Bjbpe

It's getting colder up here in the Rockies and that brings to mind a problem I have with the heater control.

If I slide the temperature setting bar anywhere from dead off to just a squigen (definition - very small amount) from the hot end, I get no heat. Then moving it a squigen more brings the heat on full blast. This is not associated with the fan control **** which can be on or off.

I'm an old guy who used to change engines back in the fifties with a block and tackle hung on a limb of an apploe tree. I've never seen so many different controls in my life. Wonder why Porsche, in it's infinite wisdom, couldn't have come up with a central electronic system similar to the fuse panel. We did it with Hamilton Standard control boxes on our jet engines and nobodey is going to tell me that this is more complicated.

Thanks in advance guys.

MainePorsche

Though we don't know the year of your 928, the control for the heater core is the same. There is a vacuum controlled (surprise) heater valve under the airbox. First check vacuum line for any defects/leaks. Then check if the arm on the valve moves up/down cleanly. The heater valve, if needed is 25.00.

MainePorsche

Here's a pic of the valve.

Alan

I think its clear the heater valve works well. however the sensor loop is broken - these are the classic symptoms.

There are micro switches at each end of the sliders that overide the temperature control loop.

At the left end the override is full cooling (almost like Max AC if you like). The water valve is closed and mixing valves direct flow around the heater core and the bypass flap send air direct to the center vent.

If the sensor loop is broken - when you move off this point you will get full heating - that is the default if the loop is open circuit.

Check the ext temp sensor wiring in the drivers side front fender under the headlight (wheel off and splash shield removed)

Alan

Bjbpe

Hello Alan:

That sounds like the problem. How would one make this "check"?

Thanks for your help.

Barney

Alan

first look for the ext sensor and see if its even there - you can search on it here.

Its in the air pipe intake to the alternator cooling hose - should be attached to the rear of the splash shield - if its there make sure the 2 wire connection to it looks good...

lets go from there.

Alan

StratfordShark

Is the original description of problem correct?

The classic sensor string problem is no heat on full cold (microswitch closed) but full heat anywhere off that position.

Poster says it's cold up to just below full heat, then full heat if temp moved slightly higher.

If say ext sensor was bad then would be full heat all positions except full cold.

I would start with checking all the vacuum lines/actuators, but if they're ok I would check resistance of total string at mixing motor, then check what the mixing motor is doing at different slider positions (could be problem with the comparator electronics in the mixing motor)

Bjbpe

To Mr. StratfordShark:

Yes, the description is correct. No heat unless I push the slider all the way to the right. Anything less results in no heat at all. I'm going to eventually have to fix the AC also (it does not work) but that's not the problem now. Just want to offset the Wyoming wind right now. My 76 year old bones are chilly. Thanks again to all of you guys.

Barney

Alan

OK well I did missread that a bit... thought I read it was @ the left end - sorry - however the description here is similar too.

There are actually 2 slider micro-switches one at the hot end and the one at the cold end (already described).

The cold one is basically no temp control feedback full cold (with good AC its really cold - but without AC its ~external ambient).

The hot one is unconstrained no temp control full heating - the water valve opens and the mixing flaps go fully open and stay there.

In between is where the automatic closed loop temperature control is supposed to work - based on the interior & exterior temp sensors.

That it doesn't work and you get no heat until you hit the full hot microswitch at the the right end suggests that the cold microswitch may be sticking on or that the temp loop is shorted out to something - rather than being connected properly or open circuit...

If both microswitches are on - the hot one wins... If the sensor loop is shorted to ground it will go cold - or certainly colder than intended anyway (in the middle slider range) - depending where the short actually is...

So slightly different story - but mostly the same diagnostic steps.

I'd still first take a look at the outside sensor - see if there is obvious wiring damage there.

Otherwise you may need to open up the head unit and see what the switches are doing - the unit is actually easy to dissassemble and the operation is fairly evident...

Alan

StratfordShark

Thanks for clarifying Barney.

It's turned chilly today in England too!

If your ext sensor (or internal) connector was open then the setting motor would "see" total resistance demanding full heat.

While ext sensor or wiring could be faulty (often is) it can't be deduced as such from symptoms.

You will need to start with checking health of vacuum actuators though. Good first and skimple step is to access main feed to hvac which is black line from 4 way vac connector under airbox (fed from brake servo), pump vacuum, see if holds with ign and hvac off then turn ignition on and move position slider. What happens to vac pressure?

StratfordShark

Alan is absolutely right - a short in any of sensor wiring would have opposite effect I described with an open circuit i.e. Full cold demanded unless hot microswitch overrides.

I have learnt something too - never knew of the hot micoswitch. Thanks Alan!

Bjbpe

I'm learning a whole lot too. Thanks to both of you guys. I'd be happy to teach you aerospace engineering but this car is lpossibly even more complex.