Autoheat and footwell blowers

porschenut · 01-18-2008, 06:07 PM

I have one rant and one question. First, the rant.

The heat exchanger temp sensor will not permit the footwell blowers to operate until it thinks the air flowing past it is warm enough. In other words, you can't have maximum heating until the car is warmed to operating temperature. That makes no sense, because the colder it is outside, the longer it takes the engine to warm up sufficiently to provide you with the heat that you desperately need as soon as possible!

The scenario I have every day is that I start feeling warm air from the vents LONG before the stupid relay will let the footwell blowers run. Those German engineers were brilliant but lacked any notion of common sense.

What happens if I disconnect the heat exchanger temp sensor from the control unit?

That was the rant. Now the question:

When driving around town (not highway) on a 32 degree day, about how long should it take the engine to get to operating temp? Because on such a day I can drive around town for an hour and never get to 90 C. As a result, I'm usually at my destination long before the footwell blowers can ever start. Why won't my car warm up faster? One thought is that the front oil cooler thermostat is stuck open. Any other ideas? Or is it normal for the car to never really get warm on a winter day?

I did a search and read for about an hour but couldn't find anything that addressed my concerns. Any ideas?

Mysterytrain · 01-18-2008, 06:51 PM

I think that was one rant and two questions. Anyway, I would guess you could bypass the flapper box sensor. I'm not sure what the circuit wants to see. It might be something as simple as shorting two pins or providing a ground. Back in the days when my 78 had a rear blower the heating system sucked. Once I removed the rear blower and installed the footwells it was like driving around in an oven. If it were my car I would check a couple of things. First make sure the rear blower is working. If it isn't it will block the airflow. Even though the controller is designed to prevent the footwells from working if the rear blower doesn't work there is always to possiblity that the controller has an issue. Second, check the flapper boxes, make sure they open and close with the heat switch and make sure that the flex hoses are connected and not leaking heated air. Inside the autoheat console there is a rod that operates the flappers make sure its connected.
Back to the rant. Most new cars have the same'feature' on the heating system that prevents cold air from blowing into the cabin until the preset is reached...those crazy germans!

porschenut · 01-18-2008, 08:10 PM

Quote:

Originally Posted by Mysterytrain

Back to the rant. Most new cars have the same'feature' on the heating system that prevents cold air from blowing into the cabin until the preset is reached...those crazy germans!

I appreciate your input Ron. However, the problem in this case is that the air is already plenty warm and ready to be forced into the cabin LONG before the engine is fully warmed up to operating temperature. Like I mentioned, I have a heck of a time even getting the engine warm enough on a long drive to allow it to engage the footwell blowers. So, the cabin takes forever to heat up. Today, the blowers came on at about 80 C of engine temp, which took me 45 minutes of driving to reach. It just seems ludicrous to me that maximum heating is not available until after you don't need it anymore.

BTW, all blowers, relays, and flapper boxes work perfectly. I also adjusted the heater control potentiometer to spec, according to the Bentley manual.

javadog · 01-19-2008, 03:53 AM

I don't know what is different about your car but I have two '86 Porsches with autoheat and the blowers work on both of those if you turn the dial past 6 or so. The higher the dial is turned, the higher the blower speed. No waiting at all.

You'll never get a 911 to 90 degrees in the winter, where you live.

JR

Mysterytrain · 01-19-2008, 05:42 AM

Dave,
We might be blaming the wrong sensor. Maybe the interior sensor is telling the autoheat that the cabin is warm enough. Can you unplug the interior sensor and see if the system will function without it?

cmonref · 01-19-2008, 06:45 AM

If you adjust the autoheat to full ON, also labeled DEFROST, it should ignore all of the sensors and provide max heated air thru the flapper boxes. At least that is how my '80 works.

