HVAC Problems and Staying Comfortable In Aging Cars

Every year when winter rolls around, you’ll see tired cars advertised as a ‘beater with a heater’.  This is definitely a good thing because when you drive a beater, you can usually make it through summer with the 440 air conditioning cranked up (that’s four windows down, at 40mph by the way).  But when your heater isn’t working, it venture’s out of personal comfort and starts to tread water in the deep end.  A car without heat can be extremely uncomfortable, and in some cases unsafe depending on how close to Duluth MN you live.  You’ve never experienced cold until you’ve been to winter-time Duluth.  Here’s a list of common HVAC problems to keep an eye on during the cooler months.

All manners of animalia can wiggle into your heater box and cause havok. This particular blower motor has been home to a family of rats for a while

#1 Blower Motor

The most common issue I’ve seen with old cars is failure of the blower motor.  Occasionally referred to as the ‘Squirrel Cage’, the HVAC blower motor is a high wear item that is generally running to some degree at all times while the car is on.  This leads to extremely high hours when you pass that ever joyous 100k mi mark.  Symptoms of a failing blower motor generally begins with a buzzing or grinding noise, and ends in complete failure.  When that happens, grab the old jacket and gloves, its going to get chilly.

The blower motor is usually housed inside of an enclosure called a heater box, which will need to get opened to access it.  The heater box is usually accessed from underneath the cowl at the base of the windshield, but occasionally you’ll have to get at it from behind the glove box, which is the case in new SAAB 9-3’s.

#2 Blower Motor Controller

Fan Speed Controllers at eEuroparts.com

Because of the resistance the controller puts on the fan power, it heats up to the point of requiring a heat sink to keep it all from melting.

AKA Fan speed controller, or blower motor resistor.  By default, 100% of the power is funneled into the blower motor to cause it to run at it’s default setting: Max.  A variable resistor is installed in line to control the speed of the fan by increasing the resistance on the blower motor’s power input.  When the resistor gets old, it’s common for the controller/resistor to malfunction, causing the motor to stop working under some settings.  Sometimes if a fan speed controller fails a certain way, it’ll cause the fan to stop functioning all together.

It’s common to misdiagnose a failed fan speed controller as a bad blower motor, so if you suspect a motor failure, remove it and bench test it before ordering parts.  It’s possible the motor is just fine, and you actually need a blower motor controller instead.

saab bmw mercedes Heater control valves at eEuroparts.com

A couple different types of heater convtol valves, the one on top from SAAB and the one below is for a BMW

#3 Heater Control Valve

AKA Heater bypass valve, or coolant bypass valve.  This control valve diverts engine coolant through a series of tubes that pass through the firewall and into the heater core.  The diverter valve function can be controlled either manually by vacuum, or electronically via solenoids, but both are prone to failure.  The SAAB 9-5 is notorious for bypass valve failure, which will not only cause your heater not to work, but will also leave you stranded when it blows all the engine’s coolant onto the road.  A bad heater control valve is less common in other models, but is important to keep in mind while troubleshooting poor heat or coolant loss.

On most BMW’s, it’s not the valve that fails, but the brittle plastic fittings crack and require replacement of the entire valve.

#4 Heater Core

On the end of the coolant hoses passing through the firewall is a small radiator that heats up to the temperature of the engine, providing your bypass valve and thermostat are functioning properly.  A heater core is a series of very thin coolant passageways that can get gummed up if you don’t change your coolant often enough.  A gummed up heater core will provide lukewarm heat and not be able to transfer the engine heat to the air efficiently.  The fragility of the core can also lead to pinholes if you have gotten into an accident or the plastic has gotten brittle and cracked.  You will get a strong coolant smell (think maple syrup) in the cabin if your core is leaking.  If you have a heater core leak, I recommend changing it immediately, or you risk blowing all of your coolant into the inside of your dash, and trust me you do NOT want that to happen!!

