Yet another "too hot" HVAC thread
06-23-2012, 09:28 PM
#16
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The standard 928 HCV closes with vacuum, so it slowly opens as the vacuum bleeds off when the car is parked. There was some discussion about putting in another valve in series that opens with vacuum, so hot coolant circulation is stopped. Search should find the discussion, with the vacuum arrangement.
I'm just thinking that, if there's really a significant amount of hot water running backwards through the (supposed to be) closed off heater core, CWV valves on both sides, with both their "exits" towards the core, may help cooling....
06-23-2012, 11:02 PM
#19
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This is with the probe inserted into the center vent - I have not diverted all the air to the door outlets and tested that - I normally run with 70-80% air from the ctr and the rest to the doors -
Since I get 2-4C at the evaporator the rest is up to how well I can block and redirect air from that point -
I'll put the probe into the door outlet the next time and see what numbers I can get there
Since I get 2-4C at the evaporator the rest is up to how well I can block and redirect air from that point -
I'll put the probe into the door outlet the next time and see what numbers I can get there
06-26-2012, 10:43 AM
#20
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I dug up the invoice for the R12 work. I was charged for 3lbs of R12. Obviously they rounded up to the nearest pound. I think I'll have to call them, though, because that still doesn't tell me anything... stock charge level is 2.09lbs w/o rear A/C and 2.53lbs with...
06-26-2012, 01:17 PM
#22
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Note that some of the small cans are 14 oz., like this one I used about three years ago. (Check out the price tag, too! Those were the good ol' days. 
)
)
06-26-2012, 01:36 PM
#23
I would also make sure that the setting motor is moving to full cold when you select a temp below ambient on the slider.
If it doesn't then you need to diagnose whether motor problem, or a fault in the sensor string (external sensor, internal sensor, hvac controller sliding resistor) input to the motor.
Very recently I suggested to another owner that his "too hot" problem was likely internal cabin sensor thermistor drifting out of spec in high direction. That's what it turrned out to be (if it goes too high, then the setting motor is always being "told" that cabin is too cold).
If it doesn't then you need to diagnose whether motor problem, or a fault in the sensor string (external sensor, internal sensor, hvac controller sliding resistor) input to the motor.
Very recently I suggested to another owner that his "too hot" problem was likely internal cabin sensor thermistor drifting out of spec in high direction. That's what it turrned out to be (if it goes too high, then the setting motor is always being "told" that cabin is too cold).
I would still check the setting motor. Mine became stuck in a mid setting that blended warm air into the cabin with cold AC air. Made sense because it would not get very warm on cold days.
Really easy to check and elimenate, it sits beside the steering column just above where your right knee would be driving. Use a mirror or get on your back, then slide from full cold to full hot. You should see and hear the motor moving. If it is stuck, it will give you the symptoms you describe.
06-26-2012, 02:04 PM
#24
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Already checked the setting motor. It moves correctly. I did tighten up the lever a bit (four turns or so), but the reduction in cooling when rear A/C is engaged is a problem regardless.
06-26-2012, 02:33 PM
#26
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I got so damn tired of the whole heater core heating crap that I just took the delivery hose off at the front and put a damn marble in the line. If I need heat again in Nov, I'll go take the marble out again.
06-26-2012, 02:36 PM
#27
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That's pretty annoying. My existing HCV is installed correctly. I suppose it could be leaking... I didn't replace it (yet) because the number I found referenced when I searched here (84707) is actually a hand-operated valve, not a vacuum-operated one.
Is there an article you know of, or can you comment on what's involved in replacing the foam on the mixer door? I looked for and asked about this before swapping my dash, but didn't get any definite suggestions on it. While I had the dash out, I checked out as much of the foam as I had access to, but didn't see where the mixer door is.
I have torn into VW HVAC's before, and they actually use a door made of perforated metal. Once the foam is gone, the door is totally useless to redirect air, as all of it goes through the 3/8" or so holes. Hopefully 928's don't have such a setup...?
Is there an article you know of, or can you comment on what's involved in replacing the foam on the mixer door? I looked for and asked about this before swapping my dash, but didn't get any definite suggestions on it. While I had the dash out, I checked out as much of the foam as I had access to, but didn't see where the mixer door is.
I have torn into VW HVAC's before, and they actually use a door made of perforated metal. Once the foam is gone, the door is totally useless to redirect air, as all of it goes through the 3/8" or so holes. Hopefully 928's don't have such a setup...?
These cars are almost all old enough, that virtually all of the A/C units have the foam falling off and leaking. Rob Edwards just finished "re-foaming" a unit, for Dan Herbert's "new" car. While there are "ice cubes" blowing out of that vehicle (38 degree air from both the front and the rear system.....without any windows or doors on the car. It is a "test" car and has a bunch of "updates" we are testing to improve the A/C systems), I'm not convinced the foam that Rob used will work "long term" and I'm currently working on getting a different type of foam. My goal is to be able to both "rebuild" these units and also provide a kit for "home guys" to re-foam their own units.
06-26-2012, 02:46 PM
#28
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Here's a quick video that illustrates how the temp slider, mixing motor, and mixing flaps move as you go from cold to hot and back. At the start of the movie, the space 'front and center' in the opened HVAC box would be where your heater core sits. So you can see that if the flaps don't fully open or close, or the foam is toast (literally), and there's any heat coming from the heater core (leaking heater valve, etc) you won't be as cold as you otherwise could be.
06-29-2012, 02:08 AM
#30
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OK. Replaced the HCV with one like this (OE for a last year of production (1988?) Audi 5000).
I think the old one may have been leaking. There was a loose thin bit of white plastic inside the valve. Anyway, between this, and the flap adjustment, I am now seeing 5 to 8 degrees Fahrenheit of gain in the side vents compared to ambient with the car fully warmed up. Much better than the 20F I was seeing.
I think the old one may have been leaking. There was a loose thin bit of white plastic inside the valve. Anyway, between this, and the flap adjustment, I am now seeing 5 to 8 degrees Fahrenheit of gain in the side vents compared to ambient with the car fully warmed up. Much better than the 20F I was seeing.