HVAC Heating Ventilation Air Conditioning, known issues

Disclaimer: The content of this document relates to the Porsche 993 (911 1995 to 1998). I am not a professional automotive technician, and some of the opinions expressed in this document are just that, opinions; therefore, please feel free to use this document at your own risk. Many times, a lot can be learned by simply watching qualified professionals service the HVAC system on your 993

 I once had a mouse living in the AC evaporator chamber of my '86 Carrera, where he stashed all sorts of stuff from Frito-Lay and had a ready supply of fresh water from the evaporator coil's condensation. Lived the fat life until the Cleveland winter came around, and we found him in the Spring, freeze-dried with all his accumulated goodies. Haven't had such an issue with my 993, but you may run into the following.

HVAC Servo Motors – The heating/air conditioning system design in the 993, unlike early 911, replaced all the hand levers and cables to control airflow with servomotors attached to air flaps in the ductwork.

These servo motors are prone to failure, and in most cases, when one fails, the AC seems to not run so cold, or the heat doesn’t work well, or there are big differences between the left & right-hand dash vents' temps, and the OBDC System throws a code pointing you to the offending unit. I have opened a few of these servos up and found that the internal components are not DIY repairable. Typically, one sees fried switch brushes with matching burns on the circuit board that the brushes contact. New or used, these are expensive parts, and in the case of used, they likely exhibit the seeds of failure already.  Luckily, I stumbled onto a company that services these servos: Partisan Autoteile. If one lives in the EC or the UK, one can ship their servos directly to them in Germany, and they will install the kit for you. Shipping them out and back to the Americas is a bit expensive, slow, and these days unreliable, so emailing Partisan about buying the rebuild kits that include basically all the internal electric-related guts is the way to go. You won't find the kits on their website, but they do offer them via email request.

 

Tore from www.bergvillfx.com has some additional insights into what happens when a servo fails:

It is normal that a [failed] servo might spring to life [temporarily], either after a short stop, overnight, or a pause due to [an] OBD session. The servo will then run fine until [the brushes] hit the burnt spot [on the circuit board] again.

Burnt Switch Brushes

Identifying which servo motor you need can be squirrely, as they are called different things in various literature. This may have to do with some being relocated from where they are found on the earlier 964 series cars. Here is a table of part numbers to help you identify the part number of your failed device. 

The left and right Mixing Flap Servomotors in the footwells are easily accessible. The front Cowl Center fresh air intake flap servo in the frunk is not too bad to remove. However, the defroster Flap and Front Well ones under the cowling will take a bit more effort to disassemble the stuff around them to get them out. If you are already committed to the huge cost of removing the gas tank and the behind-the-dash HVAC assembly, euphemistically called "the suitcase" to repair an AC evaporator refrigerant leak, I would just go ahead and have the three cowl-mounted servos rebuilt at the same time.  They are all similar in design and prone to failure, and by doing this, you should not incur any additional labor cost to speak of in their installation. There is a really good listing of 993 HVAC issues and their likely root causes at Bergvillfx.com.

Here are my notes on installing the rebuild kits for the HVAC servos.

 Improving Air Conditioning performance -- Seeing that the 993 is 25+ years old, lots of dust and lint have accumulated on the AC evaporator (cold coils), likely compounded by black mold growing on this debris. This has the potential to clog some percentage of the evaporator's cooling fins. Some potentially to a great extent. This can dramatically lower the capacity of the AC system even if it is properly functioning in all other respects. A clogged evaporator usually presents as inadequate cooling, as the system is used in warmer weather at 90+ degrees, when one is reaching the performance limits of the overall system, when one needs it most. This situation will not throw an OBD Code so it is one of those stealth situations that exhibit slowly over time.

I went ahead and cleaned the evaporator coils on my 993; it was like an AC upgrade. This is how it is done:

• I purchased a garden pump bottle sprayer with wand and;  
• A spray wand used for general outdoor washing that is connected to a garden hose. 

• I also purchased a 90-degree- 1/4 inch schedule 40 brass street elbow so the garden hose sprayer nozzle can be re-mounted 90 degrees to its wand shaft.