#5 Interior Air Temperature Sensor

Automatic climate control systems rely on this sensor heavily to understand how much heat or A/C it needs to divert.  If the sensor gets dirty or damaged, the climate control won’t be able to accurately gauge what temperature it is inside the cabin and you’ll have a lame system.  On the later SAAB 9000’s and NG900’s, this sensor has a fan inside of it that draws air in.  The fan can get noisy and fail, so it’s good maintenance to take it out every once in awhile to put a drop of oil in the motor to keep it running smoothly and quietly.  Sometimes, however, you will need to replace this sensor to get the ACC to keep the cabin at exactly the temperature you set it at.

#6 Engine Thermostat

An engine thermostat is a simple device that plays a huge part in the regulation of your engine.

The coolant thermostat is a simple little device that keeps the coolant from circulating through the engine until it warms up completely.  Different cooling systems are designed to divert coolant in different ways, but in some cars if you have a bad or sticky thermostat, the amount of hot coolant that gets to your heater core can be decreased, causing less than hot airflow coming out of your vents.  This is usually not the case, but a properly functioning thermostat is very important to have in your car, and in most applications they are fairly cheap.  If your engine has trouble coming to full temperature quickly and your heat is not super hot, your thermostat might be stuck open.  Replace it and you should be golden.

#7 Cabin Air Filter

This cabin air filter didn’t put up much fight against a rat looking for a warm place to sleep

OK so this one wont actually keep you from having heat, but it will keep you from having quality ventilation, which is important to your health.  Working at eEuro, I’ve seen some nightmares inside people’s cabin air filters, up to and including dead animals, feces, and black mold.  Those are of course worse case scenarios, but many people don’t even realize they HAVE a filter they can change, so when they buy a European car they put thousands and thousands of miles on it gleefully unaware of the horrors gathering inside the filter.  Remember that all the air you breathe passes through it.  If your car stinks, target this filter and change it ASAP.

An exploded diagram of a typical heater box, showing the control panel (1), the fan blower motor (4), the fan speed controller (5), and various stepping motors for diverting airflow (23 and 26) with associated linkages (11, 13, 15, 17, 19, 20)

#8 Stepper Motors and Associated

This one is a bit tricky because there are so many different ways that auto manufacturers approached the diverting of air around the cabin of a car.  In some cases, they are controlled by blend doors on vacuum pods, and the air channel doors (you can hear them all clicking and switching when you change airflow to different vents) are triggered with a switch with a huge bundle of vacuum hoses coming off them.

More modern cars use blend door arms attached via stepper motors and a series of arms/linkages.  When you have a door or flap in your dash fail that is meant to divert hot or cold air into the vents, the functionality of your defrost, top, or bottom vents can go away.  Sometimes the ability for outside air to blend with heat (or vice versa) will be reduced or gone altogether, resulting in very hot or cold feet, or no airflow out of a certain set of vents.

SAAB 9-5 Bonus:

If your 9-5 is having HVAC problems, you can run a self diagnostic on the ACC panel by holding AUTO and OFF simultaneously.  A series of codes will appear after it runs it’s tests, and you should hopefully be greeted with a big goose egg.  If you get something like 1 – 08, you’re looking at replacing a blend door arm and shaft before you get quality heat in your car again.  Do a simple check by removing the driver side kick panel under the steering wheel and see if any little pieces fall out.  If they do, then pick up our repair kit here: http://www.eeuroparts.com/Parts/8032/Blend-Door-Repair-Kit-Driver-Side-BDKIT/

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33 thoughts on “HVAC Problems and Staying Comfortable In Aging Cars
    • Adam Goral

      Hi Russel, it sounds like you have a problem with coolant flow to your heater core, or a blend door in your dash that is failing to mix the hot air with the cold outside air. A good first step (cheap) is to replace your thermostat and do a cooling system flush with our Citric Acid descaler. That will hopefully help solve coolant flow problems through your heating system. Here’s an article about putting the right coolant back in to prevent corrosion and buildup, see the link at the bottom for the descaler. [click here]

      Check to make sure the coolant lines going OUT of the coolant bypass valve TO the heater core are hot after the car has warmed up and the heat is on (if not, the problem could be a bad bypass valve). Then check to see if the core itself is hot. If the hose is hot and the core is not, you might have a gummed up heater core, which a flush can help, but sometimes you need to replace a heater core. Hope that puts you on the right track to staying warm out there.