The autoheat system does not care what the engine temp is -- it looks only at the temperature of the air coming off of the heat exchangers. That is the air that is ducted into the cabin. That air is getting warm by the time you reach the first stop sign on your daily commute, and downright hot after a mile or two. If no air is moving thru the heat exchangers, that sensor sees only cold ambient air.

Again, from the '80 system, the flapper box sensor is an NTC thermistor (resistance varies inversely with temp). The spec is 135 ohms at 100C. For the '84'89, I'll bet the thermistor is the same (maybe a different R-T curve) but the spec is 1.7 k-ohms at 20C.

For the '84-89, if the flapper sensor is an open circuit, the system commands full heat. If the sensor is shorted, it commands zero heat. Maybe your sensor is "almost shorted"?

My flapper box sensor was "almost shorted" years ago -- a build-up had created a bridge across the sensor, reducing the resistance to the point that the autoheat never came on. I cleaned the sensor and the autoheat began to work better. Later, it shorted internally and the autoheat simply quit.

I recently replaced the sensor with a new NTC thermistor with specs close to the factory spec. $2 for the sensor vs $229 for the factory part. Autoheat now works.

911 in SC · 01-19-2008, 06:57 AM

Now that's a thought Ron! In my car, even on 1, it will roast the passengers! Of course, to cool it down a little, I cracked my window - not good, as the interior sensor is concluding that it's COOL inside the car and gets even hotter!

Any ideas?

Brian, I'm not understanding what parts you cleaned (sorry, I'm learning - very, very slowly!). I'm curious if I could have this problem too.

Thanks,
Mike

imcarthur · imcarthur's Garage

for clarity . . .

Ian

porschenut · 01-19-2008, 08:04 AM

Quote:

Originally Posted by 911 in SC

Now that's a thought Ron! In my car, even on 1, it will roast the passengers! Of course, to cool it down a little, I cracked my window - not good, as the interior sensor is concluding that it's COOL inside the car and gets even hotter!

Mike, it's also possible that the potentiometer isn't adjusted properly. I had the same problem a couple years ago, and I followed the procedure in the Bentley manual to adjust it, which fixed the problem. From the troubleshooting table in the manual:

Symptom: Too much heat, does not regulate correctly.
Probable Causes: Open circuit in heat exchanger temperature sensor -OR- Temperature control potentiometer set incorrectly.
Corrective Action: Test sensor resistance at connector. Should be 1.7k ohms at 68F. Test resistance at temp control potentiometer terminals with switch in position 5. Should be 950 +/- 20 ohms.

porschenut · 01-19-2008, 08:09 AM

Quote:

Originally Posted by Mysterytrain

Dave,
We might be blaming the wrong sensor. Maybe the interior sensor is telling the autoheat that the cabin is warm enough. Can you unplug the interior sensor and see if the system will function without it?

True, I just assumed both of the sensors are working to spec, which may not be the case. Next time I have the passenger seat out, I'll test the sensor resistance at the connectors.

porschenut · 01-19-2008, 08:18 AM

Quote:

Originally Posted by Brian K. Haggard

If you adjust the autoheat to full ON, also labeled DEFROST, it should ignore all of the sensors and provide max heated air thru the flapper boxes. At least that is how my '80 works.

That's how I thought it's supposed to work in my 86 also. At this point I'm convinced there must be something wrong.

Quote:

Again, from the '80 system, the flapper box sensor is an NTC thermistor (resistance varies inversely with temp). The spec is 135 ohms at 100C. For the '84'89, I'll bet the thermistor is the same (maybe a different R-T curve) but the spec is 1.7 k-ohms at 20C.

Now that does make sense. I would expect the sensor to signal the relay to call for heating very soon after driving away with a cold engine. I do get heat, but the only blower in operation is the engine compartment blower, even when my switch is turned to DEF.

