928 Vents Always Have Heat

[Porschenut]

Hello Dr. Bob, Greg and All,
Upon starting and driving my 1988 928 S4, warm air appears to flow through the vents even if the air circulation or temperature controls are turned off. In order to mitigate this issue, I find it necessary to turn on the air conditioning to assist in cooling down the warm air.

To address this concern, I am kindly seeking guidance on proper diagnostic procedures.
thank you
car in question: 1988 928 S4, 184,000 miles.

[Tom]

Hi Porschenut, if we haven't officially welcomed you to Carpokes yet -- Welcome! While waiting for @dr bob or other 928 experts to arrive, I'll offer suggestions based on my experience with the 944, which shares some key parts. I would start by checking whether the heater valve under the hood is working. It is a vacuum operated valve that controls the flow of hot coolant into the heater core under the dash. It should be pulled closed by engine vacuum when the heater controls are set for no heat. If the valve is not getting and holding vacuum for any reason, or if the valve is just stuck open, hot coolant will flow to the heater core and will radiate heat into the cabin's air flow no matter how you set the hvac controls. I'd start by checking the valve operation with a Mity-Vac to see if the valve closes when it gets vacuum. There's a good chance yours does not. See video below. There are many YouTube videos showing how to change the valve, but this one is just a nice visual on how the valve works.

The 944 and 928 use the same heater valve, which tends to fail over time. Its factory part number is: 928 574 573 04. In your case, you need to remove your air filter box at the back of the engine to locate the heater valve underneath by the firewall. Your problem could be caused by other issues, of course, but this is probably the most likely culprit and certainly worth checking. 928 and 944 owners have substituted all kinds of different heater valves over the years, including one by Mercedes, but below is a link to the factory version. I would avoid the aftermarket replicas by URO and Meyle personally.

https://www.pelicanparts.com/catalog/Su ... -parts.htm



P.S., I split this out into its own topic so that hot 928 owners can find the info easier in the future.

[Zirconocene]

As @Tom said, these are designed to fail open, as a safety feature, so it may well be that you have a problem with that valve. I don't think it's too onerous to replace, but it's helpful if you have long arms, it's right in the middle of the engine bay, tucked up by the firewall.

My S4 also shows the same issue, but I have been advised that this is not totally out of the ordinary, even with the vacuum system in tip top shape. This constant hot air comes predominantly from the center vent and is best mitigated by closing the shutter. That seems like an odd "fix" to me, but I have this information from people I trust, whose cars are in fantastic shape, so take that as you will.

If you're inclined to dig into this deeper, look up Dwayne's Garage; he has a pretty great article on going through his HVAC system to get it back into proper working order.

Cheers

[Tom]

As @Tom said, these are designed to fail open, as a safety feature, so it may well be that you have a problem with that valve. I don't think it's too onerous to replace, but it's helpful if you have long arms, it's right in the middle of the engine bay, tucked up by the firewall.

My S4 also shows the same issue, but I have been advised that this is not totally out of the ordinary, even with the vacuum system in tip top shape. This constant hot air comes predominantly from the center vent and is best mitigated by closing the shutter. That seems like an odd "fix" to me, but I have this information from people I trust, whose cars are in fantastic shape, so take that as you will.

If you're inclined to dig into this deeper, look up Dwayne's Garage; he has a pretty great article on going through his HVAC system to get it back into proper working order.

Cheers

Did you see my 944 dash removal "while you're in there" thread? I ended up replacing all the foam weather-stripping material on the various flaps, to keep all the flaps as as tight as possible. On my garaged-from-birth 944, the foam was completely deteriorated and not sealing the internal duct flaps at all. The fact that @dr bob was the one who gave me just the right foam for the flaps makes me think the 928 has the same issue.

viewtopic.php?t=326

[Zirconocene]

I'm sure you're right and it's also likely to be playing a part. That work you did is on my list of things I'd like to take care of on my 944 and I'm sure the concept will carry over into the work I'll inevitably have to do on my S4.

It's hard to explain this madness to people, sometimes, so I'm glad this place is here and populated by people that get it.

