denhat

This must be my weekend for AC problems. First my Lexus SC300 had no AC,only needed a recharge and now the 928. The 928 is a 1987 with dual AC. The rear AC works fine(measured 45F) but the front on this 90 degree day blows only cool to somewhat warm air. The sight glass has a continuous stream of bubbles indicating low freon??, but why would the rear be cold. The other thing is the flaps controled by fuse#23. With the fuse in they make a constant rapid flapping (fluttering) sound. I pulled the fuse, flapping noise stopped but the front air was still cool-somewhat warm) Any ideas???I did a search on AC problems but with my limited mechanical ability I thought someone might be able to quickly narrow the possibilities down.

Thanks!!!

Rich9928p

Hello,

It sounds like you may have several issues. I'll discuss the easy to more difficult issues.

a. Cool rear A/C: This is good news. It shows the compressor is working and you have enough Freon to cool.

b. Cooling flaps. The S4 cooling flaps should be open when the A/C is turned on. If the flaps are acting strange, such as fluttering, remove the fuse when the ignition is off. The flaps should be in the full open mode then. Double check to confirm that they are open. There is no need to concern yourself with fixing them your 928 is able to manage engine temperature without them.

c. Warm air out of the front vents. It could be that there is sufficient coolant, however, there could be an actuator problem with either the air mixers or the heater control valve. Problems here would dump hot air into the cold air output and thus you'd get not so cool air. Things to check:

- heater control valve: this is the main problem for two reasons. It is plastic and wears with age, and leaks either at the water valve or at the vacuum motor. It also should be changed every 4 years or so along with the short bit of hose that connects it to the engine block. It is located under the air filter box at the rear of the engine. I'd bet this is your problem.

- A/C dash vacuum motors. The 928 has a vacuum controlled air diversion system and electronic controlled air temp mixer door. The diaphrams in the vacuum motors leak with age and you may not get a full opening in the center vent - cool air will be diverted into the heater duct and defroster vents. This may be noticed under acceleration when engine vacuum drops. A "cheap" fix is to remove the center vent, pry the center vent open, and zip-tie the rod to keep the door open at all times. This is OK for warm climates, but it also means that hot air in the winter blows from the center vent. The "real" fix is to tear the dash apart and replace the vacuum motors. This can be a fairly expensive task and you should only have a shop that has successfuly done this before do the fix because there can be a lot screwed up on the process!

d. Freon level. You are best to charge the system by Freon mass vs. pressures (which very with ambient temperature). A professional shop has a system that will purge the system and refill it with the proper amount of Freon and also add the necessary amount of refrigerant oil.

denhat

Thanks Rich,

Just so I got this. Will the flaps have anything to do with the front cooling? (I did remove fuse#23 but they had no affect on the front cooling) Also,where are they located? Your last statement indicates it still is possible to have a low freon problem with the rear air appearing to work correctly?? Thanks!!

Rich9928p

The flaps were developed to reduce the wind resistance by letting in only as much cooling air as needed. You need as much airflow to cool the condensor, so the logic is to open the flaps to full when the A/C is on. If the flaps are not open fully, the condensor may not get the necessary airflow. So, the flaps should be fully open.

The rear air has a much smaller evaporator than the front system. You may still have good cooling from the rear even if there isn't a full charge to cool the front evaporator.

Your A/C man would tell you if you have other issues, such as the need for an expansion valve.

denhat

Got it!!!

Thanks,

Daniel Dudley

It is true that if the heat is on even the tiniest bit it will seem as if there is an AC problem.

WallyP

denhat,

Send me an email at 928 Specialists, and I will send you a detailed paper on the HVAC system.