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Pelican Technical Article:

Heater System Repair


2-10 hr


$10 to $500



Applicable Models:

Porsche 911 (1965-89)
Porsche 930 Turbo (1976-89)

Parts Required:

New flapper boxes, gaskets, hoses, new heat exchangers, blower motor

Hot Tip:

Fix any oil leaks prior to fixing your heating system

Performance Gain:

Warmer car, better defroster

Complementary Modification:

Install stainless steel heat exchangers
101 Projects for Your Porsche 911

This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.

The heating and defrosting system on the 911 is one of the most confusing and complex systems on the car. It's also one of the most common systems to fail. On top of that, these systems didn't really work too well when the cars were new. Heating and cooling systems on air cooled cars have always been difficult. Combine that with the fact that the engine is in the rear of the car, and you have some added difficulties.

Nonetheless, there is plenty that you can do to restore your heating system back to its original, somewhat lackluster performance. The heating and defrosting system for the 911 relies on hot air that is channeled from the motor to the exhaust pipes, through the sides of the chassis, and into the area behind the dashboard. Needless to say, all this traveling causes the air to lose some of its heat and velocity by the time it reaches the passenger compartment.

Throughout this trail of winding heated air, there are plenty of spots for it to leak out and lose pressure. If there is a significant break in the system then the heating power will be greatly reduced. If you have an open-air car like the cabriolet or a targa, then you will immediately recognize the importance of having a working heated defroster on a humid day.

The source of all this hot air starts in the engine compartment. Depending upon which model and year you have, the orientations of the hoses and blower motors may vary. On most cars, there is an adapter plate on one or both sides of the engine fan that funnels air down into the heat exchangers. The heat exchangers are nothing more than a exhaust header system with sheet metal wrapped around the pipes. The air from the fan is funneled through these heat exchangers, and heated by the fact that it is flowing past the hot exhaust pipes.

With some of the Porsche 356 models, there were problems with the European heat exchangers rusting through the pipes. This caused the exhaust to become mixed with the air that was fed into the engine compartment. As a result of this, a few people died from breathing in exhaust gases. Although no reported cases of poisoning have been reported with the 911, it's important to remember that your fresh air is circulating past the exhaust system. If there is a problem with a significant amount of rust in the exhaust system, then you might have some leakage from the pipes. Make sure that you inspect your heat exchangers carefully and replace or repair them if they look like they might be developing leaks from rust holes. For the early heat exchangers, there are separate replacement parts available that can be welded in for repair.

Another important consideration is that the heat exchangers are located right below the engine. This means that any oil leaks from the engine will have a likelihood of dropping oil onto the top of the heat exchangers, mixing with the air that is channeled into the cockpit. When the car warms up, this oil will start to burn and smoke, and you can actually have smoke coming out of your dashboard vents if you are not careful. With this in mind, it's a wise idea to repair all of the oil leaks that might drip on the heat exchangers before you activate your heating system. It's not a wise idea to use your heating system if you can smell burnt oil in the air.

The hoses in the engine compartment are manufactured out of aluminum and paper, and have a tendency to crack and break over the years. Inspect and replace these hoses with new ones if you find any damage to them. Insuring that these hoses are intact is the first step to improving your heating system. Underneath the engine, there should be additional hoses that connect the heat exchangers to the hoses in the engine compartment. Inspect these too, and replace them if necessary. In particular, the A/C bracket has a tendency to poke a hole in the paper hose that runs right next to it, in-between the fan and the bracket.

On the 911s made after 1978, Porsche installed a separate blower motor in the engine compartment to give an extra push to the air that's channeled to the cockpit. This blower motor is connected to the air intake of the fan through a plastic hose that often develops cracks over the years. It's a wise idea to replace this hose if it's cracked, as a lot of extra air can slip out of this hose.

Exiting out of the front end of the heat exchangers, the warm air enters what is known as the flapper boxes. These two boxes, located on opposite sides of the car, control the flow of hot air into the cockpit. They have flaps inside that are either open or closed. These flaps are connected via a cable to the two levers that are located in-between the passenger and driver's seat. These levers also have an electrical switch built-in that operates the engine compartment blower motor on the later 911s. It is important to inspect these boxes, as they are often susceptible to rust and deterioration. Also inspect the hoses that lead from the heat exchangers to the flapper boxes.

If you decide that you need to replace the flapper boxes, exercise care when removing them from the car. They are attached to the chassis via small studs that were welded into the car when it was assembled in Germany. If you accidentally break off one or more of these studs, then you might have a difficult time reattaching the flapper boxes later on. When installing a new flapper box, make sure that you use a new seal between the flapper box and the chassis.