Cheers

[dr bob]

I'm a day or two late to the party, but...

The advice you've read so far is excellent.

When I experienced the same symptoms, I ended up going through all the vacuum clients to figure out what was leaking. My tool of choice is a hand-held vacuum pump like a miti-vac. My plastic Miti-Vac has been superseded by a metal version in a kit that you can buy under the Craftsman label. But I also discovered that the same kit is at Harbor Freight for bunch less.

Each of the vacuum actuators, including that heater control valve, had a connection at a solenoid "manifold" that lives in the forward part of the center console. Access is via the carpeted side panels, and there are three screws holding the support bracket. The center one demands some kind words and a short Phillips screwdriver, maybe an offset screwdriver with a ratchet if you have one. The whole manifold pulls out the driver's side in the footwell. Once there you'll see a rubber "manifold" with the white vacuum source hose connected to all of the solenoids. The hoses and solenoids are colored, so take a few pictures before disconnecting anything.

The basic test for vacuum integrity can theoretically be performed from the supply hose where it connects near the brake booster. But it takes more fiddling than it's worth, and since you already have symptoms of a leak, you'll be testing the individual actuators anyway. Take the hoses off the solenoid manifold one at a time, and use your handy vacuum tester to pull vacuum on each hose one by one. The actuators are soft spring return, so it might take a bit of pumping to get full travel. But eventually the pumping should cause vacuum to stabilize. Leave the pump connected and watch the gauge for a minute to make sure each one holds vacuum. The WSM has a color chart for which hoses go to which actuator. I can get those colors for you if don't have the WSM available. Regardless, note which ones don't hold, and grit your teeth if your suspected circuit isn't the hot water valve.

The water valve sits at the rear of the intake area in the V on the passenger side. Short stub of hose connects the valve to a fitting, then the valve and the hose across the firewall to the heater core connection. Remove the whole air cleaner housing for access. The little vacuum can on the valve has a link to the valve itself. As others share, it's a fail-open valve, plus they have a known history of leaking through even when the actuator says closed. Any vacuum leaks in the system seem to manifest as the heater valve not going to full closed position. Pull vacuum from the manifold end of the hose if you can to test the hose itself, then connect your hand vacuum pump to pull the valve to full-closed position. I've never seen the vacuum part of the heater valve leak, but I have replaced a few that don't seal the hot water loop while closed.

The valve itself is common to many Mercedes cars, and for some reason the Merc owners get a better valve. I don't have the Merc part number handy. But I can say that the one in my car now has a couple decades of service on it.

While I was living in SoCal, I'd use a cable tie to keep the valve closed all year except winter, which was the week between Christmas and New Years. Here in the high desert of central Oregon we do get winter, so the valve is now free to heat the cabin. Look hard at the valve and you'll quickly figure out a way to use a cable tie to hold yours closed and avoid the hot air from the vents. ASSuming that it actually closes; you get to test the valve that way too I guess.


The system is safety-oriented, with defaults to full heat, defrost vents, floor vents on loss of vacuum. Tying the valve closed for summer means that the valve stays closed when you stop the engine, and your AC doesn't have to pull all the heater heat out before cooling the cabin. Great on a hot restart after you've loaded your beer and groceries in the back. The blend doors still need to work correctly and won't with vacuum leaks, and that will trash your AC performance. So fixing any leaks is pretty key to total system performance especially if you depend on the AC much. I drove my black S4 in Los Angeles area, no window tint, so everything needed to work really well. Here in Oregon, the heater gets about as much duty as the AC, even though the car hibernates for the real winter season.

Fixing leaking vacuum actuators generally involves some degree of console and may dash disassembly. I just replaced them all when my blend door and center vent actuators were leaking. Once the tools are out and you are part way there, it's not too hard to pull the whole dash for the other two actuator cans WYAIT. They are all the same age, so failure is pretty generally a concurrent event.

