Tails

In view of some previous posts on the aircondition system within the HVAC system, especially claims of low tempreature from cabin vents, I did some research and report as follows

R12 freezes at -140 degrees C (-280 degrees F)
It exists as a liquid between -140 to- 30 degrees C (-228 to -22 degrees F)
It exists as a gas above -30 degrees C (-22 degrees F)
R12 leaves the expansion valve around -26 degrees F
At maximum cabin cooling, the evaporator cools the ambient air and the freeze switch is set to release the compressor clutch at 0 degrees C (32 degrees F). And re-engage the clutch at around 3 degrees C (37.5 degrees F) , so in theory, the lowest temperature at the outlet could only be 32 degrees if the system is working very efficiently, however the maximum cooling effect of the incomming air is around 25 decrees C.

The actual outlet temperature depends upon:
1. Ambient air temperature
2. Air velocity through the evaporator
3. Air capacity flow through the evaporator
4. External and internal cleanliness of the evaporator – affected by blockage externally or contaminants internally
5. Relative humidity of the ambient air – the higher the relative humidity the less efficient of the evaporator in cooling the air
6. The design efficiency of the evaporator
7. The efficiency of the expansion valve in achieving the maximum value of the latent heat of vaporisation of the liquid to a gas, in removing the heat from the air.
8. Quantity of the recirculated air – recirculating already cooled air.

The cooling effect of the R12 system is influenced by:
Outside air temperature:
1. 40 degrees C (104 degrees F), the outlet temperature at the centre vent 12 to 15 degrees C (54 to 59 degrees F)
2. 30 degrees C (86 degrees F), the outlet temperature at the centre vent 6 to 10 degrees C ( 42 to 50 degrees F).
3. 15 degrees C (59 degrees F), the outlet temperature at the centre vent 2.5 to 5 degrees C (34 to 41 degrees F).

To high an Outlet Temperature:
1. Fresh air vent not closed (recirculating flap not opened),
2. Air conditioning system not at maximum output,
3. Not enough or too much refrigerant in system,
4. Condenser heavily soiled restricting flow,
5. Anti-icing switch switching off compressor clutch to early,
6. The inserted depth of the evaporator temperature sensor not correct
7. Drive belt incorrectly tensioned (slipping).

Outlet temperature too low:
1. Anti-icing switch faulty, or
2. Temperature sensor inserted too low in evaporator,
3. Correct quantity of R12,
4. Correct quantity of lubricating oil.

Evacuating the System.
Good vacuum 0.0145 psi.
1. Connect vacuum pump to suction and discharge side of compressor to evacuate all the gas and water.
2. Add 100 grams of R12 liquid metered into system to dry out system.
3. Evacuate system again, moisture and R12 evacuated
4. Recharge system with R12 by gas into suction and discharge side of compressor until gas pressure stabilises on both gauges without running compressor.
5. Connect gas inlet hose to system, but first fully flush air from charging hose by cracking open bottle valve and then nipping up the charge hose connection to the suction side of the compressor.
Charging with R12
1. At ambient temperature between 86 to 95 degrees F.
2. Engine speed 2,000 rpm
3. Temperature setting – Maximum Cooling.
4. Normal operation the pressure gauges should indicate approximately:
5. Suction: 2 bar ( 21.75 to 29 psi).
6. Discharge: 15 bar (210 to 217.5 psi).

Insufficient Cooling
Symptons:
1. Suction and discharge pressures too low, suction 11.6 psi – discharge 116 to 130.5 psi, as a guide.
2. Air bubbles in sight glass
3. Air from centre vent not cold enough.

System overfilled with R12.
1. Cooling in condenser is insufficient,
2. S4, GT & GTS two cooling fans not operating at full speed or only one cooling fan operating
3. Condenser air flow restricted due to restriction of dirt or oil or too much oil in system
4. Insufficient oil in system (excess friction in compressor).

Excess air in system.
1. Suction and discharge pressures too high, suction pressure 36.25 psi and discharge pressure 333.5 psi,
2. Suction outlet from condenser not properly cold (insufficient cooling in condenser),
3. The causes of air in system- system not properly evacuated.
4. Remedy:
5. Renew dryer (moisture saturation),
6. Check oil for quantity and contamination,
7. Evacuate system.
Important note: If system is charge without evacuation the system the suction and discharge pressures could be approximately 2.5 bar to 23 bar respectively will be shown on the gauges.
Moisture in system.
1. Moisture will cause periodic interruption of A/C system operation. The symptoms are:
2. The suction and discharge pressure will fluctuate- the suction pressure will fluctuate from arund 20”’ to 21.75 inches of mercury and the discharge pressure will fluctuate from 101.5 to 217.5 psi.
Causes:
1. Moisture freezes and melts, and
2. Dry oversaturated.
Remedy:
1. Replace dryer,
2. Evacuate System
3. Recharge with R12

If dirt in system.
1. Clean expansion valve with compressed air,
2. Flush system and replace dryer,
3. Evacuate system, and
4. Recharge with R12.

