Heater Control/blower issues

[ash_2009]

Dear all,

Car is w202 c36 Amg 1995.

Recently I have developed two faults which could be related or not and just pure coincidence.

1) When the car is up to normal running temp 80C, if I have the thumb dials on the heater control unit turned to blue (cold) I have hot air in both the passengers side and drivers side...side vents. There is only cold air in the centre vent. Having read around on here there is a suggestion the that duo valve in the engine bay near the fuse box is either faulty/sticking hence solenoid not closing/opening allowing only hot air in the side vents. Or it could be faulty heater control unit in the centre console.

2) At the same time and more randomly, when the car is running, the round fan control and air position control dials and the heater control unit are flashing in a continuous manner a bit like the hazards. When I turn the main head lamp beam ON the flashing stops....when I turn the beam off the flashing starts again. After searching around for this I have come across several posts saying it could be a faulty alternator voltage regulator which is attached to the back of the alternator and the brushes are shot and hence when you increase load the flashing stops.

So is this just two separate things happening or could it point the the heater control unit on its way out!!

So before I go about buying new heater control unit and/or duo valve is there some things I can do to eliminate one or the other to get to the bottom of both these issues?

Thanks

 

[BlackC55]

Id be checking the earths as a first port of call. Near the due valve and the engine earth.

 

[ash_2009]

I have seen one earth near the duo valve. Where is the other places I can check for earth please? the engine earth??

 

[BlackC55]

IIRC near the PS pump there is an engine earth too.

 

[ash_2009]

PS (power Steering) pump where is that located please?

 

[BlackC55]

The passenger side at the front of the engine.

 

[ash_2009]

Will do check there

 

[219]

Dear all,

Car is w202 c36 Amg 1995.

Recently I have developed two faults which could be related or not and just pure coincidence.

1) When the car is up to normal running temp 80C, if I have the thumb dials on the heater control unit turned to blue (cold) I have hot air in both the passengers side and drivers side...side vents. There is only cold air in the centre vent. Having read around on here there is a suggestion the that duo valve in the engine bay near the fuse box is either faulty/sticking hence solenoid not closing/opening allowing only hot air in the side vents. Or it could be faulty heater control unit in the centre console.

2) At the same time and more randomly, when the car is running, the round fan control and air position control dials and the heater control unit are flashing in a continuous manner a bit like the hazards. When I turn the main head lamp beam ON the flashing stops....when I turn the beam off the flashing starts again. After searching around for this I have come across several posts saying it could be a faulty alternator voltage regulator which is attached to the back of the alternator and the brushes are shot and hence when you increase load the flashing stops.

So is this just two separate things happening or could it point the the heater control unit on its way out!!

So before I go about buying new heater control unit and/or duo valve is there some things I can do to eliminate one or the other to get to the bottom of both these issues?

Thanks

If you have a digital multimeter you can check the system voltage , and hence the charging system .

If you measure voltage across the battery , this should nominally be 12V before you start the car ; with the car started , this should rise to something like 14 or 15 V .

If memory serves , never had one , but the 202 has the battery in the boot ? You could expect a little voltage drop from front to back of car .
Oh , also the battery negative terminal will ground to the body near wherever it is . Check that for a solid connection too , not forgetting the battery terminals , since either being loose won’t help .

The system voltage with the car running should stay near constant as you turn on consumers like headlamps , rear window heater , seat heaters ... up to a point . If you see a notable voltage drop as you turn things on , it may point to a charging issue . Does the car start readily?
How old is the battery ? You might just have a tired old battery ?

Low system voltage can cause all sorts of strange things to happen .

Re your duovalves , set your heater to a mid temperature and listen under the bonnet for a clicking sound as they turn on and off to regulate temperature. They are normally in the scuttle area near the bottom of the windscreen . They do sometimes go bad , but get proper diagnosis rather than throwing parts at the car .

oh , on many Mercs , the dashboard centre vent , by design , only blows cold air - to keep the driver alert , regardless of temperature settings elsewhere . On some cars , my R129 and W140 being examples , the centre vents can be selected between heated/cooled air and unheated/uncooled air , that is just ambient air from outside .

