leak then it will take a little longer to shut the heater valve, so the cool air will first have to cool down the heat exchanger and overcome the heat being given off by the heat exchanger. If the valve is leaking, it could take longer still,
7. If you have a poor seal on the double flaps in front of the heat exchanger, or they are not adjusted correctly then there will be heat seep into the cooled air, and"

If the mixer flaps are well adjusted and have a decent seal, the cold air won't cool down the heater core, because the cold air won't be going through the heater core. The result should be a heater core that remains hot for a long time, but doesn't have too much effect, because it's cut off from air flow through the plenum.