Please read this entire procedure below and its notes of guidance from beginning to end before attempting it. 

• 

  elbow with nozzle

  The first step is to slide the CCU free of the dash.

• behind the CCU open the louver to the evaporator chamber by hand and spray a good amount of water on the evaporator coil behind it.  Look under the car to see if the drain is open and the water drains onto the ground under the car. If not the drain is clogged and needs to be snaked out before continuing with cleaning the evaporator.
  

Snaking out a clogged drain:

Make a plumbing snake out of a lenght of 12 gauge primary electrical wire dulling the end so it does not puncture the lines

The drain lines follow a rather convoluted path so if the the clog is near the end of the drain line it may be able to be cleared with the snake. Gently slip the snake into the drain outlet under the car to release the clog. Be gentle and do not use a high level of force if the snake hangs up a little. If it does back it out and try again possibly with the end bent in a slightly different direction as the goal in this is to not damage the drain line. Do not expect the snake to travel all the way up to the condenser. It should be expected to travel only a few inches. It may be necessary to detach the guid piece mount at the end of the tube to snake the tube out.

Drain Tubes

An alternative process could be to use a little compressed air at the drain outlet to blow free debris in the drain back up and out of the top of the drain tube into the condenser chamber.

 In any event one will want to test the drain again and flush any debris that came loose out of the system.

 

CAUTION: To avoid breaking the rather delicate louver it is best opened by manually pressing on the arm leading to the vacuum actuator it is attached to. One can also detach the actuator's vacuum supply hose and replace it with a short length of air or fuel hose and draw a vacuum with one's mouth to cause the actuator to pull the louver open. In early production 1995 and previous years' cars, the louver is not connected to an actuator and is simply spring loaded and can be directly opened by hand.

• 

  Louver behind
   the CCU

  Take the pump bottle sprayer refilled with a weak detergent solution, such as diluted baby shampoo or Dawn dish detergent, open the vent louver behind the CCU in the dash by hand, and spray the detergent on the evaporator coils behind it, completely saturating the evaporator with the solution. 

CAUTION: Do not use a strong detergent, bleach or any acid cleaner as the evaporator coils are delicately made of aluminum, are 25+ years old, are under refrigerant pressure and will be damaged by a strong detergent.

• Then go to the front and remove the left-hand cabin filter or the blower housing cover on a 964. Note that the alignment pin on the filter points straight up when putting it back in after this procedure.
• One of the ducts where the filter was located leads towards the

  

  Evaporator coil
  . seen on left

  
  center of the car, inside that duct. One can see the other side of the evaporator in the picture to the left. Use the bottle sprayer wand to saturate this side of the evaporator coils with detergent as well.
• Then attach the 90-degree elbow to the end of the hose-attached spray wand and insert it into the duct, and spray clean the evaporator. I used the full-on pressure of the garden hose to loosen any debris in the coils. I sprayed in short spray bursts, allowing time between bursts for the drain to catch up to the copious amounts of water involved. 
• Be sure to cover the entire area of the evaporator face, especially lower down where the clogging debris seems to accumulate the most, and the force of the water will, in turn, dislodge the highest concentration of debris from the other side of the coils.
  

CAUTION: Avoid touching the evaporator's fins with the wand as they can be easily bent. A lot of water will flow from under the car. Some may think if a 40-60psi garden hose is good my powered pressure washer would work even better. I strongly discourage using a powered sprayer as it may physically damage the coils' fins and has the potential to splatter water into places it doesn't belong.

• Then remove the 90-degree elbow and reattach the strait nozzle tip to the garden hose sprayer.
• Use it to spray through the louvers behind the CCU to rinse the other side of the evaporator. 

CAUTION: Moderate the flow with the sprayer so you do not encounter a back-splash under the dash into the cabin.

• It is unlikely you will see any dust, lint, or mold on the cabin

  

  A dirty Evaporator Coil

  facing side of the evaporator as most of the debris will be on the lower half of the evaporator out of view., The frunk-facing side did not seem to show debris on its face.
• I took the extra step of putting a drop of light oil on the blower shaft bearing while the filter was out. This was to displace any moisture that may have landed on it during the process, and let's face it, the last time it was lubricated was over 25 years ago, when it was made, couldn't hurt.
• Replace the cabin filter and the CCU, button things up, and you are good-to-go.

