merchauser

update: in early morning, with temps in low 70's, air feels nice and cold from start up. (nice). later in the day (85'), on start up AC air vent temp is 54' and not as cold as it should be.
getting up to any speed north of 35, air gets nice and cold, and a few minutes at a stop light has minimal effect. I understand this is a 30 year old car, but I don't think this is proper.

with increased air flow, AC blows ice cubes and is wonderful. at start up, with higher ambient temps, the cooling fans should take car of the air flow. I have had some voltage problems, and wonder
if that might be an issue, since the fans need full 12v signal to be at highest speed. or is it possible that I have too much R134 and pressure is a tad too high?

griffiths

I would not say just yet you are overcharged based on your past observations and notes.

Put your service gauges on the car and note ambient outside, your low and high at idle, as well as vent temp; as suggested in past thread posts.

If you have an IR gun shoot the connection point of the temp switch on the drier or bypass it and see if condenser fan speed increases, or run the fans with direct power.
Or, remove the temp switch and test it.

Note your observations, write it down, put it in a notebook. Refer to your past notes.

merchauser

will do thank you

dr bob

Paul --

Note that on your S4+ car, you don't have an actual "temperature switch". On top of the drier there's a pressure transducer that effectively translates high-side (condenser) pressure into equivalent temperature. From that reading the fan controller adjusts target fan speeds. The connections on that transducer need to be clean, especially since it's a very low-current loop. Do Not Jumper The Transducer as you might a switch. I think I remember reading somewhere that opening the loop (lifting one of the wires) will cause the fans to run fast.

The fan controller looks at AC pressure only when the system is running. ie: When the AC button is depressed. I'd have to go back through the electrical drawings to see whether it's the AC button itself or the second set of clutch relay contacts that connects to the fan controller. You can casually test for this with a cold engine-- If the cooling fans go immediately to at least low speed on AC button push, that circuit and the controller are working.

-----

Getting into a car that's been out in the sun for a bit, especially on a warm day, means that the AC system needs to pull accumulated heat out not just from the air in the cabin, but also from a lot of the solid masses in there. Plus in my case, a large gelatinous 100º mass plopped in the driver's seat. The actual temperature drop through the evaporator is fairly consistent, but on that warm soaked condition you are starting out with warmer air at the passenger's feet (air inlet to the system) so the air coming out will be similarly warmer. To get a good idea of how well the system is actually working, you'll want to be looking at the delta between the inlet near the floor and the outlet at the center vent rather than just the center vent temp. The center vent temp determines the apparent comfort level but is only part of the whole system performance determination.

This discussion then tangents to when using the rear system might help. In a heat-soaked car, you want to eventually pull heat from front and rear. From a total heat removal perspective, it makes some sense to run the rear system. With nobody riding back there though, I want to go to front-only to keep the gelatinous mass in the front seat exposed to the lowest possible vent temp.

FredR

If I have had the car out in the sun for any length of time I always run the car with the windows open for the first few minutes- the black bulb temperature is 84C over here so air at 45C helps get rid of that heat soak quite nicely and quite quickly. It also takes a little while getting the centre vent temps down as well so a win win situation.

Leaving the windows open initially sounds a bit counter intuitive but it works as does a good quality ceramic heat rejection film. You cannot stop the solar radiation unless you are parked under a sun shade [very common here] and the ceramic film helps the car cool down much quicker as it reduces noticeably the rate of solar heat absorption.

merchauser

weather has gotten warmer, but not able to take proper readings since my gauges have committed suicide

what I can say, is that while my AC is perhaps near a very good rating, it just does not seem as cold as it should be. at speed, any speed, with good air flow, AC feels colder. my cooling fans are working correctly, so we can take those off the table.

driving around in nominal traffic, I never get to turn the fans down, and I never get that "its freakin' cold in here, I need to turn the fans down" feeling. guessing that my components are all in good repair?

shopping for a reasonable digital gauge set, so I can get some accurate readings and report back properly.

dash temp readings are all over the map with my HF temp gun. if I pull the center vent, hold the gun against the opening, and shoot the beam at the coils, I can get 34', but I don't think that is how I should be getting my readings?

Speedtoys

Fifty-seven posts and two weeks ago, the advise to not use a temp gun was given, for such reasons.

GregBBRD

Wow.

There's a whole bunch of "Internet Myth" in this thread.....almost too much to even begin to attempt to dispel. However, I'll start and you guys can......argue.