I could swear my footwell blowers came on yesterday, but I better check the inline fuses to be sure. I'm suspecting sensor failure as the likely culprit.

porschenut · 01-19-2008, 08:24 AM

Hey, I just found an interesting note in the owner's manual. See the third "warning" point below, about maximum heating output:

cmonref · 01-19-2008, 09:25 AM

911 in SC: the part that I cleaned was the one labeled Heat Exchanger Temperature Sensor in the outstanding picture posted by imcarthur.

To get to the sensor, disconnect the driver's side flexible duct from the flapper box. Look up inside the duct towards the flapper box and you will see the sensor -- a mirror will be necessary. The sensor sticks out of the end of a 1/2" tube where it can sense the airstream coming from the heat exchanger. An old toothbrush can brush off the sensor.

Porschenut: The third warning is quite literally true -- the harder you drive the car the hotter the exhaust headers get and the hotter the air coming out of the heat exchange wrapped around the headeres. But -- a 911 heater system will produce lots of heat at 2500-3000 rpm in town well BEFORE the engine gets to "normal" temperature. Mysterytrain's use of the word "oven" in an earlier post is spot on.

The problem with 911 heat has ALWAYS been its inability to deliver usable heat -- meaning defrost on a cold morning -- while sitting in the driveway.

911 in SC · 01-19-2008, 12:59 PM

Brian, Dave, etc.,

Great info. I had the pic that Ian so graciously posted (also on his descriptive website), but didn't know which item was which. Thanks for the clarification. Thanks again guys. I really do appreciate it. I really do need to get out my Bentley and start reading...

porschenut · 01-19-2008, 03:18 PM

Took the car out for a long drive today and got it nice and warm. I checked very carefully for operation of the footwell blowers, and neither of them ever came on even though the air was hot. The factory manual says they're supposed to be on at all times when heat is called for, and the inline fuses are fine. After reading through Ian's troubleshooting guide, I'm betting it's the heater relay in the engine compartment.

imcarthur · imcarthur's Garage

Quote:

Originally Posted by porschenut

After reading through Ian's troubleshooting guide, I'm betting it's the heater relay in the engine compartment.

You should test the fans. Run a 12V line to each & see if they run. The motors can seize & the heat can effect them adversely too.

If they work on their own, then the relay could be the culprit. Ron (Mysterytrain) is our resident expert on it.

But are you sure the main blower is working? My footwells don't work (I'm just lazy) but I still get lots of heat. Even top down. And have you checked the actual flapper inside the flapper?

Ian

imcarthur · imcarthur's Garage

The complete heat exchange duct & valve.

The flapper inside:

Ian

porschenut · 01-19-2008, 04:46 PM

Quote:

Originally Posted by imcarthur

You should test the fans. Run a 12V line to each & see if they run. The motors can seize & the heat can effect them adversely too.

They're both about a year old. I replaced them last year with brand new units after the originals started to squeal. I haven't checked them recently, but it might be a good idea before shelling out for a new relay.

Quote:

But are you sure the main blower is working? My footwells don't work (I'm just lazy) but I still get lots of heat. Even top down. And have you checked the actual flapper inside the flapper?

I checked the flapper boxes this afternoon and they seem to be opening/closing normally. The main blower works great. It's not hard to hear at all. I do get warm air output at all the vents, just not blasting like I remember it doing last year.

By the way, I've been reading for a long time that the footwell blowers didn't get inline fusing until the '87 model year. In fact, my car has a build date of 6/20/86 and it has factory inline fuses. The factory must have started adding them shortly before the end of the '86 model year production.

Rot 911 · 01-19-2008, 05:02 PM

If you are turning the control up to 8, 9, or defrost and are not hearing the footwell blowers come on then the problem is not in any sensor. They should always come on when set to those numbers.

imcarthur · imcarthur's Garage

No, mine never blasts air. It seems to seep pretty well though. Before you shell out for a new relay, try the jumper solution. It did nothing on mine, but I never went further.

Re the inline: Porsche dealers might have installed them automatically as cars came in for warranty. I don't know. And the start/stop date for changes was often fuzzy & not at MY launch as well.

Ian