Heater playing up

[thechasmanblue]

I have a problem with my W124 300CE that seems to be a variation on a common theme.

In any position other than the heater control wheel in "off" the heater is on full on both sides (regardless of which control wheel has been moved). The only way to turn the heater off seems to be to turn off the ignition and put both control wheels back to "off" then fresh air is resumed when the car is restarted. But as soon as these are touched again...

I have seen the various posts about the duo valve but as there is power to it (so fuses fine) and I can get fresh air (as long as I follow the procedure above) it doesn't seem to be a total failure of the unit.

Any suggestions as to what the best place to start is? Thx.

 

[television]

Only by connecting a volt meter to the the valve, and then turning the controls will you know if it is the control panel with a poor connection or the valve sticking

 

[thechasmanblue]

Finally got round to sorting this out.

Electrical skills not up to it so decided to spend some money on parts rather than professional help. Initially replaced duovalve with secondhand one for £20 one but same problem so found a used control unit for £10 and 5 minutes later we have controllable heat.

Fantastic and all just in time for autumn too.

 

[maxicab]

All or nothing!
Sounds like a great cure. Mine has the opposite problem! No heat comes out at all until thumbwheel temp controls are nearly at their maximum, then it seems to produce maximum heat all of a sudden, and I can't turn it down without turning the wheels back down which causes the heat to disappear completely with minimal movement of the wheels.
From your experience it sounds like I should go for replacing the heater control unit first up. If you did it in 5 minutes it must be pretty easy to remove and replace. Any tips on doing this job? From where did you source the parts?
Thanks, David

 

[creamy]

Slightly different issue?

Hi,
I cannot get any more than a slight warmth from either side of my heater (W124 200TE, no A/C).
The aux pump appears to be working, the duovalve certainly 'clicks' positively on both sides, and the feed and return heater pipes connected to the matrix seem to be warming up.

There was a lot of dust on the interior temp sensor (by the sunroof control) but I have cleaned this out and it makes no difference. Checked resistance across this sensor, and it certainly changes with temp (around 5-6k ohm - is this correct?). However, there doesn't seem to be any suction from the pipe connected to the sensor. I'm sure I read somewhere that there is an air pump to draw cabin air across this sensor, but where does this live? I don't believe this is working.

The LCD temp display shows between -20 and -30 degrees (yes, this is 'minus') - could this give a clue? However, even with the duovalve disconnected (should default to 'open', i.e. hot) and the aux pump running, it does not seem to blow hot.

I have flushed through the matrix with a garden hose, which started off with little flow, but increased, so this should have cleaned out some blockage, but it doesn't seem to have made much difference to the heater temperature. Should I persist with this? Maybe I should whip the duovalve out and give it a thorough check?

Any other suggestions?

 

[television]

As these valves fail in the fully hot mode I would still go for the pump,,it should measure 3 ohms but that is no indication that it is running. I take it that the engine temp is getting beyond the 80c

 

[42864]

Hi,
I cannot get any more than a slight warmth from either side of my heater (W124 200TE, no A/C).
The aux pump appears to be working, the duovalve certainly 'clicks' positively on both sides, and the feed and return heater pipes connected to the matrix seem to be warming up.

Any other suggestions?

I believe that you will find that the heater system is similar to the 140 series S class, in as much as there are 2 sensors that live in the centre Consul, if these go faulty this will give the symptoms that you describe, if you can obtain (main Dealers have copies ) of the Novemer eddition of the Mercedes Benz Club Gazette you wil find full details on page 60. Hope that this will be of some help.

