fossberry

My wife's 968 A/C has been blowing hot air for some time, and after doting on my 928 I thought I'd better give hers a good once over. Found that the little plastic clip that controls the temp mixing flap was broken. A-ha! I thought...that'll fix it. Fitted the replacement only to find it still blew hot air. Someone previously had disconnected the rubber hose that goes between the cabin temp sensor and the temp controller, so refitted that, cable tied it so it wouldn't come off again. Vacuum lines were all good but still no joy. Being tall and lanky I was able to shine a light up at the mixing flap control arm and when turning the ignition key from off to accessory, watch the control arm go to a full heat state, irrespective of what the A/C temp was preset to. Seeing as a replacement temp control unit was going to cost $700+ USD I thought I haven't anything to lose, so out came the unit which is fairly straightforward - just take care as the clear plastic surround behind the black faceplate may be brittle and split if man-handled roughly, removed the black plastic housing (all the buttons simply pull off), and had a look at the electronics on the PCB.

First suspect was anything mechanical, as after 24 years these typically are first to fail. Checked the temperature selector, which is a linear potentiometer, from the solder points on the circuit board and that measured fine with a multimeter. I then suspected the 3-legged BC337 NPN transistors (2 of). With a good light and magnifier you'll be able to identify their markings. After procuring 2 of these through Jaycar (NZ's equivalent to Radio Shack) I very carefully desoldered the old ones from the PCB. You'll need access to a fine point soldering iron and solder wick, plus some good side cutters. Fitted the replacements, and cut off the excess leads, reconnected all the wire harnesses , set the control arm manually to full heat, along with the temp controller on the A/C control unit, started the car and when I adjusted the temp to full cold...viola! The mixing flap moved in sync to full cold. Left the car running for a few minutes, moved the temp control and the mixing flap moved accordingly, producing warm air. Moved it back to full cold and nice cool air came out the vents. Whilst there I removed any oxidation off the posts the wire harnesses connect to on the temp control PCB using a small wire disc attached to a Dremel.

Now this of course may not work for you but in my case it did. A problem that may have cost nearly $1000 NZD to buy a brand new Temp controller for your 944/968 was (in my case) fixed for $1.20 NZD ($0.85 USD). WIN!

curtisr

Nice!

jhowell371

Well done sir

mytrplseven

Good on ya.

Jay Wellwood

Great information - thanks for sharing!

fossberry

Just as a side note - these were the same transistors in the G10 fan relay that failed in her car. Once again a cheap fix without replacing the relay. Here's the post from 4 years ago:

https://rennlist.com/forums/924-931-...urning-on.html

FRporscheman

Thanks for sharing! My 968 also has a problem with the HVAC - it blows cold air no matter what, except for occasionally it'll change to the setting of the **** (which I leave on hot so I can notice it).

I'll try opening up the control unit and checking those transistors!

fossberry

Worth checking definitely! Could also be the potentiometer that controls the temp (cold solder joints). Definitely clean the posts the wire harnesses connect to whilst there

culture

I believe that the CCU is what's malfunctioning in my car. The car will switch to cold or hot or medium on its own time and not always to what the dial is set to. BUT, the fact that it does change eventually means that the motors/flaps/vacuum is working. its wither either the sensors or the CCU. my bet is on the latter.

fossberry

Yeah my bet is on the latter for the same reasons you outlined. May be the cabin temp sensor by the glove compartment (long shot) but I'd suspect a bad solder connection on the temp selector, oxidation on the posts the wire harnesses connect to or those transistors. I'd reflow the solder on the selector, replace those transistors and clean the posts. Cheap and easy enough job. Let me know how you get on