  1. Bob Newton

    ’99 9-3 SE with ACC. My heater fan seems to be dead. If I smack the side of the console I get a weak whirring sound and minimal air flow, but it cuts out completely after a few minutes. How can I tell whether the problem is the blower fan or the controller without taking everything apart?

    • Adam Goral

      When the controller starts malfunctioning, usually you will still get fan functionality on some settings, and nothing on others. It definitely sounds like a blower motor or related wiring for you. The best way to know for sure is to take it out and apply battery power to the fan contacts. It’s not uncommon to find other problems during that phase, such as melted connectors and burned wires. There’s a lot of power running through this circuit on a constant basis.

  2. brian

    2004 9-4 Arc. When the heat is on high for a period of time or when I punch defrost, the ACC display goes all wonky. Every pixel lights up, the brightness goes dim. Sometimes only a few things display and it will say Centigrade but it’s really Fahrenheit. Turning the system off and back off sometimes resets it, sometimes not. I am baffled. Can’t find anything on Google search, either. Once I figure out how to pull the ACC panel, I may swap it with the one in my 1999 9-5 to see if there’s any change. Thoughts on what’s going on?

    • Adam Goral

      It sounds like an electrical problem that would be extremely hard to diagnose over the internet. Initial estimation would point to loose wiring, corrosion, or a bad ground. It’s not what you want to hear, but it will take some time with a multimeter to track this down, most likely. An easy first step would be to gain access to the wiring harnesses involved, and jiggle them to see if you can provoke a change. If not, clean your grounds to ensure maximum current flow.

  3. jerry

    Pcs breather pipe has corroded and split, engine light on and no heath . Heater was working normal very warm then stopped in a three hr window I did pull away from lights quick before these problems car down on power for few months now saad93 hot hatch 2.0 2002

    • Adam Goral

      It sounds like there are multiple compounding problems going on here that would be very hard to diagnose sight unseen, I recommend seeing a mechanic before damage occurs.

  4. Steve Anderson

    I have an ’03 9-3 linear with manual climate control. Yesterday my blower stopped working completely and I have had zero issue until then. I pulled the glovebox out to check on everything and the connector that plugs into the resistor that doesn’t go to the blower is partially melted. Did a bad resistor cause that to overheat and melt? What other possible issues could there be? Thank you for any help.

    • Adam Goral

      Hi Steve. First thing to do is to check your fuses and make sure the correct fuse is installed. If there was some kind of short or surge, and too large of a fuse was installed, then the connector is a bottleneck that would take the brunt. A faulty resistor would be a prime candidate if the connector on it is melted. How is the blower motor working? Hope we can find you the solution we need. Contact customer service with a photo of the connector if you need to splice a new one in! Thanks.

  5. George E. Valentine

    We have and 03 9-5 Linear, and for a long time, the blower made noise at high speeds. It finally quit, and a shop replaced both the blower motor, filter and resistor. Now, we end up with a smooth sounding powerful blower and weak cabin air flow in all positions. The weak air feels cool enough, however it’s not getting through. The ACC calibration and fault code test results in 0 diagnostic trouble codes. I am hesitant to let a shop proceed further until I can at least understand the issue. What do you think? Thanks for your assistance!

    • Adam Goral

      Hey George, It sounds like either the blower motor has a problem, or it wasn’t installed correctly, possibly missing an air seal somewhere allowing it to leak. Do you feel airflow in weird places, like dash buttons or out of the windshield cowl/surrounding area? What brand blower motor was installed?

