AC-vent-gheater valve ?
10-03-2018, 05:33 PM
#1
Race Director
Thread Starter
AC-vent-gheater valve ?
So I am getting the AC repaired on Seans 87 S4. The charge was low, so that was easy to fix. The air temp coming out of the main center vent was much warmer than the air out of the side vents, so I assumed the heater valve was bad. It turns on the heater valve does work and stops the water flow like it is supposed too. I remember an adjustment to the mechanical flaps on the "air blender" that can go out of adjustment and cause issues. The only other thing I can think of is some of the other vacuum actuators are leaking, causing a loss of vacuum over time and making the heater valve open? Its been years since I worked on an 928 with AC, I just can't remember
10-03-2018, 06:16 PM
#2
Chronic Tool Dropper
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Check for vacuum leaks in the system, then focus on whether the comb flap is opening completely with system vacuum. The comb flap, in concert with the motorized mixing valve, decides whether un-cooled air makes it out that center vent.
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I had a vacuum leak in the system. With the HCV tied closed, I was still not getting full-cold air at the center vent. It was around 45-55º, a long way from the "normal" sub-freezing temps I see with the freeze switch jumpered. Plus, the system pressures were way unbalanced for measured ambient and cabin conditions. Vacuum checks found that the HCV vacuum diaphragm was leaking. Capping the HCV vacuum hose masked the problem and cured the symptom. A new HCV solved the problem. I felt pretty lucky that the vacuum leak was so easy to fix. But it was not foresight; more like fortuitous whim instinct that I decided to look at the vacuum system. Everything pointed to a plugged expansion valve.
I'm convinced that AC system diagnosis starts with system pressure checks, goes immediately to gross vacuum system testing unless the refrigeration side shows really low. Still do the gross system checks, with MitiVac at the hose from the splitter.
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I had a vacuum leak in the system. With the HCV tied closed, I was still not getting full-cold air at the center vent. It was around 45-55º, a long way from the "normal" sub-freezing temps I see with the freeze switch jumpered. Plus, the system pressures were way unbalanced for measured ambient and cabin conditions. Vacuum checks found that the HCV vacuum diaphragm was leaking. Capping the HCV vacuum hose masked the problem and cured the symptom. A new HCV solved the problem. I felt pretty lucky that the vacuum leak was so easy to fix. But it was not foresight; more like fortuitous whim instinct that I decided to look at the vacuum system. Everything pointed to a plugged expansion valve.
I'm convinced that AC system diagnosis starts with system pressure checks, goes immediately to gross vacuum system testing unless the refrigeration side shows really low. Still do the gross system checks, with MitiVac at the hose from the splitter.
10-03-2018, 09:57 PM
#3
Race Director
Thread Starter
The comb flap makes sense....that is what I remember, any leak in the vac actuated flaps when “used’ causes lots of problems.....Sharky had a rare dead solenoid that always opened the heatervalve. Once fixed it blew ice cold...bill ball was impressed at the temp differential I achieved with a functioning freeze valve...my thoughts were freeze valve was dying, towards to freezing side.
Another symptom that makes sense is odd noise From center vent...a sort of clapping...which is clearly the center comb vent failing and recovering, which also causes heat to sneak in past heater valve....which isn’t fully sealed from leaks....
so need a comb flap actuator.....does the dash have to come off to change it? If so I’m leaning towards replacing all the vac valves....
of course me being ME the dark blue dash and pod are cracked and need repair....
Another symptom that makes sense is odd noise From center vent...a sort of clapping...which is clearly the center comb vent failing and recovering, which also causes heat to sneak in past heater valve....which isn’t fully sealed from leaks....
so need a comb flap actuator.....does the dash have to come off to change it? If so I’m leaning towards replacing all the vac valves....
of course me being ME the dark blue dash and pod are cracked and need repair....
10-03-2018, 10:15 PM
#4
Just remove your glove box and center console. You can access all of them that way. Taking the instrument pod out can ease getting the defrost actuator but it’s not necessary. Get with Roger and get the pots he has, then just replace the rubber on the center and floor actuators. For the defrost disassemble it and replace the innards reusing the old cap and spring. Recirculating requires the box being removed from above and below, taking the CE panel down a bit.