. After the cleaning, I found my vehicle's AC runs cold now, even in the 90-degree weather. So, before one explores more invasive AC upgrades, I would consider this to beat the summer heat. Recently, I have read about folks experiencing icing of their condenser that exhibits as the AC working well, and then as it runs, diminishing in performance as ice builds up on the condenser, disrupting airflow across it. Could this problem be the result of a dirty condenser where airflow is diminished, encouraging icing? Who knows?

Replacing the cabin air filters:
The 993 has two replaceable air filters called pollen filters that are located on the left and right side of the car below the windshield in the frunk. Their design suggests they have a long service life. Installing new filters can be very frustrating if you are unaware of the secret that the plastic alignment pin on each filter needs to be pointed towards 12:00 o’clock (straight up towards the ceiling), not 3:00 or 9:00 o’clock, as some try to do during reassembly. Knowing this will turn a one-hour job involving the mutilation of the filter with a mat knife into a five-minute job!

Note the alignment pin
on the rim of the
filter housing above

Once I figured this out, I made a note of it with a “this way up” arrow on the replacement filter with a Sharpie pen so years from now I don’t have to relearn this secret.

It may also be a good idea to put a drop of light oil on the blower motor shaft to add a little lubrication as the blower motor shaft bearing was probably last lubricated over 25 years ago when it was originally manufactured.

What is that buzzing behind the dash?

While you have the Climate Control Unit (CCU) pulled out of the dash, you can consider cleaning out the lint that has accumulated in the little temperature sensor blower attached to the back of the CCU. As the blower builds up lint, it draws too much current, and the CCU throws an OBD Code. 

• A dust plug may have also formed in the tube it is attached to. 
• Cleaning the blower involves removing and disassembling it (2 screws), removing the cover, and pulling the impeller off its needle bearings. 
• Then one may see some dust around the impeller shaft that needs to be removed. 
• Before reinstalling the impeller, put a drop of motor oil on your finger and touch the two needle bearings on the ends of the impeller shaft. If you see oil on the bearing, you applied too much and it will attract dust, requiring future cleaning sooner.
• Once reassembled, the blower can be tested for quiet operation by attaching a nine-volt transistor battery to its pigtal wires. Be aware that the impeller only appears to turn in one polarity of the connection, so when attaching the 9-volt battery, reverse the connection if needed to see the impeller move. 
• Then you can put your ear up to the blower to see if it runs silently. 
• Also, when the battery is disconnected and the bearings are good, it should continue to spin for a few rotations, and just before stopping, it should rock back and forth from the magnetic field in the motor.
• If it rattles or doesn't spin freely, the bearings are shot, and the blower will need to be replaced. Be aware that if you purchase a used replacement, it will at least need to be cleaned, or it may also have a failed needle bearing..

Doing the above should eliminate the source of the buzzing sound coming from behind the dash that some find irritating. The lint will return over time, so some have found ways to cover it with filter material, eliminating the root cause of the buzzing. If cleaning doesn't clear the issue, the blower's bearing is bad, and the blower can also be easily replaced with an exact replacement or an aftermarket fan solution that eliminates the noise problem going forward.

Low cost exact replacement:

If your sensor blower is non-functional or its bearing is worn so it can't be made to stop buzzing it is a very common Denso part used on may contemporary European cars to the 993.

See the section titled "Hunting for the best value in "Service New" replacement parts:" on the page this URL points to. The document discusses the most economical route to replacing the CCU blower with a part of the same design as originally supplied with your CCU. 

Aftermarket CCU sensor blower fix: 

Tore  from Bergvill F/X offers an aftermarket fix:

https://www.bergvillfx.no/en/products/ccu-fan-964993

The value-added aspect of it is that the blower/fan is a much larger and significantly more robust design where age, use, or minor accumulation of lint/dust on it is likely of little consequence to its durability, function, or how quietly it runs. Tore's solution  is unlike the original blower where the original blower design  may need to be visited again in the future for cleaning or replacement.

