Eric Kessel

I've been trying to diagnose a problem with the heating on my 90 C2. Any help will be appreciated.

Symptoms: After sitting overnight outside when it's below freezing, the first drive of the day, the heat would stay on full blast, and after a 15 minute drive, the climate control would then go to the requested settings. While the heat is stuck on full, the CCU won't respond to blower speed, or temp, or the slider settings. Just full heat out the foot wells, and side vents.

Newest Symptom: It just stays on full heat, even after a nice 1 hour drive........ but, it's got a new trick. If I press the defrost button, the vents direct the heat to the windshield, and side vents. When I turn off the defrost button, it reverts back to heat on the floor and side vents, and just a little to the windshield.

Troubleshooting so far: The rear blower and front blowers go to full as soon as the car is turned to on. If I remove the CCU, then no blowers come on. If I put the CCU in, but unplug the front final speed regulator unit, then only the rear blower comes on. When I turn off the car, all blowers stop immediately (I haven't had the car hot enough to cause the rear blower to stay on now that it's cold out)
Earlier in the year, I had a loaner hammer, and the problems listed were a bad fresh air servo, which I have since replaced, and a rear blower problem which has also since been replaced. During the summer this heat stuck on issue didn't happen, but it did last winter.

(I took out the CCU, and found the internal temp sensor wasn't soldered in correctly, so I resoldered that, but that's as far as I've gotten)

Any thoughts on what to check next will be appreciated. I'll send the CCU out to be checked, but figured I'd see if there is anything else to check first.

Thanks,
-Eric
90 C2
NER/PCA

Randall G.

Hey Eric,

Wow, that's a lot of things going on at one time! If I understand things correctly, for a period of time after starting the car cold, you have no control of:

-fan speed
-temperature control
-direction of air flow (no slider response)

Or, in other words, your CCU controls are completely non-responsive, other than the defrost button.

Wrt to the defrost, that's what it does normally, regardless of slider setting. That is, the footwell and center vents close, air is directed to the side vents and windshield. So, at least this part is working correctly.

The response to the defrost button demonstrates that--at least--your windshield/center vent and footwell servos are fine.

Just out of curiosity, what happens when you press the recirc button? Does the fresh air supply servo close?

I'm trying to think of something the fan speed control, servo motors (air direction) and servo motors (temperature control) have in common, besides the CCU itself.

-The servo motors all share the same sensor ground (from the CCU) and 5V reference voltage.

-The final stage regulator (FSR) has nothing in common with the servo motors.

The rear blower going to full as soon as the car is turned on means it thinks (via the CCU) that you've commanded a fan speed setting greater than 2. The rear blower running in the first place on a cold engine means the rear blower thinks it needs to supply heat to the cabin. Soooo....it sounds to me like your rear blower is responding correctly to what it thinks it's seeing >>> fan speed greater than 2, heat requested in the cabin.

Now, I am wondering if your CCU might have a bad ground? I dunno', but I imagine that could cause all sorts of strange stuff. And, ground is the only thing I can think of that the FSR and servos (via the CCU) have in common. Also, I wouldn't be all that surprised if Porsche designed the system to default to full-blast heat in the case of bad (open) ground? That is, better to have full heat, than to freeze to death. But, this is conjecture on my part.

One problem with this theory, though. The front blower circuits (left and right) are grounded to the chassis at the same point as the CCU. If the chassis ground is open, the front blowers won't run (open circuit). So, if there's an open on the ground wire, it has to be upstream of their junction, or inside the CCU.

Another problem with this theory--I wouldn't expect the center vent/windshield servo to respond to the defrost button (no path to ground).

Your ground plug on the CCU is K-1. The chassis ground should be at the base of the windshield, in the trunk, right side of the car. You might try verifying K-1 in the wiring harness has continuity to the chassis ground. Or, before even doing that, check the terminal to make sure it doesn't have an open internal to the CCU?

Well, this is all I can think of for now. The only other thing I can suggest is borrowing a friend's CCU long enough to see if your problem goes away. Or, I can ship you my spare CCU, if you'll return it when you're finished. It works fine, other than not starting the oil cooler fan automatically.

Hope this helps, and good luck!

Eric Kessel

Thanks for the help Randall. I'll check out the ground, and also see what the fresh air servo does.......