Don't tell anybody, but I left that center screw out of the solenoid mounting bracket when I reassembled. I did tape the screw to the bracket so it doesn't get lost, but I decided that the extra hour and language lesson weren't really needed. Maybe someday the guilt and loss of sleep will drive me to put that screw back in. Not this week though.

[dr bob]

Colors for the vacuum solenoids and lines going into the console, from the workshop manual:

footwell flap (yellow)
Defroster flap (green)
Center nozzle stage I (orange)
Center nozzle stage II (brown)
Mixing flap and heating valve (red)
Fresh air bypass flap (blue)

This page 87-44 is from the 1980 printing, so take it with a grain of salt when applying to your S4.


Here's diagram that shows the vacuum and actuators.

Vacuum Diagram.jpg (221.98 KiB) Viewed 203 times

[Porschenut]

I would like to express my gratitude to each of you for extending a warm welcome and taking the time to provide me with detailed information, pictures, and videos. Your efforts are greatly appreciated, what a nice bunch of Porsche lovers!!. I will follow your suggestions and hopefully provide a positive update in due course

[DonaldS4]

I also, had this exact problem. The air sensor, in the alternator cooling hose was corroded, and was an open contact. I fixed this connection, and my temperature settings worked as they should, after this fix.
This sensor tells the module what the air temperature is.
GL

[dr bob]

The temp sensors do play an important part in the way the system works. The sensor in the alternator duct, behind the LF headlight, is a thermistor. That sensor changes resistance with temperature change. In the dash is a small sensor assembly with another thermistor and a fan to draw cabin air across it. The other key parts of the balancing act are the temperature slider in the head unit, that includes a rheostat (variable resistor), and the temperature blend door servo actuator. The servo sees a single signal input from the sensors in series with the temp slider, and from that decides how much cold and how much hot air get mixed together for the cabin. It's a straight gain-only controller.

On that same blend door servo actuator, there's a series of contacts that operate the fresh-air flap and the heater control valve solenoids. In addition, there are microswitches at the ends of the temp slider travel in that head unit. Full-cold position keeps the heater valve closed, assuming there's sufficient vacuum to pull the heater valve closed. That override is in spite of the position of the blend door servo motor. So if the slider is full-cold, the blend door might open to allow air across the heater core, but with the heater valve closed it won't he hot, just ambient temperature air into the cabin.

At the center vent, there's an actuator that mixes ambient air with blended air. If that vacuum actuator fails or there isn't enough system vacuum to operate the flap there, you can get warm air from the heater box. Remember that everything 'fails hot'.

Someplace I have the details on the two temp sensor elements. One is a "reverse temperature coefficient) or "RTC" thermistor, and that's what enables the simple yet effective balancing for the blend door servomotor.

You can test the various sensors, using temp/resistance data that's in the workshop manual. You can verify the temp slider in the control head using an ohm meter.

You can also verify the operation of that servo, but I've only heard of one of those that actually failed. The actual "blend door" inside the air box uses thin foam to seal at position extremes. The original foam has long-since crumbled and fallen down to the bottom of that box. I have some of that if you need some, but I'll confess that the stuff I have is getting older too so maybe new is needed for this particular duty. Mine is great for sealing assemblies and connections, but is likely too tired for actual wiping duty. PM me if you need some and I'll stuff some in an envelope for you.

The control head slider is accessible with some screws and brittle plastic tab latches once the control head is out. Clean the trace on the control board inside with a white rubber eraser, no more. There's an excellent mod for that control head that replaces the way-undersized compressor relay too, something to consider even if your compressor still comes on reliably when you push the AC button. I've got the original documentation I did for my own car 20+ years ago when I performed this modification. It's one of only a few actual modifications I've made to the car, along with converting the system to R-134a at about the same time. PM if you'd like a copy of that too.

For the R-134a doubters, The system will easily pull the center vent temp below 20ºF on a 90º day cruising. The anti-freeze switch failed, and now I can freeze fingers holding the steering wheel. I have an improved freeze protection plan in mind, and need to get building to get it ready for prime time. For now in the very dry desert climates I've lived in, the cold air is a bit more welcome especially in my black car without tint.