How does the heating system work for 74-89?

[Dublinoh]

Originally Posted by ceejay1135efe

Hi Can you help,
Ive just replaced the fresh air blower motor and this works!!!!
The footwell blowers have never worked - if I remove one and add 12V they work fine. does anyone know where they are switched from ?
Please help
Thanks Chris 1986 911 turbo bodied cab

My 87 Carrera footwell fans come on when the heat knobs are lifted. Round knob is part of the auto heat. I have the round knob...but not auto heat...and as near as I can tell it doesn't do anything.

[javadog]

There's a third system that looks like auto heat but isn't. It has a similar rotary control to the auto heat option but it is basically just a fan speed control. I'be betting that's what Chris has, although auto heat was an option for a cab in 1986. Either way, you turn the knob and the fans should come on.

JR

[Nick Triesch]

I love my car but even a yugo would blow cool air on the drivers face! But not my 85 Carrera! 27 fans and 14 levers and no cool air on my face! How did that happen? How did they make a $35000 car have terrible heating and A/C?

[donagain1]

Originally Posted by ceejay1135efe

Hi Can you help,
Ive just replaced the fresh air blower motor and this works!!!!
The footwell blowers have never worked - if I remove one and add 12V they work fine. does anyone know where they are switched from ?
Please help
Thanks Chris 1986 911 turbo bodied cab

There's a thread somewhere on here which explains the circuitry for the footwell blowers very well, better than I am able to, but here's the overview:


The middle fuse of the 3-fuse panel behind the plastic cover in the engine compartment provides the power for the fan heating fan control unit, which is a plug-in device directly above it. The fan control module routes the 12V to the footwell blowers through the rotary switch between the seats. Here's the key to it all: The 12V for those routes through contacts of a relay in the module. The winding of this relay is in series with the engine compartment blower motor.The relay will NOT operate (and thus provide power to the footwell blowers) unless there is current flow operating the engine compartment blower. In normal operation, this means you must raise one or another of the red-knobbed levers between the seats, which provides the 12V to flow through the relay, onward to the rear blower, and back to the module to ground. In other words, any open in the circuit which powers the rear blower, fails the footwell auxillary blowers because the fan control relay won't activate.

In my case, I could raise the levers in the cabin and see 12V across the pins of the rear blower connector, but the rear blower was dead when it was plugged in. This placed the trouble in the rear blower motor. I removed it, opened the motor case in an attempt to repair it myself, and found that the entire plastic structure within it that holds the commutator brushes was burnt to a cinder. Replacement was the only option. When you think about it, that blower assembly is subject to a tremendous amount of heat over its lifetime, it amazes me that they last so long.

My recommendation: Open the engine compartment, switch the ignition on, raise the red-knobbed levers, listen to discern whether the rear blower motor is operating. If it isn't, pull the 2-pos. motor wire connector and meter betwen the pins. If Voltage is there, replace the motor. If Voltage isn't there, check fuse number two under the cover. If the fuse is good and has the 12V through it, you probably have a bad fan module. All the parts are available here.

Don

[javadog]

Originally Posted by Nick Triesch

I love my car but even a yugo would blow cool air on the drivers face! But not my 85 Carrera! 27 fans and 14 levers and no cool air on my face! How did that happen? How did they make a $35000 car have terrible heating and A/C?

Cool air on your face comes from the A/C. If your car was built without A/C, you could get fresh air through the dash vents. If you've removed your A/C components, you can change what you have to allow fresh air to be ducted to the dash vents. It's been discussed here before, in detail.

Porsche's system seems complicated if you don't fully understand it. Most of the complaints come from people who don't have a good understanding of how it works and half of those have never even read the section in the owner's manual about how to use it. AND, there were several minor variations over the years, depending upon the market the car was built for, whether it had factory A/C and a few other things, so it helps to know something about the specific system in your car, and not what someone else has. And, if you've modified it, as many do, it helps to not just do "the removal" part and quit there.

Likewise, the heat is great, if everything works and it's adjusted properly. You can heat up the car much faster than a normal car.

JR

[baloo]

Originally Posted by Nick Triesch

I love my car but even a yugo would blow cool air on the drivers face! But not my 85 Carrera! 27 fans and 14 levers and no cool air on my face! How did that happen? How did they make a $35000 car have terrible heating and A/C?