Cabin blowers acting squirrely: 

If the HVAC cabin blowers run intermittently, and when wiggling the sliders on the Climate Control Unit (CCU), here is a page that discusses what I did to remedy this situation.

If your AC appears to have stopped blowing cool at the beginning of the cooling season, or really any time, and it's not over 90 degrees outside, don't look too deeply into the probable causes until you have verified the refrigerant fill level.  In my experience, 99% of the time, low refrigerant is the cause of inadequate cooling. Also, the 993 has a low-pressure refrigerant switch. One of its functions is to cut off the compressor if the refrigerant fill is too low to avoid a vacuum on the low-pressure side that can pull the external atmosphere into the system and damage it. If your refrigerant level is really low, the compressor won't run.

First of all, a quick test as to whether the compressor is functioning and that there is some level of fill of refrigerant is to: 

• Run the AC on AC-MAX  on a 70-80 °F day, with the windows open, and observe the compressor. If a useful running range of refrigerant is present and the compressor is working, the compressor should cycle on and off every 20-30 seconds or so. Cycling on for a period and then off for a period. This demonstrates that when the compressor is running it is building effective pressure in the system to the point where the temperature sensor on the evaporator coil drops to 4 Degrees C and the Climate Control Unit (CCU) disengages the compressor's clutch. Once the temperature on the evaporator coil warms up, the CCU reengages the compressor.
• The factory literature says that at 77°F. ambient temperature, a properly working system should produce temperatures between 45 and 52 Degrees at the vents. At 95 °F, it should be between 48 and 56 °F. Beyond this ambient temperature, the vent temperatures rise significantly as the system's cooling capacity is reached. The above values are reached after driving around for ten to twenty minutes with the engine above 2000 RPM. This is so the compressor is turning at a higher rate, there is good airflow across the condenser coils in the front wing of the car, and any heat soaking of the ductwork has cooled down.
• If the compressor never cycles off, the system is probably slightly undercharged as the compressor is having trouble building pressure and dropping the evaporator to 4 °C.. In hot weather, say in excess of 90 degrees F, it may not cycle off even with a full charge of refrigerant as the system nears its maximum effective capacity. 
• If the compressor is not running at all, the system is most likely undercharged with refrigerant, tripping the low-pressure switch designed to avoid having the system pull a vacuum and potentially drawing external atmosphere into the system, with the potential to damage components.
• If with each on/off cycle the compressor cycles on for just a few moments only as it cycles and then cycles off for a much longer period, the system may be overcharged with refrigerant as the overall pressure in the system may be too high, triggering the high-pressure switch. This should be addressed immediately as it can damage components in the system.
• Be aware that the 993 has a lot of cabin window glass, and therefore, in warm sunny weather, the cabin and the HVAC ducts and components get easily heat-soaked. Therefore, the AC system should be judged by; 
• The temperature coming from the dash vents, 
• Preferably with a thermometer and its degree offset from the ambient temperature outside.
• Evaluated after ten minutes with the engine RPM over 2000 RPM.  
• In very hot weather, the cooling needs of the cabin can easily exceed the rate at which the AC can remove heat from it.

Some 993 owners have had good success improving cabin cooling by having window tint applied to their cars. Even the clear tint material seems to improve overall cabin cooling to some extent for some. In theory, applying tint film works by blocking wavelengths that induce heating of the glass and interior surfaces. It is claimed that the less effective solar rejection films block about 40% of heat, while the highest performing films block up to 70% of thermal energy. Not knowing how these numbers are calculated I leave it up to individual car owners to decide if it is worth the installation expense and the ongoing maintenance of these films for the promise of a cooler cabin. 

