124 Heater defaults to no heat or too much heat

[Ben300SD]

The heater in my 91 300D (W124) is acting up. The fan speed seems normal, and the air comes out of the center vents when it should be cooling and out of the floor vents when it should be heating.

The problem is that the air is either unheated or super heated. There is no in between. The system defaults either one way or the other and stays that way until the car is shut off and restarted. Luckily (it is winter) it usually defaults to heat, but the air is so hot that even with an outside temp of 25 F the car becomes too hot after a while.

The amount of heat is not correlated with vehicle speed, it is either really hot or just the outside ambient temp.

Where should I start looking?

Ben, I've been chasing a similar problem on my 1989 for 4-5 months now, without any sucess. The problem is not exactly similar to yours but very close. Mine turns cold when I need hot and turns hot when I need cold (in the summer), only when the car idles. It reverts back to proper temperature by simply accelerating or having the engine turn at 1,200RPM or more.
So far, I have spent close to $800. on unsuccessfull diagnostic, replaced the aux. water pump, mono-valve, checked the circuit board on the climate control panel, checked the temp. sending pump.
My local Mercedes dealer says he has never seen this problem before........
Good luck
jackD

[Ben300SD]

Somebody?

[J.HIDALGO]

Most likely, monovalve or a/c control unit.

[rmasteller]

I was taking a 1994 E420 for a long test drive tonight and it seemed to do the same thing. When at idle in the driveway, cool air would blow. Once I revved the engine, or moved the car, very warm air would blow. I am putting this on my list of concerns when negotiating. The car will have a Starmark warranty. Please post any solutions discovered!

Thanks,

[Ben300SD]

I read many monovalve posts. As I mentioned in my post, the air temperature is not correlated with the speed of the car. This leads me to believe that it is not the monovalve.

Is the A/C control unit part of the push button unit? That was replaced less than two years ago, so I may have a warranty claim. If it is separate then I may be pulling stuff apart.

What now, ladies and gents?

[pmizell]

I don't fully understand your exact problem, but if the temperature inside your car is not rising or falling by turning the temp wheel, you could have a bad temp sensor (behind dome light) or your sampler blower motor (behind right dash vent) may be bad.

If you use the EC button when heating, you'll never get truly cold air when you turn wheel to all blue, even if its frigid outside.

Do a search for "sampler blower motor" and a couple of threads should pop up.

G'luck:p

Yes what was previously called the "sampler blower motor" could very well be the problem.

There is a small fan motor that draws air from a little vent near the dome light. It draws that air across a sensor that determines if the car is too hot or too cold. When it fails it does what you are describing.

The problem is that this little fan motor is in several different locations throughout the 124 series. You might try removing the glove box and looking and feeling to the right. This assembly is about an inch or inch and half in diameter and a few inches long. There will be a tube connected that goes off on its way to the grille near the dome light. This fan gets voltage any time the key is on.

You might try making some smoke somehow, such as a cigarette (yuck!) and when holding the smoke near the grille it should be drawn into the grille. If it doesn't draw into the grille, then it will be more important to find the motor.

Good luck,

[Ben300SD]

Excellent idea with the smoke, Larry. I tried it right after I read it and it was sucked into the hole. I wish it wasn't, because then I would know what is wrong with my car.

It is now 25 F outside and my car is defaulting mostly to no heat. I can usually switch it to full heat after driving for more than 15 minutes and after switching the ignition on and off a couple of times.

My heater doesn't work when I need it the most, and it doesn't work like it should when it is working. Any more help, please?

Sincerly,

The coldest commuter in Colorado

[rmasteller]

The smoke got sucked in the vent and past the sensor, but that does not mean the sensor is working. Has that been tested/replaced? Perhaps you could force cold or warm air past the sensor some how and "trick" the car into blowing warm or cold air respectively. Is there a way to test this part imperically? That is, with an multimeter or something?

[haasman]

Larry is correct, the sensor could be the problem. I was told years ago to use the hose of a vacuum cleaner and put it over the sensor and such out the accumulated dust.

This helped with an older car where the sensor sucked but was covered with crap.

[Gary!]

Hello,

I have try that before! I susepect that is the heater value internal leak, and ask the dealer for part, and they ask me to try to check again, as the parts are expansive... (I am in England)..

Afterward, I found that is the temperature control knot cause problem.... The problem for me is come from the potentialmeter for the temperature control, the ECU of the temperature control use the value of resistance for the temp control to control the heater value, so try to clean it up with some electronical solvent, same as what I have done for mind, I think u could solve the problem...

hope this can help...

Gary!

Since I have the very same problem, I have spent over $1000. and many hours trying to find the cause.
ECU: tried a brand new one on, no change.
MonoValve: Replaced.
Air pump tested OK, good air circulation.
tem. sensor: Tested OK.
Tried to fool the system by blowing hot air from an hair dryer through the air intake. system responds perfectly by blowing hot hair through the air intake. Full cold blast on automatic.
My mechanic (doing only Mercedes since 25 years) is baffled.
My Mercedes dealer spent 7 hours of diagnostic @ $85.00/hour (testing and trying replacing different parts) and admitted he had no answer.
Very strange.
jackD

[400E]

Do you have the repair CD ROMs? The climate control section has a very thorough diagnostic section, including specific resistance values for the cabin temp sensor and others.

The system is sufficiently complex that to try to diagnose it without the manuals would be pretty difficult.

I doN,t have the CD manual but my Mercedes dealer tested everything with theirs opn hand.
They also tried new parts (sensors, climate control module, monovalve, etc.) with no results
JackD