      • George E. Valentine

        Thanks for getting back to me Adam. Invoice shows “OUTCOOLING” blower motor and “OUTCOOLING1” blower motor resistor. Does that sound like a brand? No, I do not feel any airflow from anywhere except the vents, which is weak. Is there any way I can personally check the blower, even by removing the cowl?

  6. 2002 Saab 9-5 linear wagon. Cannot get the heater to work. I don’t get any codes and the heater Matrix inside pipes are hot, well the top one is hot the bottom one is warm. The AC worked fine in the summer and I was wondering if maybe the inner flap in the heater box was broken only allowing cold air? Or maybe the cabin temperature sensor is broken, is there anyway to test that?

    • Adam Goral

      Hey Mark, how often have you flushed new coolant into the car? The heater core (as mentioned in the article) can accumulate buildup and deposits over time, especially if the antifreeze is never changed. If both pipes aren’t HOT, and you don’t have any codes show up in the ACC diagnostic (hold AUTO and OFF with the key on), then either the valve isn’t sufficiently routing coolant to the heater core, or the heater core is so gummed up that coolant can’t get through it anymore.

  7. Tom See

    I have a 1994 900 SE Turbo hatchback with ACC auto control that I bought new. I have a noise while the heater fan is working and the engine is on. If I just turn the car on without starting it, the fan works beautifully without the extra noise. As soon as I turn the engine on, the noise comes on. If I then turn the heater off, the noise goes away.

    The pitch of the noise changes with fan speed. At the default speed, it’s quite noisy. If I increase the fan speed, the pitch decreases and vice versa. There are some speeds (low and high) at which I don’t hear the noise. I pulled the stock audio head and removed the antenna and the power and speakers connectors. No change. Some time ago, I oiled the blower motor with 3 in 1 oil. No change. I bought and installed a used ’96 blower motor. No change. A mechanic says it’s a ground loop, but with no change with the audio head removed, it’s hard to see, BTW, my Haynes 1993-1998 manual shows the ground points for the blower motor and the audio head to be the same (LH A pillar). The WIS shows them to be on opposite A pillars. When I removed the audio head, the harness appeared to go to the left. The blower motor harness appears to go to the left. Anyone know the truth?

    I’ve also looked at the cabin sensor. I get air flow and no noise from it. I tried blowing air into it and then spraying a little electric razor lube into it. No changes.

    • Adam Goral

      I’m not sure about the head unit causing a noise with the HVAC system. The system is a series of interconnected ducts, and more than likely either you have a gap somewhere where the airflow is causing a noise (I assume it’s like a howling noise, like blowing over a jug or bottle in which the tone is denoted by both the angle of the airflow and speed). It probably won’t be easy to find, but I would have a quick poke through speaker holes and other areas you can pop off and access the ducts, vent elbows, junctions, flaps etc. while the fan is making the noise. The other possibility could be the vibration of the fan not seating correctly on the gasket isolators, or just being plain worn out. You mentioned trying to fix the fan by lubing it, or putting in another used fan. Let me know what you find, thanks for reading.

      • Tom See

        Thanks for the reply. I took out the glove box and the passenger panel and poked around with a piece of tubing. Couldn’t find anything definitive except it sounds like it’s way back by the matrix or maybe (as you said) in the ducting. I did part the duct from the center out to the side vent. It didn’t seem to matter. In some ways, it sounds like coming through the firewall from the motor. I took the scuttle cover and the cabin filter off and listened with my tubing and there was nothing from the motor. Previously, I have run the motor with everything off and it’s not from it. I took the opportunity to listen closely to the temp sensor and there was no noise. I did take it out and clean and lube it. I’m going to take the drivers panel off next and see if I can narrow it down some more. I might also pull the control unit. I’m beginning think that you are right and it’s to do with some ducting problem in which case I will turn up the radio. Two things that make no sense to me are: 1. if I just turn on the key without starting the engine, the fan works great with no noise. It’s only when I turn on the engine that the noise comes; 2. Increasing the speed lowers the pitch of the noise.

