Hello all,

Ever since we got the car there has been either no heat/poor heat. It kinda comes out when the blower is put on a low speed, not enough to comfortably warm the car though. When the blower is turned up the air becomes much cooler.




I have taken apart the dash and inspected the blend motors M2/6 and M2/7 which seem to be operating fine. No noise is coming from them either that would suggest they are not working properly.

I have confirmed that the "M2/15 - Left and right footwell flaps positioning motor" is stiff or blocked through the use of a computer diagnostic system and doing the defrost/recirc test which never shuts off.

My question for anyone with knowledge of an issue like this is could the footwell flap not working somehow block the airflow from the heater core from coming in.

I have read that the footwell flap motor and those little plastic pieces are notorious for breaking and have seen many threads for it. People have issues with the motors causing noise when turning the car on and such but I have no seen anyone mention about it causing the heat to work.

The picture I posted shows that the footwell flaps are very close to the blending motors and flaps and may have something to do with it. I just wanted to get further opinions weather I should look into other issue on top of this.

Thank you

 

Check the engine temperature, sounds like the thermostat failed open. Does the engine take longer to get warm?

 

I've had the car since last March. I've never noticed a problem with the car temperature and have kept an eye on it periodically. The car gets to operating temperature in a reasonable time and during the summer did not go above 95. Stays within range and I have never seen it act up.

Could it be possible that the thermostat is still faulty? I've felt the pipes under the dash which go to the heater core I'm assuming, and they are very hot.

 

Sounds like the thermostat is OK.
A sneaky thing I found on another car was 15 years of dust in the vent for the interior temp sensor. So the heater controller could not get accurate interior temp. Haven't figured out where it is on my coupe.

 

Check that the auxilliary coolant pump has not failed.

 

Glyn, does the pump run if the engine is running? To boost hot water circulation? I thought it was for the residual engine heat function that warms the car after the engine is off.
And I'd like to see that whole schematic for the coolant circulation, please.
My car heats up nicely, but sometimes blows cold air from the dash vents. The vent at the left end of the dash that blows onto my left hand on the steering wheel gets my attention. Are the dash vents supposed to be outside air?

 

It runs if you want the heater to work. If you demand heat after switch off it will continue to run in the Accessory position of the key. The heater will never work properly without it running.

All settings give outside air until you switch to recirculate cabin air.

 

Guys thank you for all the responses. I have looked at the aux pump previously and even had my mechanic buddy check it out before.

There is some wacky wiring going on to it as the wiring is not insulated completely which indicated that something might have been replaced it and half assed it but when touched, we can feel the pump vibrating indicating that is does indeed work. I will look further into this as I assumed the aux pump was only for the REST function and was not needed for heat when the car is running.

Also, can anyone give any insight to the M2/15 footwell flap not operating properly causing no heat or blockage? My friend has one of those professional scanners which works similar to the star diag system and the M2/15 was the only actuator which was not operating properly. All the others seemed to be fine.

 

On some W204's the Aux circulation pump was deleted to save cost & the heaters worked like crap. Make sure the pump actually pumps.

Regarding M2/15 you need to check the flap for tightness, that the stepper motor works & that the arm is not broken.

 

I just got back in after doing some more work and checking. I had the car running for about 2+ hours while inspecting everything. Around the hour mark, it feels like the heat does work better but when the blower is turned all the way up, it is not hot at all, although it is warmer than if I were to turn both the knobs to the cold setting.

I checked the M2/15 and it appears as if the motor does not work at all. I manually moved it around and it seems that if the motor were working the whole setup should work fine. When I moved it to one side I checked both foot blowers and they worked then when I moved it back they stop blowing. Im assuming the arm works since I checked both sides for air but the whole setup was not very tight?

I looked into the aux water pump some more and it seems to work and I can feel water flowing but I am just not sure of how much. I plan on taking it off tomorrow and checking how much water it actually pumps by putting a hose in a bucket and attaching it to the pump.

I noticed the return line from the heater core did not seem too hot and the one going to it was obviously blazing hot. Is there supposed to be a large loss of temperature?

 

After a bit more research of the topic, I believe that either the aux pump even though it works is not running strong or that the heater core is clogged due to the difference in temperature from the line in to the line out of the heater core which traces back to the aux water pump. I do not believe there is any leaks as I do not smell any coolant inside or outside the car and have not seen any leaks under it.

 

My previous car was a 2003 C240 & the heater worked very well. Seems you need a new stepper motor if it's getting signal from the controller. You should get a temperature drop across the heater matrix. With modern longlife coolants like Benz use you should never suffer a clogged heater matrix (radiator) ~ but who knows.

Your pump test is a good idea. The impeller might be damaged or have come loose on the shaft.

 

My M2/15 was broken also but the heater still blowed very hot air (to my feets tho) even at -25c weather.

I tackled a lot with w203 heating issues and got it working remotely okey with changing 2 thermostats (CDI's have 2) and replacing that aux pump ($50). The stepper motor did not really change the cabin warming up speed time.

Can you get the temp. readings for core heater? Star can show lots of information regarding the temperatures. Also Star can show if any other stepper motor/flap is dead and causing this problem

 

After some more work I have finally (hopefully) concluded my issue. The aux pump after taking it off and doing some testing works, and works really well. This really puzzled me for a bit. After turning the car on and letting it sit for 20 minutes with the heat (or lack of heat) on, I felt the return line from the heater core right on the inside near it and it was ice cold.

This whole time I have assumed that the heater core must have been working as I was getting some heat and there must have been some sort of airway blockage. Now I am convinced that the heater core is clogged.

I will attempt to disconnect the return hose and the inlet hose and run water in forwards and backwards to attempt to dislodge whatever is in along with using CLR and letting it soak along with more water running and possibly use some compressed air but nothing too strong to damage anything.

Today I tried to get the inlet hose off and spend hours wrestling with the goddamn clip till I had to stop in fear of breaking something or taking some anger out on the car lol.



I will be borrowing a set of grips? or whatever they're called specifically for this and tight spots which has part you attach and hooks to the grip with a wire

If this process does not work I guess this will be a summer only car as I am not about to take out the entire dash and everything to replace the heater core as I seen the video on youtube.

 

Yea I concluded that the M2/15 has nothing with my issue. Might as well fix it while half the dash is apart. All the others seem to be working.

The diag thing my friend has has a bunch of options possibly even the heater core but at this point its deff the heater core.

 

OK ~ now you at least know where the issue is.

Try to back flush the heater matrix. Removing it is a crap job.