Faulty Expansion Valve, open too wide.
1. Frost and heavy moisture on suction side of condenser, replace expansion valve.

Last item.
Faulty Compressor. Replace compressor.

Tails 1990 928S4 Auto

ROG100

Tails awesome info and already added to Wally's HVAC write up.

I guess I am still confused about temperature from the center vent.
At what point at or infront of the center vent do you measure the temperature?
Most owners and workshops here in my area use a IR gun. Very easy to use but impossible when trying to read the temperature in mid air.

As already said I have readings of lower temperatures when using an IR with the beam on the matrix of the heater or plastic vent fin - solid object.
Prior to using the RF I used an analog thermometer with a 6" long probe pushed into the center vent.
In my R134a GTS it would always read low 30's with full recirc at speed on the highway in 100F conditions. Until the compressor clutch burned out that is.

So based on your factual statement (54 to 59F) why do we see lower tempertures of circa 30 to 35F on an ambient 104F day.

When do you take the temperature measurement? When the car is static or moveing? Does not the speed of air passing through the condensor have an effect also.

Porsche in the WSM expects us to see much lower vent temperatures for R134a than R12.
Is this saying that R134a is better than R12?

I am now waiting for the temperature to rise to 100F and go and take measurements on my 87 with R12. I will use the thermometer and IR to see what results I get.

dr bob

Roger--

some quick hints. Use the probe thermometer instead of the IR. More consistent, reads air temp instead of plastic vent. Probe is more accurate. I stick the probe all the way back in the space between upper and lower vent grills.

The WS. Shows different/better GTS results with r-134a at ambients above about 90F. I'm not sure why they would show that unless the condenser design changed. It may also be that the multiple compressor upgrades between early and late cars are finally documented to show the performance improvements.

blown 87

I have done back to back r12 to 134a to r12 conversions as a test, granted not in a 928 but my findings were that the r12 was a the best choice.
I use a digital temp probe for checking the air temp, not a IR gun.

Bill Ball

Rog: Forget the IR gun for vent temps. It is heavily influenced by the shininess of the surface. Use a common AC temp probe.

Tails - comprehensive as always. My vent temps do get down to 28F, so I already suspected my freeze switch is off calibration. Some of the other Porsche models have adjustable freeze switches. Not us.

borland

The 0.0145 psi vacuum stated in the above post is about 750 Mircons of mercury.

You really need that kind of vacuum to make sure your removing any traces of flush solvent and water.

System leaks can go undected for weeks unless you have one the thermistor vacuum gages. Using one of these will provide assurance that your vacuum pump can reach the level of vacuum needed to remove water and ensure that you have no significant system leaks. Otherwise it's just guess work.

Tails

Roger & Bill
Thanks of the comments.

If you use a digital thermometer or an mercury in glass thermometer I would first place the thermometer at the centre outlet as this will be the reading of the mixed air, in case your heater flaps are leaking.

Secondly I would then place the thermometer as far back over the heater core to the evaporator as possible to obtain the temperature of the air exiting the evaporator.

I would take these two sets of readings with the recirculating flap open and the flap shut, that is the get the best results with the ambient air intake and the recirculated air (ambient air shut off), also recorded the ambient air temperature when the car is stationary at 2000rpm and then again on the highway at speed to get the cooling effect of the forced air induction through the condenser. I would also record the fan setting for each set of reading as the higher the velocity of air through the evaporator the less transfer of heat from the air into the gas through the evaporator.

I would then build a matrix table of the results for comparison.

On an 104 degree day on recirculating air, highway driving with both electrical fan operating at full speed and the HVAC fan operating at it lowest speed, the recirculating air is dry, as the moisture has already been removed, so you should see a lower temperature at the outlet, if the heater flaps are fully shut and sealed correctly.

If you actually get a 30 degree F outlet then the anti-icing switch is not working correctly, as the temperature at the evaporator sould be at it lowest around 32 degrees F, as the compressor should switch off.

As you have seen from the above there are numerous variables that can impact on the efficiency of the system and to get 32 degrees F the evaporator would have to be very clean internally and externally.