        • Adam Goral

          With the engine off, what if you let the fan run for awhile, will the noise eventually come on? Could relate to the amount of power the motor is getting from the battery. If the motor bearing or shaft is getting on in age and isn’t perfectly spinning, or a spinning part that used to be lubricated is now running a bit dry, a vibration can be amplified by all the surrounding plastic housing and ducting to translate to a deep thrum. The vibration would change with rpm, resulting in a different tone. If you haven’t found any ducting that’s letting air out, I would sincerely point the finger at a blower motor. We currently have a Valeo unit 715061, which is a brand I would highly recommend. The unit comes in under $100, and is made in Germany.

          • Tom See

            I’ve been pulling stuff off and on the past few days (snow off and on). I know for sure that the Temp Sensor is fine. I know the fan itself is fine (took everything apart and ran the fan fully exposed. No noise at the fan). I know the recirc motor is fine. Pulling the control unit, the audio head, the glove box, and the side panels, as best I can tell, the noise comes from the center rear of the heater unit. It seems like it is coming through the firewall, but there is no noise in the engine bay even with everything pulled off of the fan. Besides, if I just turn the key on w/o starting the engine, the fan works perfectly (increase and decrease fan speed just fine). But as soon as I start the engine, the noise is there. Interestingly, when I start the engine, the noise is at a higher pitch and comes down to the pitch of the normal fan speed.

            Without taking the whole consoles, dashboards, etc. out, I don’t see a way to find the culprit. I’m going to turn up the radio. Actually, I’ll manually adjust the speed to one that doesn’t whine. I have a high speed (anything from full on to 3 bars down if I go full on then down) and a low speed (2 bars).

            I’ll keep replacing the motor in mind. I’ve looked at the ProParts blower motor that you sell and would probably get that one. My SAAB is a ’94 and I’m 76 and trying to keep it running at a reasonable cost until one of us goes. I did spend $25 for a used motor from a ’96 and it had the same problem. BTW, do these motors have brushes that wear out and might cause this?

          • Adam Goral

            Tom, your resilience is commendable, I think most people would have given up at this point. Here’s my theory. Yes the motors do have brushes, as well as bearings (of course). When the motor wears out, it begins to cause a little bit of vibration. Some blower motors are actually insulated from this vibration with different types of gasket material, as in this MINI blower motor https://www.eeuroparts.com/Parts/75581/Heater-Fan-Motor-64113422644/. If you are sure this only happens with the engine on, then perhaps the vibration of the heater box and the different independent vibrations of the engine bay might have just enough tolerance to close the gap and rub on the site of the housing. Because the housing is integral to the amplification of the noise, this is why you did not hear the vibration with the motor out. If you do decide to replace it, I would highly recommend the ACM unit, made in Germany. https://www.eeuroparts.com/Parts/168976/Heater-Fan-Motor-08575115/ The price is comparable to the Proparts unit. If you do decide to order, think about making it quick, there are none left here and only 2 in external warehouses for drop shipping. Have a great day Tom.

  8. Kris


    I am experiencing a different smell(similar to metal surface burning) inside the cabin even with recirc door closed. My blower motor was making chirping noises last year and applied lithium grease and the sound went away. However, for past few months, i am experiencing some smell which is causing is dry throat and nasal congestion. Any thoughts on the cause for smell/and on the fix. I am checking the same on other forums.

    Thanks in advance !

