safulop

When I first got my S4 in Nov., the HVAC was intermittent, and now it seems to be dead or dying. The symptom is, no power to climate control at all in any position. It does work well when it works. It used to work 99% of the time, but now it is less and less. I am wondering should I just buy a replacement HVAC head and get my shop to install it? Is it worth poking around to see if there is some other problem? Or I should just skip the nonsense and buy a rebuilt unit? It's going to cost me hours of labor to have the guys mess around with it, so I'm thinking it will save money to just buy a climate control head if there is not a reasonable chance there is some other issue.

-Sean

WallyP

"The symptom is, no power to climate control at all in any position. It does work well when it works. It used to work 99% of the time, but now it is less and less."

That could be almost anything, but might well be as simple as cleaning a fuse holder or replacing a cheap relay.

While each owner is able/willing to do different levels of troubleshooting and repair on their 928, it seems to make sense for you to learn how the system works. At that point, you can describe specific symptoms and do specific checks (non-mechanic checks, such as moving the lever to defrost and telling us what happens) that might well allow you to either do a quick cheap repair on your own, or to tell the repair shop what you think the problem is so that you don't pay for extensive, expensive (and sometimes unnecessary) troubleshooting.

As an example - if the blower relay fails, it appears to virtually kill the system. If you crank the engine and move the mode lever to DEFROST and the blower comes on at high speed, that tells us that fuse #17 is OK but the blower relay is probably faulty. If the blower doesn't come on, fuse #17 or its connections might well be faulty. (Given your description, that would be the first thing that I would check.)

If you are interested in taking the time to learn about the system, go to our web site, go to the tips section, then Wally's World, then the HVAC system. Print that paper off and read it.

safulop

Well, I am aware that the defrost position is governed by a separate relay, and it does not work in that position either. There appears to be no power to the system at all, when it is down. Moving the lever does not actuate any flaps, the AC button is dark when depressed. It is completely dead. So, I will check fuse #17 and see about it? If the connections are faulty, that would cause the complete loss of power to the head? I will also see the paper you've sent me to, I like to learn all about the car even if not able to work on it myself.

Thanks,

Sean

safulop

Forgot to mention, the car has a new blower motor from a couple of years ago.

soontobered84

Sean, is the head unit plugged in inside the console? 2 plugs there.

safulop

Well since it works still most of the time (it was dead today, then fired up on the way home from work, typical behavior), I guess it has to be plugged in. But it is more than possible that those plugs are coming loose, the PO had the HVAC system worked on to fix vacuum issues, new blower fan etc. It is likely something was not plugged back in correctly or a loose-fitting plug was not corrected. How do I get behind the console to look at the plugs? Do you have to pull the head from the front, out of the console?

linderpat

The job to pull the head unit is major - dash, pod and consol must all come out. I can't imagine the cost to have this done by a mechanic, and I wouldn't trust that they would put it back together correctly. You read Wally's post - he is the dean of all things HVAC, so I suggest going to the site he references, doing the research and eliminating the easy stuff (relays, fuses, etc). The head unti is the last thing I would suspect has failed.

Landseer

Our 86 failed to blow on defrost or regular modes this weekend.

Logic suggested blown fuse, as its shared between both relays.
I surmised that the blower fan was dragging on low speed and may have blown the fuse.

Wrong.

Fuse was fine. Replaced then, the relay for blower regular mode.
Regular mode then worked, but defrost didn't.

Replaced the defrost relay with another new spare I had.
Defrost mode began working.

BTW, these were original relays --- the one car where I didn't follow my rule of "replace all simple "53" relays" when doing the obligatory new owner panel polish and fuse replace service.

The car is complicated and old. There are only 2 choices --- dig deep to diagnose and fix, else, ante-up.

Doesn't make sense, but its solved.

Ed, I thought you could pull the head be removing the two side covers, the console surround trim and 4 philips head screws.

the flyin' scotsman

The head unit is attached to the center console with 4 screws; these screws are accessible after the H plastic cover is removed.

Once the screws are out gently pull the head unit out and observe the connections.

Benton

Ed, I think you are imagining pulling the complete AC/heater box out. The A/C control head is relatively simple to remove as others have stated.

Sean-
Easy with the upper console trim piece when removing so that you do not break it where it necks down around the radio. Once that is out, you will see the four screws. You will want to unscrew the A/C & central locking switch below it to allow the A/C control head to be removed. I have a nice control head available for sale if you end up needing one, let me know if you would like more details.

soontobered84

Sean,
You can pull the head unit as Mark and Malcolm have stated above. Check the connections as you pull it out, but it sounds as if the internal relay is going bad (as they ALL do at some point). There are a lot of threads about the repair for that. You can either repair it with an external relay or replace the head unit.
Just know this: if you replace it with a head unit that has not had the internal relay already replaced, at some point THAT relay will go bad also. YMMV. Good luck.

WallyP

First thing to check (very carefully!) is Fuse #17 and its connections.

linderpat

You guys are correct - I was thinking of the a/c heater unit with the evaporator. The head is simple to access and pull. In fact, we just put a new on in my old car last weekend- my friend bought it and had all the dash parts recovered in leather. Huge job putting it all back together, but it is coming along nicely so far.

safulop

OK, this is a funny story. Keep in mind that while I love to know all about Porsches (especially when I own them) I not like the rest of you guys taking it apart all the time. I'm all thumbs. So I have never yet seen the inside of my fuse panel.

Today at work I started the car and had no climate control again, so on a silly whim I leaned over and hit the fuse panel cover. It fired right up! Then later, the same thing happened and the same trick worked. It's like Fonzie getting his free song out of the jukebox.

A little ridiculous, but when I finally opened the panel I realized the guide diagram is missing from the cover. So I don't even know where fuse 17 is! Plus it is really tight in there. Anyway I have the WSM and so I guess I should be able to find a diagram of the fusebox.

Thanks for the tip Wally, saved me a new climate head.

-Sean

linderpat

I wouldn't do that. While you might have momentarily got your HVAC back, you could cause something else that is ready to fail, but hasn't yet, to fail now. Think fuel pump relay or something else that will leave you stranded. Plus, the car's brain is behind that wooden panel, and you don't want that to get rattled.