whale driver

Just had my A/C system checked for proper level of Freon R134A. Curious why the front output is only cool and the rear unit is frigid, regardless of panel selection. Is there an issue in the dash HVAC assembly or expansion valve?

dr bob

The Most Likely Cause is heat introduced into the front system. A leaking or non-functioning heater control valve is a common cause. The valve is inexpensive, and it's located under the air cleaner housing on the right (passenger on US cars) side of the engine at the back of the head. Vacuum is applied to close the valve, so look under there to verify that the actuator is fully retracted by vacuum with engine running and AC on. Many in warmer climates go through the effort to cable-tie the actuator in the retracted position.

Also: The front system is sensitive to vacuum leaks in the actuator system. A leak in almost any section will affect the heater valve as shared above, and also the other doors and flaps that manage heat and air movement in the front of the cabin.


The rear system has no outside heat source and no vacuum actuators, so it's a good indicator of refrigeration-side capability.

merchauser

^^^ many posts mention that when using the rear AC, the front AC system becomes much less efficient? Those posts continue to discuss the condenser as the culprit, and the remedy is a better or additional condenser. I was under the impression that when using the rear AC the condenser is strained and thus, front temps are compromised. Dr. Bob, are you saying that if all components are in good order, that is false and that similar temps should be present at the front and rear?

Kevin in Atlanta

Does the AC start out ice cold on startup? Does it warm up as the car warms up?

whale driver

No, not icy cold, but the front does start cooling on start up. The rear if selected starts cold on start up. The front doesn't cool at its best when the rear is on. The rear still supplies cold air regardless.

Kevin in Atlanta

Disclaimer: I know nothing of the history of you AC system.

I am not an AC guy. I come to my knowledge from many years of owning more than a few 928s ('95-now) All my cars have been converted to 134a by previous owners and blow ice cubes. Typically, I don't use the rear AC as I don't need it.

If you are not starting out ice cold, feel at the pipes at the expansion valve. They should be too cold to touch.

I would start by finding a good AC guy. My 87, 90 and 91 do great in the GA heat. They freeze me out and I have to run the AC fan on 2 or 1.

The most important lesson I learned was to clean then entire system from stem to stern. This involves removing the expansion valves, bridging the pipes with hose and clamps and spraying the crap out of everything. SeanR uses a AC system cleaning setup. You cannot imagine the amount of contaminants that build up over the years. This means dropping the AC compressor and dumping out the contents and getting it clean as possible. It means disconnecting and replacing the AC drier, too. Renew all the o-rings if they are not already 134a compatible.

I spray brake cleaner into the condenser coil and blow it out with air. If you hold a shop rag over the other end you will be stunned with what comes out.

Pressure wash the radiator, too. From the back to the front. Loss of airflow to the condensing coil impacts overall preformance.

Have the proper type and amount of oil put into the compressor, button everything up, vacuum it and confirm it holds vacuum.

My AC guy is a wizard. I bought gauges and vacuum pump - but I have no use for them now. He fills the system to the gauges. You can ignore the cabin temp. Look at the pipes at the expansion valve. That should be too cold to touch.

To me that's the starting point. if the AC - front only is still not cooling it's time to look elsewhere.

Speedtoys

No..the front will still be 'warmer' when running the rear even with a perfect system on one condenser.

Speedtoys

Lots of words in the thread about issues..etc.

First, get some air temps after 10min of highway driving from the front and rear, both front only and with front and rear on.

Use a proby style, not an IR gun for temps.

you could be...normal.

dr bob

The system in the 928 is condenser-limited in hottest conditions. The front system especially on restart has a lot more heat sources. But it also has the largest capacity to move heat, so it's hard to make a universal statement about which should be colder.


Jeff has a Greg Brown system modification that adds more condensing capacity. In very high load conditions, like high ambient temp and/or high relative humidity, the modification moves the "weakest link" away from the condenser, and you can have both systems available to grab heat from cabin air to boil the refrigerant liquid in the evaporators.


I drove my black S4 yesterday in high-90's ambient conditions, humidity in the mid teens, slider at 70ºF, fan on speed 2, and the system was cycling the fresh air flap to maintain 70º cabin temp. More capacity with just the front on than the heat load demanded. Unless someone is sitting in the back seat and wants the cool airflow right there, there's no reason to run the rear system in my car at all. If the car has been hot-soaking a while, open one window all the way, open and close the opposite door a few times to rotate the greenhouse air out, then close everything and let the front AC do it's job. You can turn the rear on and move slightly more heat out of the cabin a little faster. If the car is that hot inside before the system balances temps out, I like that cold blast of freezing air from the front blowing right on me, ratyher than wait for the two systems together to drop temps more slowly in the rear where I'm not sitting. Just sayin'... Converted R-134a system, freeze switch has failed closed, so it;s full-tilt cooling the evaporator.

I took a little tour on Wednesday, about 90 mins or so, in 45% RH conditions. When I got home, the ice in the evaporator took about 15 mins to melt and drain out onto the catalyst crossover and hiss in my cool workbay. Airflow was fine when I drove in, but still there was ice to melt.

Roger, I need a freeze switch on my next parts order please. Evaporator is just too cold without it working.

Speedtoys

I always liked the rear AC on always on low..kept the back of my head/neck cool. Felt more like home where it wasnt just a frontal assault of cool air.

Michael Benno

Bob, I have a spare used, but calibrated freeze switch if you want it. I can mail it, but I suspect you can just re-calibrate yours and it will be fine

dr bob

mine is afu, doesn't open on cold regardless of any adjustment. It was fine in SoCal, and generally fine here. An hour and a half and no noticeable airflow reduction on a desert humid day. I'll take another shot at the one I have.