  9. Peter

    I have a problem with 2003 Saab 9-5 Aero AC not cooling sufficiently. Last summer air at the vents was as low as 45F when on max, this summer it is only cooling down to 65F.
    Pressures measured when at idle are 250psi on high side, 50 on low side with ambient air temp at 85F.
    These pressures seem to be in the correct range. The low pressure line also does not feel very cold to the touch when the system is running.
    Calibration of system indicates no errors. Cabin Filter has been replaced. Cabin air seems to blow correctly with good flow whatever direction it is set to.
    Heat turns on or off as expected. Econ mode does not blow any warmer than outside temp.
    Any help with solving this problem before the GA summer really gets into full swing would be greatly appreciated.

    • Adam Goral

      Inside the AC system is something called a Receiver Drier, what this part does is store excess R134a and remove moisture that naturally works its way into the system over time using a desiccant. If your receiver drier is original, it could be to moisture capacity, and not doing it’s job by cleansing the refrigerant properly. A side effect with moisture in the AC system is the formation of hydrochloric acid, which is obviously extremely detrimental to the longevity of your AC components. Unfortunately this will not be an inexpensive service, as after you replace the drier (and contemplate replacing other parts that are getting on in age like the compressor), you will need to go to a service center to flush and circulate new fluid in, which takes some time and specialized equipment.

  10. Stephen

    Hi, hoping one of your Saab wizards can give me some direction on this puzzling issue. I’ve been struggling with a strange hvac problem for a while on my 1998 9000. The car will blow heat for about 10 -15 minutes before the heat stops and it begins to blow colder air. When it does this…and this is the puzzling piece…switching the acc to HI doesn’t blow heat.

    Thinking that there was air trapped in the core, I nosed the car onto my steeply angled driveway then jacked the front even higher so the coolant tank was the highest point in the cooling system. Cap off the tank, let it idle for about 30 minutes, acc on HI, while I did some work in the garage.

    My observations have left me as puzzled as when I began

    First, I was expecting the coolant to expand and bubble up in the tank. It didn’t. The thermostat opens when it should and the waterpump must be doing its job…neither are that old and the engine doesn’t overheat.

    Basic “feel” feedback on the hoses was really hot going to the heater core, less hot coming out so that leads me to believe the core is doing its job and the flow is good. Top hose on the radiator was warm, bottom hose seemed like it should have been hotter. I was able to hold the hose for longer than I would have thought I should be able to.

    Shutting off the car for about 20 minutes or so seems to “re-set” the issue…blowing heat for 10 to 15 minutes then cool air and stepping the acc to HI doesn’t blow heat.

    Thoughts and/or direction? Thanks much!

    • Adam Goral

      Here’s a thought, if your Saab 9000 has ACC then you have an interior air temperature sensor that relays temp information to the computer. If you are having issues with this sensor, which they do tend to have problems when they get old, then the computer might be getting the wrong signals and automatically adjusting the heat out. Listen to make sure the fan is still drawing air in. If the little fan stopped working, the sensor will be sitting in stagnant air in the dash (which heats up) and potentially telling the ACC computer that the cabin is hotter than it actually is.

      • Stephen

        Hi Adam, thanks for weighing in. I’ve actually tried 2 cabin sensors as well as 2 ACC units and had no change to this weirdness.

        About the only thing I have not tried yet is undoing the heater hoses and trying to flush the core, but that might be next

        • Adam Goral

          If you have a bubble in your cooling system, I would suspect that you would just have poor performance all the time. The way it swings back and forth still points to an issue (for me anyway) with the operation of the blend door flaps in the climate control system. If you do end up taking out the heater core, it’s such a bear of a job (or Behr of a job, pun intended), that it might be better to just replace it with brand new. They are not too expensive.

          • Stephen

            Replacing the heater core is a job I hope will be the ‘nuclear option’ haha. I know how that one can go. The blend door/stepper does make some sense. It would provide a clue as to why setting the system to HI doesn’t over-ride the ACC and just revert to blowing full force heat

  11. Stephen

    One question I have regarding blend door failure…would a failure cause the ACC to throw a code in a calibration test? Asking because I’ve tried recalibrating a couple times and it doesn’t give one. This is true on both ACC modules

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