surgiblade

Firstly it has been great meeting othger 928 owners and discovering planet Porsche 928- These forums and technical tips on line are excellent and have been most helpful.

A number of the technical tips describe the air conditioner not working when it is really hot, and suggest replacing the heat bypass valve on the right passeger side behind the air filter. Unfortunately these otherwise excellent articles do not mention how to get to the air filter and valve- although they describe in great detail how to remove the air filter and replace the valve.

How does one get to the air filter and bypass pump with a minimum of disscetion ??

Thanks
Mike
hmikej@sbcglobal.net

Bill51sdr

Hi Mike,

The valve you are referring to is actually the heater control valve and can be somewhat notorious for leaking internally when old, thus allowing hot coolant into the heater core at all times. A more common problem is a fault or leak in the HVAC vacuum system that allows this valve to remain open at all times.

This valve is located under the airbox and can be easily seen and worked on once the airbox is removed. If you have a fault in the vacuum system, the easiest (read: temporary) solution is to strap the valve shut, and assuming the valve is not leaking internally, stops the flow of coolant into the core. Use the search function as there are many articles posted here about how to troublshoot and repair this all-too-common malady. Good luck and welcome to the fold!

Jack '84 928s

open your airbox, the big black thing at the back of the motor, take airfilter out. Remove the 4 bolts holding it in place. move it to the driver side of the car. There is a thing that looks like a ball kinda of with a vavum live going to it and little lever looking thing going in it. On the passenger side.

surgiblade

Bill-

Thanks much- still, when I open the hood and look furthest backtoward the windshield, there is a 4 inch black ledge protruding under which I asume is the air filter and valve- how do I get this ledge off with minimum damage/dissection.Figured I'd ask before trying to wing it.


Mike
87 927s4

Bill51sdr

You're looking too far back if I'm reading you correctly. The air filter is within the large, rectangular box that is attached to the air inlet tubes, in FRONT of the firewall. You need to remove the tubes, undo the 4 straps holding the top of the airbox on, remove (and I'm sure clean or replace) the air filter, which will reveal the nuts/washers holding the airbox on. You don't mention what year 928 you have, but if it's an '87 or newer S4, you should be able to just lift the airbox bottom from the mass airflow sensor. If you have an 86 or older, you will probably have to deal with a clamp holding the AFM (or whatever version thereof) tube to the box. The heater valve is a black and white plastic POS with 2 hoses and a vacuum line attached to it. HTH and good luck!

EDIT and correction, I see you have an '87 S4, just lift the bottom 1/2 of the airbox once you have removed the retaining nuts

SharkSkin

Have a look at Tony's excellent page for parts ID... here.

The heater valve is made up of items #10 and #11 in the pic below.

Jack '84 928s

Hm if that picture is right mine on the wrong way and has been since i had the car. But im removing it soon when i do my ac. My AFM doesnt have a clamp i can pull it out with the airbox bottom peice no need to remove it.

SharkSkin

One side has the German word for "Water Inlet" cast in one side.... I think it's "Wasserinlicht" or something like that. That side goes toward the engine.

Kaz

Is there an Audi part number for that valve in metal?

Bill Ball

Mike:

Come on by this weekend and we can take care of that, if that is the issue. I was out of town and could not get to your email earlier, but I'm back now.

surgiblade

Thanks to all replies so far

Well- first a hearty thank you to Bill Ball-a real treasure to the 928 community.

After reading the tips in Nicholson re :replacing the heater bypass valve and Theos on fixing a vaccuum leak, I proceeded to Bills place where today Bill(with minilmal assistance from me) replaced the valve, found the vaccuum to work, checked the high and low pressure outlets and found there was 28 psi on low, 150 on high, added freon and

STILL NO COLD AIR

Any wisdom re this kind of problem appreciated and any recommendation of a
928 air conditioner expert in SF Bay Area appreciated.

Thanks so much-

Mike
hmikej@sbcglobal.net

Bill Ball

As Mike says, it was not the heater control valve. His looked fine inside and was sealing OK and responding to vacuum. Replaced it anyway since we had it off.

With AC on, air is not even cool. I hooked up gauges to his system. Initially there was pressure on the high side but not on the low. After running a minute or so, the low pressure came up and we saw 28 low, 150 high. That sounds normal. Air still not cool. Barely below ambient. I gave it a small shot of freon anyway but stopped when pressure shot up and came down to 32. No better cooling.

What do we look at next? What could explain no pressure on the low side, followed by pressure later? My car doesn't behave that way. Bad expansion valve?

Giovanni

I am having the same problem but the rear ac blows cold air. Have you checking yours? I replaced the leaking valve and that did not help any...:-(.

Bill Ball

Well, if rear AC blows cold and the main doesn't, then expansion valve is likely, although the heater control valve we replaced would be another. I don't know if Mike has rear AC (optional on 87).

bd0nalds0n

Sounds like you checked vacuum and replaced the valve, and forgive me for being pedantic, but did you confirm that the heater valve actually closes when the car is started?

I had a bad mixing motor, which didn't ever energize the heater control valve vacuum solenoid, so even though my AC worked, and my system held vacuum, the heater control valve didn't shut no matter where the thermostat was set, and it never blew out cold air.