When I put the recirc button, the little light on it turns on, but I haven't checked that the fresh air servo closes......

-Eric
NER/PCA
90 C2

Eric Kessel

I checked, and the ground is fine, both on the cable, and into the CCU. The ground point at the bottom of the windshield is also fine.......

The fresh air servo did move when I pushed the recirc button, so that's another feature that's working....... I'm going to try to find someone on the east coast that can repair CCU's next........

-Eric
NER/PCA
90 C2

Randall G.

Good luck, Eric, and please keep us posted on what you find.

Jeff Curtis

I'm having the EXACT same issues, after a nice long drive with the girlie-friend today...in 50-something degree weather - I WANT TO FIX THIS!!

Randall, I may very well take you up on your "loaner CCU" option, I'll put Eric in line first, being that I've jumped in on this thread and you offered it to me some time ago, and I'm a bit late in accepting said offer.

I hope it's a bit more simple (and cheaper) than a new CCU, but it would be comforting just to know what's up with this darned heat!

Eric Kessel

Well, there is a guy selling 993 CCU's on e-bay, he has 9 of them, but they are all the .01 versions........

Randall, are you serious about your loaner?

Jeff Curtis

Eric, is there a problem with using the 993 ".01" version?? Is this unit too "new" for the 964 or what? I'm going to have to start learning about this beast, it seems.

Eric Kessel

Adrian's book, and the following thread:

https://rennlist.com/forums/showthre...hlight=993+ccu

make it sound impossible to use the .01 version of the 993 controller. I'm going to try and get the CCU scanned with a Hammer. Do you know of anyone on the east coast that will repair the CCU's? Everyone seems happy with Otto's over in CA, but that will add a bit for shipping.....
-Eric

Jeff Curtis

Thanks Eric, I now consider myself INFORMED!

I think I may take the path of least "resistance" and send my unit to Otto's...come to think of it, the oil cooling fan has NEVER come on either, now the heat issue...blah, blah, blah...stinking CCUs!!

It would be nice to diagnose it with a good spare unit to ensure that my $300 is going to a good cause though...if any Virginia Beach residents have their 964 or early 993 at the "locals" repair facility...WATCH OUT!

Randall G.

Hey Eric,

Yup, serious about the CCU loaner, I'm sure it will make its way home to me. Though, suppose ya' gotta weigh the cost of shipping from the West Coast.

Jeff....how do you get your oil cooler fan to run on the track? Do you have a manual switch? Unplug the temp. sensor? Can't imagine not having a working oil cooler fan when driving the **** out of the car?

It sure would be good to verify with a loaner. But, with all the weird (apparently unrelated) things ya' got going on, it's hard to imagine the problem being anywhere besides the CCU. That is, you wouldn't expect a servo failure (heater flap), FSR failure (front fans) & another servo failure (sliders not responding) all at the same time. Which leaves your car's wiring/power/ground. You've verified the ground is good, fans are working, most (if not all servos) are working. So, these seem to be okay. Not my money , but it seems like you're to the point of giving the rebuilt unit a try. If it was my money, I would still try like hell to locate a loaner, before springing for the rebuild.

Good luck, and let me know if ya' want to try my (won't start the oil cooler fan but works otherwise) spare CCU.

Rich W

I had the same problem a number of years ago. My CCU was toast.

BTW Jeff, my oil cooler fan comes on just fine

Of course, that's very helpful in the DESERT!

Randall G.

Hey Rich...watcha' doing in Arizona. You aren't working at Palo Verde, are ya'?

Rich W

Staying warm without my heater.

Wife got a great job, couldn't resist.

Check your CCU (staying on topic - or at least trying to

Jeff Curtis

Hey Rich!! ...good to see your computer still works in Wildcat Country!

...I was thinking of you the other day when pondering how much work is involved in changing out the AC condenser.

Randall, I don't think my oil cooler fan has come on in over 2 years...seriously. I find that it's OK, as long as you keep the car above 120MPH.

My motor doesn't get that hot at the track, even on 90degree days...I guess the fan would only come in handy when I pull in though.

Yeah, I'd enlist the services of a good loaner to troubleshoot if one's available, maybe when Eric is done trying it out he can send it down my way?

Again, good to see you're around Rich.