HVAC problems

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dr bob
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Just noticed that this a dupe of the info I had previously shared in post 13 in this thread. I do everything twice. I do everything twice.
Tom wrote: Fri Sep 20, 2024 11:52 am Since rabbit holes are kind of my thing,

<...>

Just riff'n here, but if the solenoid were internally leaking such that A, B, and C all flowed freely between each other, one could imagine a scenario where there is still enough vacuum to pull the heater valve shut any time there is full engine vacuum (where the unlimited supply of vacuum overcomes the 'leak' at C), yet can't sustain vacuum for long under boost when relying on the oxymoronically named vacuum reservoir alone (given the leak at C), causing the valve to always be closed under vacuum and always be open under boost. No idea if the solenoids ever fail that way, however. Just thinking out loud. I'll stop now. :)


heater-solenoid.jpg
Re-reading this, I wonder if you are missing a check valve. The 928 was designed for full-speed bahn-storming, and while it doesn't have the positive manifold pressure your turbo cars have, it still needs to maintain vacuum for the HVAC (and cruise-control, plus a resonant-flap actuator on the S4+ cars) under that condition. Porsche added a venturi eductor, but more importantly it has a vacuum accumulator (which name seems just a little backwards...), plus a small check valve between the engine and that accumulator. The check valve allows air to flow only from the accumulator towards the engine, so under hard throttle or boost conditions, the check valve will keep manifold air from entering the accumulator. The accumulator will keep the whole system available, at least until leaks, seepage, and cycling actuators eventually bleed enough air into the accumulator that there isn't enough 'vacuum' to hold things in position.

In the V8 car, there's also a restriction in the hose between the accumulator and the HVAC controller. It functions to slow down the actuators, but also protects the other clients (cruise, resonant flap actuator) from leaks in the HVAC system.

I don't know all the details of the 944 systems, but I do know that Behr very likely participated in the design and probably a lot of the parts and pieces as they did for the 928. Assuming they all shared a lunch table at some point, there's a good chance that they use a similar check valve. Those are not expensive at Porsche, tend to be even cheaper at Mercedes stores, and are actually pretty generic in shape and function. A decent POLAPS (plain old local auto parts store) is likely to have a generic one.


dr bob

1989 928 S4, black with cashmere/black inside
SoCal 928 Group Cofounder
928 Owner's Club Charter Member
Former Ex Bend Yacht Club Commodore Emeritus

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Tom
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Inspired by this thread, I did a little tutorial video on the heater solenoid and how it works with the heater valve in the 944. After watching my own video, I was a bit baffled why the 944 blows hot air when the heater clip fails. See this thread and video for background.

viewtopic.php?t=371

After studying the schematics and heater console, the climate control module has no way of knowing the heater flap is open when the clip breaks. The servo motor gives positional feedback to the climate control unit, but it has no way of knowing the linkage has fallen off. So, if the system is working properly, why would there be hot coolant in the cabin? With the temp set to max cold, the heater valve should be shut (per video below) and so the heater core should not be getting any hot coolant.

Does the climate control unit open the heater valve for some reason even when the system is set to max cool? I did a series of tests and have a theory. I intentionally disconnected my heater clip linkage to see if I could figure out why the cabin was getting hot. Surprise! It wasn't. I can drive around all day with the linkage disconnected, and the vent temps never get heat (unless I turn up the heat on the dash). I confirmed under the hood that the heater valve remained shut the whole time, as expected.

This is inconsistent with all the reports of always-on heat when the clip breaks, but consistent with my understanding of the system. I then took and old heater valve and did another little experiment. I forced the valve closed, then attached a hose to it and filled it with water. No water passed through the closed valve. However, when I blew on the hose, I was able to force some water through the closed valve. Now you'd think, in the car, both sides of the valve would have about the same pressure when the system heats up, but the hot side may be higher pressure? Either way, the only explanation I can come up with is that the valves must be leaking a bit on those always-hot cars with broken heater clips. It normally goes unnoticed with the heater flap closed, but becomes a problem when the flap opens. Or am I missing something?

My heater valve is fairly new, so I'm guessing I have little to no leakage, which explains why a 'broken' heater clip doesn't cause heat in my cabin. If my theory is right, it suggests you might be able to cool your cabin a bit easier by replacing old heater valves. Anytime the heater core has hot coolant in it, it's going to add heat to the cabin whether or not its blocked by heater flaps...

In any event, here's my video. :)



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dr bob
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ASSuming your 944 uses the same MB valve as my car does (and your video confirms), the valve itself is directional. The diaphragm moves a little disk across the end of one hose connection, and when there's some differential pressure from the water pump it pushes the disk tight on the end of that connection to improve the sealing. Push water the other way, and it lifts the disk off the end of that side and bleeds through.

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There's certainly some functional 'magic' you can do with some creative solenoid fitting. A second solenoid between the original one for the HCV and the valve itself would connect them when powered on, but isolate and secure the valve under vacuum when power is removed. Connect that coil to 15 via a small fuse, and it would do a lot to solve the heat migration into the heater core when the engine is off but hot. A loop of vacuum hose might extend the time it would stay closed after engine stop on a warm day.

My biggest gripe with the fails-open HCV in my 928 is that heat migration on engine stop on a hot day. The AC gets to cool a heat-soaked cabin on restart, but only after it pumps all the accumulated heat out of the air box and the heater core. Securing the HCV closed/isolated on engine stop if it was closed while running is an easy effort with another solenoid and that wire from 15 (true on RUN and Start). A small cap, a diode and a resistor would add a delay to actuate that solenoid, at least until engine start and vacuum available to the main HCV vacuum solenoid or source valve.


dr bob

1989 928 S4, black with cashmere/black inside
SoCal 928 Group Cofounder
928 Owner's Club Charter Member
Former Ex Bend Yacht Club Commodore Emeritus

Free Advice and Commentary. Use At Your Own Risk!

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