 

If your system is not cooling adequately, and it is not in the +90 Degrees F range outside, and the AC compressor is running continuously, and not cycling on and off or not cycling on at all, 

• 
• Go to Walmart and buy one $7 can of refrigerant and a filling hose with a gauge.
• Attach the hose to the can and snug down its seal to the can top without puncturing the can's seal.  
• Then hook the hose up to the low-pressure side of the compressor, the side marked low, or the one with the fatter metal pipe attached to the compressor. 
• Then, with the can upside down, the engine running, and the AC  set to MAX, 
• Some refill kits do not require inverting the can, so read the instructions that came with the can in addition to these.
• With the engine running, look at the gauge when the compressor cycles on; it should show the fill to be just at the lower end of the good range. 
• Filling is accomplished by puncturing the can's seal by scrwing in the knob, then opening the valve on the can of refrigerant by backing off the knob, and watching the gauge while the refrigerant charges the system. 
• Filling the refrigerant level to the middle or top of the canister's "good" gauge range will overfill the system with undesirable consequences and diminished cooling. So turn the knob back in when the correct level of fill is seen on the gauge.
• The AC system's adequate fill level should be determined by vent temperatures rather than fill volumes because some have experienced overfilling and diminished AC effectiveness if the full factory-recommended weight of refrigerant is added. 
• If the refrigerant is very low, it will empty into the system quickly, and if the compressor is not running, it should start cycling on and off. 
• If the gauge reads at the lower edge of the good range without adding any refrigerant, do not add any and look elsewhere for the root cause of your cooling problem.
• If the AC compressor begins to run if previously not cycling on, and starts to cycle on and off, it is a very good sign that the system is now charging, and a low refrigerant level was likely your problem.

Avoid the temptation to put the entire can of refrigerant in the system or fill it to the middle or top of the good range on the gauge, as this can easily overcharge the system. 

 If with each on/off cycle the compressor cycles on for just a few moments only, as it cycles and then cycles off for a much longer period, the system is probably overcharged with refrigerant, and the pressure in the system is too high. This should be addressed immediately by bleeding off some of the refrigerant, as overly high pressure significantly lowers the performance of the system and can damage components in the system.

The newer refrigerant refill kits at Walmart have a gauge on them, also some of the kits where the valve comes with the can may not require inverting the can so liquid refrigerant is drawn into the system. Read the instructions that come with these kits as to how to best use them. If the gauge shows the charge as at the lower edge of the adequate (good) range when the compressor cycles on, avoid the temptation to top off the system to a greater value on the gauge, as it is very easy to overfill the system.

If vent temperatures are still not cooling into the ranges discussed above, your problem may lie elsewhere. 

• Verify the servos discussed above, located in the footwell, are adjusted properly and fully closing in AC mode; a failed servomotor will trigger an OBD Code that can be read by a reader that supports the HVAC 964/993 code set.
• Verify that the fresh air servo opens the hood vent to the outside air, as it should by the CCU. 
• When the recirculation button is pressed, the change-over valve in the frunk cowl triggers the vacuum actuator behind the CCU, and it opens the cabin return vent to the evaporator chamber. 
• Check that the condenser coils in the wheel well behind the liner are clean and; 
• If the evaporator coil is not reasonably clean, clean it using the process described above if necessary. 

I do the little recharge-the-refrigerant dance in the Walmart parking lot every 5 to 6 years.

HVAC guys will throw up over the above low-tech recommendation, but it seems to work for me over many years and two 911. I have also noticed that if one runs their AC occasionally year-round for a few moments, it seems to retain the charge for a few more years; maybe the seals on the compressor shaft get exercised and lubricated and seal better; however, this theory is just from anecdotal observation. 

If one wants to have a better understanding of what is going on with their AC or has a concern about overfilling the system, a set of AC diagnostic gauges is inexpensive at Harbor Freight, and an adapter is available so the small, disposable cans of refrigerant may be used with it if you use it to add refrigerant. There are several web resources and videos on YouTube.com on how to use the gauges for diagnostics in an automotive HVAC application.

Unlike home AC systems that are completely sealed in rigid metal tubing except for the filling ports, an automotive system is an assemblage of hoses, fittings, and seals that can leak, so some level of discharge of the system over time should be expected.  If you find you are charging the system every year more than once in the summer season, the source of a leak should be investigated. This is done by adding refrigerant containing a phosphorescent dye and using a UV black light to find the point of the leak, also an easy DIY project. A shop may use a sniffer to detect a leak.