If the heater hoses that connect the flapper boxes and the heat exchangers are broken or ripped, then you should replace them with new ones. Be sure that you only use high temp heater hose, as most other ordinary hoses from hardware stores will melt when exposed to the high heat from the heat exchangers. There are very good aftermarket hoses available that were originally designed for the aerospace industry that will work well in your Porsche.

The flapper box cable is another source of problems for the heater system. This cable has a tendency to break on the older cars. To replace it, you need to remove the small handle assembly located in the cockpit, between the two seats. Lift up the edges of the carpet, and find the small screws that hold the lever assembly to the chassis. When you lift up the assembly, you should be able to see the catch for the cable underneath. The new cable needs to be threaded through the two holes in the chassis firewall from inside the cockpit (not always an easy process), after first being threaded through the lever assembly. Make sure that you don't damage the cable during this process. You may find it easier to work if you pull out and remove the passenger's seat from the car.

After you have fixed all the components that comprise your heater system, you should test it. Most people don't know how the heater system even works on most Porsches. As previously explained, the hot air from the heat exchangers is funneled behind the dash. The red lever on the climate control module determines whether the heated air will be directed at the windshield (for defrosting) or through the dash and lower vents for heating. In addition, there is a fresh air blower that mixes air from the outside with the heated air from the heat exchangers. By adjusting the volume of fresh air mixed with the heated air, you can change the temperature of the air exiting the vents. You can also change the amount of hot air released into the system by adjusting the heater lever arms in-between the two front seats. Pulling up the handles only part of the way will limit the amount of air that passes through the flapper boxes.

When installing new flapper boxes, make sure that you use new sealing gaskets.
Figure 1

When installing new flapper boxes, make sure that you use new sealing gaskets. The old flapper boxes should be taken off with care, as it is possible to accidentally snap off one or more of the studs that hold the boxes to the chassis. Once these are gone, it's very difficult to attach the flapper boxes to the car.

New flapper boxes are painted to avoid rusting.
Figure 2

New flapper boxes are painted to avoid rusting. When installing these new flapper boxes, make sure that you inspect the flapper box cable and the high temp heater hoses that connect the flappers to the heat exchangers. Only use hose that has been approved for high temp use. If you use normal hose available from a hardware store, then it might melt and create noxious fumes.

In the engine compartment, there are several hoses that warrant inspection and/or replacement.
Figure 3

In the engine compartment, there are several hoses that warrant inspection and/or replacement. The large plastic blower motor hose that acts as an intake from the fan often cracks and leaks air out the sides. This can actually have a detrimental effect on the cooling of your car, as this air now becomes 'wasted.' In order for your engine to get all the cooling that it needs, this plastic hose needs to be intact and installed. The long paper and aluminum hoses that connect the blower motor to the heat exchangers also often need replacement after many years of use. Finally, if the blower motor is not functioning properly it too may need to be replaced. Check the connections to the blower motor to see if there is any power getting to it before you replace it (see Pelican Technical Article: Troubleshooting Electrical Problems).

The hidden catch for the heater pull cable is located under the heater knobs.
Figure 4

The hidden catch for the heater pull cable is located under the heater knobs. These pull handles have a dual-purpose. They act as a physical switch, pulling open the flapper boxes. There is also an electrical switch underneath the pull handles that actuates the blower motor. If you wish to improve the look of your interior, the little red plastic tips on the pull handles simply pop off for replacement.