 

[creamy]

... an update

Many thanks for the responses above. I have since done a bit more investigation and found the following:

1) Duovalve appears to be working fine on both sides, and is clean inside.
2) Cabin temp sensor (by light) was pretty clogged up, but even when I cleared it there did not appear to be any suction from the tube. Resistance across sensor seems to vary wih temp, so I'd guess this component is probably OK even if the pump may not be running.
3) Aux pump appears to be pumping water OK, even if it is slightly noisy.
4) Biggest problem appears to be that there is very little flow through the matrix. With duovalve and matrix inlet pipes disconnected, I can connect a garden hose to one end, and I would expect to get little resistance to the water flow. However, there appears to be considerable resistance, and I am concerned about blowing the matrix due to the pressure build-up from the hose. Am I correct in thinking that there are no other valves between the input hose and the duovalve? (Can't see why there would be). Can anybody recommend an effective way to try to clear the matrix of any crud which may be blocking it? I would rather stay cold than blow the matrix!
5) Finally, exactly what is the temp display on the dash supposed to display? I'd guess outside temperature, but mine reads -25 to -30 most of the time.

I really don't want to have to replace the matrix, so a simple solution would be fantastic! When I arrived at work last week, there was still ice on the INSIDE of the windscreen! Also, just drove 250 miles with the family, with no heat at all. Not very confortable.

 

[television]

Why not get some de scaler into it,, something very kind to the matrix would be a 24% concentrate acetic acid,,I use this to de scale kettles,,you can buy it from any industrial chemical supplies or from your local Chemist, it comes 30% proof, just dilute with water a touch,,fill the matrix up and leave over night.

 

[wireman]

It is very likely that your matrix is choked up, I have never ever found one that could be unblocked without taking it out and rodding through the water holes after its end tank is removed, you can only do this if you can reassemble the matrix afterwards.
The gunge that blocks the water tubes will not be removed by chemicals unless there is a flow through them, if they are blocked there is no flow and therefore no cleaning.
The choking medium may be scale from too much tap water or that brown stuff that tired antifreeze leaves behind, an acid cleaner may remove some of the scale but it wont touch antifreeze, if you do use an acid it is essential that it is totaly neutralized before the cars cooling system is put back into service or horrid things will happen to your engine/water pump/cylinder head/radiator after a year or two, only use a tried and tested cooling system cleaner and use it strictly in accordance with the instructions, radflush works but again I must say that I have never got a heater matrix to come clean with it.

If the duo valve is left disconnected it should be fully open and you should get hot all the time, to be sure that your heater controller has no contribution to the problem leave it disconnected and check for warmth.

As a test to confirm blockage of the heaters water circuit,
switch off the heater fan, set the temp high (wont matter if the valves disconnected) and run the engine for several minutes
then put the blower on and feel for warm air, if its hot and goes cold quickly you have a blocked matrix or valve or collapsed pipe between engine and heater.

The auxilliary pump on the 124 will help you stay nice and warm when the engine is turning slowly and the water pump is not realy doing much to shift water around the system, it even keeps the heater working without the engine running untill the heat in the engine is used up but it does not stop you getting at least some heat when on the move if its not working.

The 124 has two heater matrices in one assembly (one for each side of the car) which get hot water from the cylinder head and return it via the duo valve and pump to the back of the engine thermostat, you may find that one side is different in behavior to the other.

I am of the opinion that most cooling problems are caused by a reluctance to change the antifreeze at the specified intervals, MB say every 2 years as do most antifreeze makers.

If others wish to avoid some of the cooling problems that affect most cars sooner or later make sure that your antifreeze is in good condition.

 

[creamy]

Many thanks for the replies.

Sounds very much like that is the problem. Certainly I can get a some warm air from the driver's side (only) if I leave it shut off for a while first, but it soon cools, especially if I turn the fan up higher. It supports the blocked matrix theory. Maybe an acid flush would work, but only if (as stated) there is any flow through it at all.

I really don't want to be taking the dash out, so until I get a chance to do the acid thing, I'll just wear another jumper!

regards,

Alex

 

[delboy777]

Finest thing for cleaning out blocked rads is caustic soda crystals available from your local chemist for next to nothing.

 

[Smudge]

If anyone needs a W124 heater matrix. I have a good one here for sale £30