Evaporator coil Replacement:

Many times when the AC system won't hold a refrigerant charge an enterprising mechanic will suggest the evaporator coil needs to be replaced.  Don't be in a hurry to change your evaporator, a very expensive and invasive procedure. The mechanic proposing the work must show you clear evidence such as a sniffer detecting gas when a probe is inserted into the suitcase evaporator chamber or dye from a gas leak is seen inside the chamber with a UV light or on the floor under the drain or around the drain. You can see the evaporator coil inside the suitcase after removing the left side pollen filter through the duct in front of the blower that runs towards the center of the car.

Once the system is working properly, consider replacing the system filling/evacuation port caps on the compressor as a 993 is over 25 years old, and the Schrader valves at the ports are always suspect as a source of a slow leak. This will eliminate this potential future path for a slow refrigerant loss. Also, replace the caps on your home AC system every year or two and make sure that they are snugged up well; this will eliminate the most common source of having to have your home AC/Heatpump system recharged.

Evaporator Icing causing Inadequate cooling:

This is an issue I came across in my 1986 Carrera many years ago, and others have experienced in a 993.

• If you find that after a 20-minute drive or so with the AC running and the A/C working well, and 
• Then, over a short time, the cooling starts to diminish as you drive; this may be the issue. 
• If the center vent temperature starts to climb above the 44°F to 56°F normal vent range for 85°F. to 95°F ambient conditions, the evaporator coil may be icing over. 
• Some experience this in high-humidity weather, although it can present under dry conditions as well, as I experienced.

In my situation, it was the result of the position of the temperature sensor mounted to the evaporator, causing it to be slow to react in sending a signal to shut off the compressor.  This resulted in the evaporator icing up and impeding airflow through it. The indication of this is 

• In some cases, very little condensation is seen draining under the car in this running condition as ice is forming and; 
• For some, when the car is parked with the engine off, a good-sized condensate puddle forms under the car as the ice melts.
• Both of the above observations can be somewhat subjective, as a properly running system can produce a lot or a little condensate under the vehicle, depending on environmental conditions.  A stronger indication of icing as a root cause of inadequate cooling is revealed after a several-hour period of sitting with the engine off, followed by a restart. At this point, the AC seems to be working normally again.

So, before deciding to replace expensive AC components, verify the correct position of the evaporator temp sensor so that it will see the lower temperatures produced sooner, and send a signal to the CCU to shut off the compressor before icing begins. 

In addition, the sensor can be tested at its wiring harness connection to the CCU.
The operating range of the sensor should be between 3.3K~7K ohms.

The plugs on the back of the CCU can be removed by pressing in on the release tab on one end of the harness connector and lifting that side of the connector where it pivots off the other end of it to release it:

• With the car battery disconnected, unplug the CCU harness connector "G" as shown below.
• Connect an Ohm meter negative, black lead to pin socket G18 (sender ground - Brown/Blue wire).
• Connect the Ohm meter positive, red lead to pin socket G22 (White/Black) Evaporator sensor.
• Reading should be about 3.3K Ohms at 20 Degrees C ambient temperature

  

 Also, could an icing problem be the result of a dirty evaporator from 25+ years of service, where airflow is diminished, encouraging icing? Consider cleaning the evaporator coils as outlined above,  as part of evaluating the sensor.

It is very important to correct this problem as soon as it is identified, as ice formation can damage the evaporator as it forms, and the evaporator is an expensive component to get to and replace.

Related subjects:

Here is a page to address the HVAC recirculation button/function issue.

Here is the list of what the CCU does in the 993.

Here is a listing of CCU common repair parts.

Here is a discussion of the AC condenser and oil cooler fans.

Here is a page on servicing the HVAC duct servomotors that control airflow.

Here is a procedure to address if the HVAC blowers seem to work intermittently when wiggling the knobs on the CCU.

If you found this page informative, return to the main page and bookmark it for future 993-related servicing, repair, and upgrading guidance & information. If it saved you some aggravation, time, or even some coin, consider a donation through the button on the top left margin of this page. 

#993 #Heating #Ventilating # AC #A/C #servomotor #Climate Control Unit # CCU