Comments and Suggestions:
Jonathan Comments: I have an '80 911SC. I pulled out the carpet to replace, and found there are flexible aluminum/paper hoses coming from the body just behind each wheel well to the underside of the dash. They are crumbling, and the drive side appears to have been "repaired" with what looks like an old school vacuum cleaner hose! What are these hoses called? I need to replace them before I put the new carpet in. The tech article on heater work didn't specifically name anything beyond the flapper boxes.
June 9, 2017
  Followup from the Pelican Staff: Heater hoses. Give The Pelican Parts parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
Greg Comments: The black knob on my fan control switch popped off. How do i get the climate control face plate off in order to put it back on? My car is a 1977 Targa. Does the plate just pop out?
May 13, 2017
  Followup from the Pelican Staff: Been along time for me. I think it is a unit and the faceplate doesn't come off. I am however going by memory. I opened a post in our forums. A Pelican community member may be able to answer your question.
- Nick at Pelican Parts
Fred Comments: Hi. I have a 1988 3.2 Carrera targa. The main under bonnet fan is running dry and noisy. How do I find the fan unit and is it able to be lubricated. I had a noisy fan in the rh foot well and was able to lubricate that one.
March 21, 2017
  Followup from the Pelican Staff: I don't believe it is meant to be lubricated, but you can try! I think you can just remove the front carpet and the fiber board cover and then start taking stuff apart in there. I am not sure you will be able to lube it though. Will post to the forums and see if another member can offer assistance. - Casey at Pelican Parts  
Allan Comments: I have 1985 Carrera coupe with the electronically controlled flapper boxes. They work and open heat access, but the footwell blowers have failed on both sides, as a result I get very little heat into the cabin.
Can I make some modifications to eliminate the blowers or some other option to get some heat? I dont need too much heat, I live in Texas.
February 15, 2017
  Followup from the Pelican Staff: I don't believe there is a delete for those the circuitry ties in to the rear blower. I suppose (but I am not familiar with this approach) you might be able to get rid of all the blowers if you don't need any heat. NICE HEADERS!! Bet she sounds sweet. - Casey at Pelican Parts  
Balloonmeister Comments: I have a 1987 930 Turbo. My problem is that I cannot get the heat turned OFF! I have hot air pouring in from both of the outside dash vents and even if I close those two vents with the knurled knobs there is hot air coming into the cabin from what seems to be the footwell area. No difference if I turn the heater knob in the center console to Off or all the way CW. Could my flapper boxes be stuck in an open position and, if so, what is the most likely solution?
February 15, 2017
  Followup from the Pelican Staff: I believe your car has the electrically operated flapper system with the knob in the center console behind the hand brake? If so it is very common for the screws which hold the assembly to the floor of the car to come loose. Make sure that these are all tight, and that when you rotate the knob to OFF you hear movement. It is also possible that the control valves under the car in the engine compartment have become corroded and stuck. You should be able to get under the car and operate these fairly easily. - Casey at Pelican Parts  
Larry Comments: My original push-pull cables that go from the dash of my '69 911E to the front flapper valves are toast. Any idea where I can get replacements? Can't seem to find them.
December 31, 2016
  Followup from the Pelican Staff: I’m not the best with part numbers.

Give The Pelican Parts parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
Ted Comments: I just had a new clutch installed in my '86 930 and now have no heat. The blowers are all working. The flappers and cables seem fine. Is there a servo motor that might be the issue? Fuse? It's getting cool here in VT!
October 22, 2016
  Followup from the Pelican Staff: Check if the pipes on the heat risers were left off the manifolds. - Nick at Pelican Parts  
Big Mike Comments: when I turn on my heat nothing happens. Had same problem last season but miraculously started working. Is there a blower motor fuse? I did have oil leaks, could that effect the blower?
September 28, 2016
  Followup from the Pelican Staff: What type of vehicle are you working on? - Nick at Pelican Parts  
Miro Comments: is it possible to wrongly mount the Rear Heat Protection Plate on a 1986 3.3 Turbo ?
I have just had it windows out repainted, but on reassembly the rear bumper now heats up like hell...
June 7, 2016
  Followup from the Pelican Staff: If it was left off. Otherwise it fits into place and would be hard to misalign. - Nick at Pelican Parts  
jamzboy Comments: i have a 1973 911t dose anyone know where i can get the heater and fresh air control plate on the dash restored to org. condition, please help
April 9, 2016
  Followup from the Pelican Staff: Give our parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
forrest Comments: I have an '89 930. The electronic heat control does not activate the blower motors when heat is selected. It does activate the heat. The blowers are working but the controller is not activating them. Is there an electronic component in the control that commonly fails? Can this unit be repaired? A new unit from Porsche is very expensive. Any companies out there that repair these? Thanks.
November 12, 2015
  Followup from the Pelican Staff: First check that the controller has good power and ground. I am not sure if there is a repair service. We may have a more affordable part available. Give our parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
Steve Comments: I have an '88 911 Carrera. I always have heat coming through one set of vents or another. With the levers between the seats both in the fully down position shouldn't that stop all hot air from moving forward? Also what is the correct position for the dash levers to eliminate any air flow? I live in Florida, just don't need that much heat.
October 23, 2015
  Followup from the Pelican Staff: Here is some info:

- Nick at Pelican Parts
john Comments: are the blowers on all time there is no on/off switches
1972 914 ???? fresh air and heat blowers
June 30, 2015
  Followup from the Pelican Staff: I recall there fan being on all the time. Controlling temp with the flap / cable actuated. - Nick at Pelican Parts  
Boudreaux Comments: Can I delete heater duct hose that
Goes from heat exchange to
Blower motor I live in Louisiana B
May 11, 2015
  Followup from the Pelican Staff: It can be done. Here is a discussion on that modification. - Nick at Pelican Parts

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Page last updated: Sun 3/18/2018 02:00:54 AM