Regarding evacuation:
Evacuation must be allowed to run for lengthy periods of time, to get the moisture out (or the system needs to be flushed with Nitrogen or Freon.) The reason is very simple....all one needs to do is "think" just a tiny bit. An open system has moisture laden air throughout the entire system. Pulling a vacuum boils the water, in the localized areas that the moisture laden air is located. That water is vapor at that point in time....with almost no circulation to move that vapor towards the vacuum pump. (If there's water vapor in the evaporator....it's virtually stuck in the evaporator.) "Short" vacuum pulls will not move that vapor. Longer vacuum pulls have more time to "pull" that vapor towards the vacuum pump. There will still be moisture in the system, when the vacuum is taken away. The "dryer" is designed to absorb this moisture. (The dryer must be replaced, each time the system is opened, because of this.) Flushing the system with Nitrogen will "push" the water out. Filling the system with Freon, recovering that Freon and running it through a recycle machine will also remove the moisture from the system.

Regarding oil:
Too much oil in the 928 A/C system is a very common problem....and the A/C efficiency drops like a rock with too much.

Charging systems:
Regardless of the Freon used, the cooling occurs when the liquid is sprayed from a high pressure area into a low pressure area, which flashes the liquid to a vapor (at the expansion valve, spraying into the evaporator.) This is how the system works. Liquid to vapor. Vapor makes cold. Regardless of the type of Freon used, if there are bubbles at the sight glass, there will be vapor at the expansion valve, reducing the amount of liquid that can flash to vapor.

928's in general.....why the rear A/C increases the front evaporator temperature:
In hot weather, there's simply not enough condenser/heat exchanger in the stock 928 to be able to deliver liquid to both the rear expansion valve and the front expansion valve (without the front "system" being extremely overfilled when the rear system is off.) In short, when the rear A/C is turned on, liquid from the front is diverted to the rear....increasing the front evaporator temperature. There's really only one way to make this work....Install a second condenser. I offer a fantastic conversion that allows both the front and the rear A/C to blow large volumes of ~32 degree air in hot weather, while reducing the temperature of the air that "hits" the radiator by 100 degrees. The front evaporator ends up with so much cooling capacity that I actually "need" to install a higher volume fan to keep the freeze switch from constantly switching off the compressor. The only downside (other than cost) is that the stock alternator, which is "taxed" already becomes even more marginal, at idle. I developed my high output alternator, entirely for this reason. This conversion will make the 928 A/C system operate as well as a late model car with fantastic A/C.

Finally:
Never jumper the freeze switch for the front evaporator. Turning the front evaporator into a block of ice will ensure that you need to spend even more money replacing the evaporator (not simple or cheap, on a 928.) If the freeze switch is bad, replace it.

Schocki

Amen to all what Greg said!

Little info: good evacuation of an AC system takes some time, half an hour "quick service" because somebody is running a promotional "after winter deal" is simply a waste of time. I don't like those fully automated AC machines, never have, never will. Old school is better.
Common mistake made, especially here in Europe with colder climates is to evacuate and charge the system before the season starts. If ambient temperatures are below 10 degrees C or 50 F, it is close to impossible to pull vacuum low enough to boil all the water in the system. You would need some sort of ultravacuum that can't be pulled with a regular pump. Wait for a nice and warm 70 F day or make sure that your shop is properly heated! Important!

I just finished a complete HVAC refurbish on my 92 GTS (evaporator leak) with a close to new 94 HVAC box provided by Roger. I only pulled vacuum in the cold winter here in Buchaest, because my garage is heated. However, I let the pump run for 3 hours, leak checked it initially for 72 hours. Closed the system for spring time and I drove the car 4 weeks around with vacuum pulled (BTW no loss in vacuum after this time ) until we had a nice spring day with 75 F. Opened the garage, pulled vacuum one more time and charged it with R-134a according to the factory recommended weight. Result: 37 F at the centre vent at idle...

Just my 50 cents....

928FIXER

What GregBBRD said.Good stuff.

One last observation,and pardon me if this has been mentioned before.

Has the actual flow of hot water to the heater core ever been pinch off to see what effect that would have on the front vent temp?I've seen LOTS of leaking water valves in my 40 years of fixing all types of cars.

In my old 79,I actually have a manual water shut off valve in the heater hoses.Once the time of year pasted that I would even think needing heat,I closed the valve.