944 heater always on troubleshooting

[s14kev]

Hoping to get some insight on another 944 heater issue. I've had my 951 for 15yrs. Was very active on Rennlist over a decade ago when this car was being built/restored. Seems like this is the new place for 944 folks. I recognize many of the same names from Rennlist.

My 951 has the dreaded heat always on issue but this is a little more unusual. When car is on and the temperature knob is turned to the coldest setting the heater valve in the engine bay will close and the temperature mixer flap will close as it is supposed to. A few seconds later the mixer flap will open and the engine bay heater valve will open resulting in hot air to the vents even though the temperature knob is left on the coldest setting. If I turn the knob to heat then back to cold, the mixer flap will close and the engine bay heater valve will close resulting in cool air but seconds later it will open again resulting in heat.

All regular items check out fine. Mixer door rod clip is fine. Heater valve in the engine bay is new and functional. All vacuum lines are intact and hold vacuum. Solenoid valve (next to the glovebox) which controls the engine bay heater valve is tested and functional. I ran through Toms HVAC diagnostics pdf and everything checks out. The only item that is hard to check is the Climate Control Unit. Visually it looks fine with no burnt components, leaking capacitors etc. My experience with the CCU is when it is dysfunctional, it completely stops working. It's odd for it to control the mixer doors and solenoids fine. Car is otherwise in excellent shape and always garaged with 80k miles. Any thoughts are appreciated.

[Tom]

Hey Kev -- very happy to have you over here. You probably figured it out already, but I was Tom M'Guinn over on RL. We started Carpokes in '21 when it seemed RL just kept straying further and further from its original mission, to the point where I just couldn't take it anymore after peaking behind the scenes as a moderator. This is indeed the new place for 944 owners (and all Porsches for that matter). Tell your friends.

I'd be inclined to start by tapping into and watching if pin A4 is switching to 12 volts and staying there when the heater clicks back on, or if the CC unit is supplying 12 volts to A4 only briefly and then losing it in short order. If A4 stays at 12v even when the heat comes back, then it's less likely to be the CC unit, and more likely to be the solenoid, the vacuum system, or the valve itself. The solenoid is driven by a chip (IC8 4201A pin 1) and has a flyback diode across its power pins (D8). If the CC unit is initially supplying 12 volts on A4, but then stops, I could imagine that diode breaking down when it heats up and shorting the voltage to ground and/or the chip might be bad. Wild speculation on my part, but just my long way of repeating myself -- i.e., monitor what's happening on A4 and go from there... Here's a video that might help visualize how A4 controls the system.

[ROB III]

@s14kev
Kev, you will likely find several former RL people here, including myself. You will also find this a much friendlier site where people can share their joy of owning/driving/working on their Porsche's. Some of the new members are first time owners and are reinvigorating/saving forgotten gems, working to bring them back to their former glory, as well as those that have been 944 owners/racers for a long time.
I'm glad you found CARPOKES and thank you for joining.

[s14kev]

Rob. I think I met you a few weeks ago with your black 951? I'm the non-PCA member at the PCA event a few weeks back. I think I was driving either my 72RS backdate or 993TT. If so, nice to meet you.

[ROB III]

Ahhhhh, I remember the 72RS backdate, blue and white parked outside...yes?

[Latitude48]

I'm having the same full heating issue with my 1990 944 S2. I'm going to pull the Climate Control Unit and replace it with one of two new spares I purchased years ago. If that doesn't solve the issue, I'll have to dig further.

[Tom]

I'm having the same full heating issue with my 1990 944 S2. I'm going to pull the Climate Control Unit and replace it with one of two new spares I purchased years ago. If that doesn't solve the issue, I'll have to dig further.

Check he heater clip first!

[Latitude48]

I'm having the same full heating issue with my 1990 944 S2. I'm going to pull the Climate Control Unit and replace it with one of two new spares I purchased years ago. If that doesn't solve the issue, I'll have to dig further.

Check he heater clip first!

The clip could be the issue; however, my records show I replaced them with metal clips... way back in 2001. The main reason for replacing the DCU (Dash Control Unit as Porsche calls it) is the clear faceplate is broken.

[Tom]

I'm having the same full heating issue with my 1990 944 S2. I'm going to pull the Climate Control Unit and replace it with one of two new spares I purchased years ago. If that doesn't solve the issue, I'll have to dig further.

Check he heater clip first!

The clip could be the issue; however, my records show I replaced them with metal clips... way back in 2001. The main reason for replacing the DCU (Dash Control Unit as Porsche calls it) is the clear faceplate is broken.

The actuator rods have plastic clips on both ends, so if you replaced one end with a metal clip 24 years ago, there's a some change the other end broke. Worth taking a look anyway, as long as you have a good chiropractor on call. Rennbay made a reproduction clear faceplate and sold that here as a group buy maybe a year or two ago. I may still have a small personal stash if you want one. The beauty of them is they fit around the buttons, so you don't need to desolder anything to replace them -- and they are brand new modern clear acrylic that's not old and brittle like most originals.

[Tom]

You've probably seen this and/or already know all about it, but in case helpful...