Heater/climate control unit issue

wolfie308 · Sep 12, 2017

Chaps,
I've got an ongoing intermittent problem with the temp control unit. Set to minimum, it's usually alright on start up and okay for either a short or long drive until you stop. On restart, the heater often goes to very hot and takes a few miles to return to ambient air temp. A couple of sensors were changed by the previous owner's Indy, but the problem persisted. I have an untested spare unit which I plan to swap over to test.

One thing that's puzzling me....this unit has a notchy action on the temp knob, about 1 click per degree but on my current and 2 previous S.2's, it's a smooth rheostat action. Maybe not a big deal, just curious......could it be from an earlier year car?.....maybe '86 - '88? Interested to hear who's got a notchy and who's got smooth knob action[

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bmnelsc · Sep 12, 2017

IIRC the temp knob on my 91 S2 "clicks" slightly between the degree markings on the dial. You don't mention fan speed during the period it goes "very hot". From reading about other HVAC problems this sounds like it might be one of the resistors for the fan circuit - when the resistors "overheat" the fan runs fast until the resistors cool down. If the fan blower doesn't act this way then its probably something with the temp control unit or the ambient temp sensor (the one hiding behind the little circular grill with horizontal openings). I'd check the ambient sensor first in this case as it seems the HVAC control thinks its very cold on re-start and after running a while it figures it out...(need to confirm that no signal from the sensor = cold and high signal from the sensor = warm; I'm just guessing).
You might also try turning your temp knob to full cold before restart (I would think that should "switch out" the ambient sensor) and see if it still goes to hot. If not - then once started dial in your desired temp and see if it gets there or if it goes hot for a few miles as you describe. Not a very scientific approach but it might show some different behavior.
I understand there are no known published specs or diagrams on the internals of the temp control units but there is a place in the US (they advertise in Panorama magazine and IIRC Excellence magazine) that offers rebuilt units. See here...http://www.ecudoctors.com/porsche-944-climate-control-unit-944-653-021-04-hvac.html. They might have some trouble shooting help to isolate the problem.
Oops! - the above website says they can provide schematics if you email them directly...
Good luck!

pauljmcnulty · Sep 12, 2017

I had the clicks on my Lux, but the smooth version on the S2 with the "climate control" temp dial rather than the simple blue to red one. I thought it odd, as the newer version had defined temps that should click between....

wolfie308 · Sep 13, 2017

Hi Bruce, many thanks for your very helpful analysis. I tend to agree, I don't think it's the HVAC unit, more likely to be related to the ambient and/or temp sensor or the dashboard control unit. Looking back through the paperwork, the under bonnet sensor was changed a few years ago, so I'll assume that's okay. I don't know how to test the ambient sensor, so having a spare albeit unknown condition control unit, that's probably the best place for me to start. I've always had the temp control set to minimum when the problem presents itself on restart.

But firstly, I'll have to fabricate a repair to the clear mounting plate of the unit which is made from a wafer biscuit.

Paul.....I thought it was going to be that the earliest units that had the clicks, but that is confounded by Bruce's click type on his '91 car, and on my current '89 car it's definitely smooth action[&:]

More feedback from other owners would be of interest

Eldavo · Sep 13, 2017

The valve that controls hot coolant flow to the heater core is a vacuum operated one down the back of the head above the reference sensors.

Check that the vacuum hose for this is secure on the valve itself and where it goes into the bulkhead.

jasonp · Sep 14, 2017

Hey Wolfie,,
my 1990 is smooth as was my 89,,
No clicks,,,,jasonp

PSH · Sep 14, 2017

Hmm..., guys, I believe that there are two types, one is normal A/C whereas the other is full climate control..I had thought/read that the full climate control was on the earlier cars whereas standard A/c was fitted to later cars as one of the many cost cutting methods implemented, perhaps climate control was still an option? someone more knowledgeable on this may know the details?

cheers

Pete

Waylander · Sep 14, 2017

I have some new thermistors to go in the ambient temp sensor and the under bonnet sensor somewhere, have to go on the search

wolfie308 · Sep 15, 2017

Thanks David, I'll have a look a those when it stops raining long enough.

Jason....smooth action further suggests the clicky ones are early model apart from Bruce's which may be different as his is a Japan/ROW model.

Pete.....my car shows option code 424....'automatic heater control' according to Clarks Garage, AKA climate control I thought due to the dash sensor? As you know,full air conditioning was sadly a very rare and expensive option on the S.2

Martin....if swapping out the control units doesn't work, I'll give you a shout re the thermistor for the ambient sensor. Would I be right in thinking the circuit works like Wheatstone bridge?...2 fixed resistors, the variable temp knob and the sensor in the dash? Just a guess based on 'O' level physics from many decades ago.

PSH · Sep 15, 2017

wolfie308 said:
Pete.....my car shows option code 424....'automatic heater control' according to Clarks Garage, AKA climate control I thought due to the dash sensor? As you know,full air conditioning was sadly a very rare and expensive option on the S.2

I think you'll find it's the other way around sir...A/C is cheap, climate control is not.....much more complicated and thus much dearer...

cheers

Pete

wolfie308 · Sep 16, 2017

Hi Pete, my humble apologies, I didn't mean to sound rude, I wasn't sure I understood your post. I thought there were only two options on our cars. A/C system with condenser and compressor which can cool cabin air to below the outside temp, and climate control which maintains a selected temp but can't cool below the outside temp. Wasn't A/C standard on Turbo's and a very rare and expensive option on the S.2?

I guess what I'm trying to ascertain is....'will my clicky temp knob spare unit be a straight swap with the installed unit with its smooth action?' before dismantling the dash. Pics show the spare with repaired clear plastic mounting. Not pretty, but stronger than the original.....which wouldn't be difficult.

PSH · Sep 16, 2017

No prob sir, I didn't take it as anything rude....climate control is basically an 'intelligent A/C' it works just as an A/C for cooling, it will go colder than outside temp but it can also keep the interior temp at any temp that it's set for by mixing both warm and cold air, this temp will stay no matter what the temp is outside. This, of course, requires all of the sensors to be working correctly, something I doubt is the case today for most of the cars so fitted unless they have had a full overhaul of the system.

cheers

Pete