Have to agree with you, Nick. The idea of needing to read instructions how to operate a heating/cooling system is just baffling -- even as far back as the early 70s. Furthermore, to view heat/cold as a superficial excess speaks volumes about the engineering department's lack of total comprehension of automobile market. Luckily it took the Japanese to come up with the outrageous concept of "user friendly" and apply it to the automobile. So nowadays, anyone without all the creature comfort options that work easily and well, are immediately out of business.

It's like the preference for single-shot rifles over full automatics -- twenty-30 years ago, full auto was seen as a waste of bullets. NOT ONE soldier today will echo that ridiculous old idea.

[javadog]

There are two ways to look at it. What some people don't consider is that the system Porsche used gives you far more flexibilty than systems in other cars. Some of the BMW's had pretty decent controls that gave a lot of options to the user, but the 911 system was better in that respect. Modern systems are more user friendly but the climate control computer makes some assumptions about what you want and tries to give it to you. Sometimes that works, sometimes it doesn't. I can teach someone everything about a 911 system in about five minutes.

JR

[Nick Triesch]

Javadog, I did study the controls on my car. I know fresh air comes out of the defrost vents. . I know you can remove the A/C and re route stuff to get fresh air on your face. I know the heater works great. My point is that you should not have to do all of that stuff to get stinking air on your face. These are great cars but the heating and ventilation system is just stupid. So is the A/C. You can teach me everything about the 911 system in about 5 minutes but it still will not put cool air on my face when I am driving. That is the one main option I want.

[tazzieman]

Originally Posted by Nick Triesch

You can teach me everything about the 911 system in about 5 minutes but it still will not put cool air on my face when I am driving. That is the one main option I want.

A flexible duct under the ashtray coming from the A/C blower can direct plenty of air at your face or other body parts. Convection of even warm air has a cooling effect. The factory fan is plenty powerful.
Ice cold is overrated, except when prefacing the word "beer".

[javadog]

Originally Posted by Nick Triesch

You can teach me everything about the 911 system in about 5 minutes but it still will not put cool air on my face when I am driving. That is the one main option I want.

So, your A/C doesn't work?

JR

[Nick Triesch]

Yes my A/C system works fine. My point is that in a car like a Porsche if you want fresh air in the cabin or on your face you should not have to turn on A/C to have it. You should just pull a lever like any other car. VW had the right idea with old Bugs. Two simple levers for heat and defrost and a wing window for fresh air. Worked great on a trip!

[javadog]

If you want fresh air in the cabin, open the outside air vent and turn the blower fan on, if you want. You can use the A/C to blow cold air on your face.

Or, open a window, like in a bug.

Or, pipe fresh air to your center dash vents. There's a thread on Pelican on how to do it.

JR

[Va_Powerman]

Dredging up an old thread with a question - can anyone tell me why, with the levers set (apparently) correctly, I get warm air out of the passenger defrost vent, and cool/cold air out of the drivers defrost vent? Could it be a heat exchanger valve issue?

[javadog]

You might not be getting hot air into the left side of the car, back at the outlet from the heat exchangers. Or, the left side distribution box may not be directing the hot air properly, either because a cable is incorrectly adjusted, or there is something wrong with the box internally, or the small duct to the defrost vent is not connected.

JR

[Va_Powerman]

Thanks for the quick response - it gets cold being the driver.

For other searchers, I found the following article which is interesting:

Porsche 911 heating systems - Philip Raby Porsche

[javadog]

What year is the car? I assume you're getting no warm air at all on the left side? You want most of it to come out under the dash into the footwells, if you are cold while driving.

JR

[dyount]

I have an 83 SC with autoheat.... my problem is the fan in the engine compartment only runs on one speed. Should it not be 3 speed based upon the dial on the control box? and would it be a control box issue or a relay somewhere?
Basically the heat in mine works damn good even on real winter days in the NE

[javadog]

Is your '83 a cab? Do you have auto heat, or do you have the similar-looking box between the seats that has a control for fan speed?

JR

[dyount]

my 83 is a Targa w/autoheat. Just not sure if the 3 stage fan speed is regulated by the autoheat box or from ? a relay in the engine compartment

[javadog]

I think you only had a one speed fan. Porsche introduced a pair of footwell blowers on the '83 turbo, controlled by a switch on the dash to the right of the steeering wheel but I don't think the '83 SC targa had that. The later 911s got the footwell blowers and the autoheat control unit was different. There were 3 blowers speeds available, depending on the dial position.

JR