My belief if 134a is used in a R12 system it will not perform as well as R12, however, in 134a system that has been specifically designed to achieved the required performace with this gas,, so it should be as efficient as an R12 system, as both system are controlled by the anti-icing switch, set at 32 degrees F to release the compressor's clutch.

Tails 1990 928S4 Auto.

blown 87

I should have said in a system designed for R12 I prefer R12.
We sure see a lot more compressor failures with 134 than we did with R12, as most of the compressors were designed from the start to use R12 and lower pressures.

How reliable are the freeze switches in our 928's?
After I have been driving mine for say an hour you can feel the outlet air go warmer and then come back out very cold in cycles.
I think it could be the freeze switch.

Tails

Greg,

I would suggest that your freeze switch or moisture in the system.

If it is the freeze switch then the evaporator is icing up and melting. You may need to replace the anti-icing switch or at least check it operation in ice water and warm water for it on and off operation at what temperatures.

You should also check the capilary tube location in your evaporator by removeing the fan and the rubber boot from the air inlet duct.

I have extracted the following from my original post regarding moisture in the system.

Moisture in system.
1. Moisture will cause periodic interruption of A/C system operation. The symptoms are:
2. The suction and discharge pressure will fluctuate- the suction pressure will fluctuate from arund 20”’ to 21.75 inches of mercury and the discharge pressure will fluctuate from 101.5 to 217.5 psi.
Causes:
1. Moisture freezes and melts, and
2. Dry oversaturated.
Remedy:
1. Replace dryer,
2. Evacuate System
3. Recharge with R12

Tails 1990 928S4 Auto

blown 87

I think it must be the freeze switch, every thing else should be good, we did a new drier and had it on a 5 CFM pump with new oil for 15 hours.

Fogey1

Handy device, great probe. Ours have been very accurate (based on insertion in a pan of boiling water and in a glass of ice water slush.) Also useful for measuring the temp of the return stream into the coolant bottle.

Great for the kitchen, as well - no more dry pork roasts - and something you can use in the garage without upsetting the cook.

Polder Original Cooking All-In-One Timer/Thermometer
http://www.amazon.com/Polder-Origina...0154512&sr=8-2

dprantl

The freeze switch in my 928 that I bought new crapped out in 1.5 years. I also use a digital temp probe stuck directly into the center vent a couple of inches.

Also, it is important to remember that the center vent temp depends on your recirculation flap. If it is 104 deg F outside and the system is cooling fresh outside air, you will never get colder air at the center vent than ~55 deg F. If the flap is set to recirculate, it will start off blowing in the 50's, and as the air gets colder, the center vent should drop all the way to 32 deg F when the freeze switch will kick in.

Dan
'91 928GT S/C 475hp/460lb.ft

GeorgeM

I'm with Will... a good thermometer is the key to the perfect pork loin/roast. Cook 'til ~25F shy of done (FDA recommends 160F min), then cover with foil and let it sit for 15-20 mins. I cook it on the grill, start with a good sear on both sides, then turn the fire down and place in a 9x13 pyrex with at least a half cup of good olive oil, on top of an air-bake pan. You can baste it several times with the oil if you want.

Result: Juicy, tender pork loin. I don't even bother with tenderloins, a perfectly cooked standard loin is tastier, and cheaper.

CraigL

Since this seems to be a 'hitting to all fields' A/C thread, I'd like to know what happens to the oil when the system is charged and used over time. For example, I recently changed the compressor, drier/accumulator, valves and flushed the lines. The new compressor was a Denso 10PA20C.

I put 140cc of oil in the system and distributed it:
- Compressor 40%
- Evaporator 35%
- Condenser 15%
- Receiver/Drier and lines 10%

After running the system, is the oil carried from one part of the system to other parts?
Or does it stay in these locations forever?

WallyP

Yes, the oil circulates with the refrigerant, and the numbers that you used are an estimate of where the oil is after continued usage.

If the refrigerant gets very low, the oil leaves the compressor and doesn't make it back, so the compressor fails due to lack of lubrication.

If you continue to add oil to a system that hasn't lost an equivalent amount, you end up with excessive oil, which cuts cooling performance.

The amount of oil in a 928 varies with the compressor that is installed, and whether or not the optional rear air system is installed.

If you replace a component, you need to replace the amount of oil that would have been in that component, and only that amount.

If you flush the system, you should replace the amount of oil that you would have removed. It is difficult to completely flush a system without completely disassembling it.

If you use poor-quality oil, it can coke in the condenser and cut performance.

If you use the wrong oil, it will not circulate with the refrigerant, and the compressor will die. Mineral oil will not work with R-134a. PAG will not work with R-12. POE will work with either. PAG is in factory R-134a systems, mineral oil is in factory R-12 systems.