 

[wrdcc01]

The duo valves close off the hot water flow into the heater matrix when powered so if you have warm air in the cold position it could be there is no power to the valves or they are jammed open. Test for power and earths on the 3 pin connector. Should be power all the time on one pin with the two switched earths from the controller.

 

[ash_2009]

If you have a digital multimeter you can check the system voltage , and hence the charging system .

If you measure voltage across the battery , this should nominally be 12V before you start the car ; with the car started , this should rise to something like 14 or 15 V .

If memory serves , never had one , but the 202 has the battery in the boot ? You could expect a little voltage drop from front to back of car .
Oh , also the battery negative terminal will ground to the body near wherever it is . Check that for a solid connection too , not forgetting the battery terminals , since either being loose won’t help .

The system voltage with the car running should stay near constant as you turn on consumers like headlamps , rear window heater , seat heaters ... up to a point . If you see a notable voltage drop as you turn things on , it may point to a charging issue . Does the car start readily?
How old is the battery ? You might just have a tired old battery ?

Low system voltage can cause all sorts of strange things to happen .

Re your duovalves , set your heater to a mid temperature and listen under the bonnet for a clicking sound as they turn on and off to regulate temperature. They are normally in the scuttle area near the bottom of the windscreen . They do sometimes go bad , but get proper diagnosis rather than throwing parts at the car .

oh , on many Mercs , the dashboard centre vent , by design , only blows cold air - to keep the driver alert , regardless of temperature settings elsewhere . On some cars , my R129 and W140 being examples , the centre vents can be selected between heated/cooled air and unheated/uncooled air , that is just ambient air from outside .

Thank you for that.

I will go and check for the voltage when car is off and then on and see where that takes me. I will also check all ground connections and yes the battery is in the boot in these cars. I found two yesterday and have made sure they are good contacts.

Regards the duo valves, I did place the heat setting in the middle (white) and tried to listen for clicking but didnt hear anything but I will double check that again after a little run.

Thanks

 

[ash_2009]

The duo valves close off the hot water flow into the heater matrix when powered so if you have warm air in the cold position it could be there is no power to the valves or they are jammed open. Test for power and earths on the 3 pin connector. Should be power all the time on one pin with the two switched earths from the controller.

When you say test them, is this the resistance or voltage please?

 

[wrdcc01]

Pull the plug off the valves and test for voltage on the middle receptor and body earth, with the ignition on there should be 12v. If that's good then turn the blower on and set the temp' to blue go back to the 3 pin socket to the valves and test for 12v between the middle receptor and the two outside ones (one at a time). You should get 12v on both if the controller has both zones on blue. The controller is ok if it is putting out an earth on each. The valves are jammed or just duff if there are power and earths at the plug socket. You could go on the valve pins and test resistance across the two coils I guess, again middle pin to outside pins.

 

[ash_2009]

Pull the plug off the valves and test for voltage on the middle receptor and body earth, with the ignition on there should be 12v. If that's good then turn the blower on and set the temp' to blue go back to the 3 pin socket to the valves and test for 12v between the middle receptor and the two outside ones (one at a time). You should get 12v on both if the controller has both zones on blue. The controller is ok if it is putting out an earth on each. The valves are jammed or just duff if there are power and earths at the plug socket. You could go on the valve pins and test resistance across the two coils I guess, again middle pin to outside pins.

Hello wrdcc01

Here is what I checked and please excuse me as I am not the best with electronics/meters.

1) Checked battery voltage with car OFF and it is 12V. Then started car and voltage is 13.90V and steady on the meter.

2) With key in the ON position I tested the center pin connection for the duo valve and got 13.90V with body ground.

3) with blower OFF and in COLD position the outside pins both when tested with ground the meter would jump between 1 and 4V.(not steady)

4) with blower ON and thumb dial in BLUE position both outside pins show steady 0.35V with ground.

5) with blower ON and thumb dial in WHITE position both outside pins show fluctuating between 0.1 V and 0.4V.

6) with Blower ON and thumb dial BLUE position testing between centre pin and two outside pins I get 13.80V steady with engine running.

7) with thumb dial in WHITE position i dont hear any clicking on the solenoids.

I hope this makes some sense.

P.S. The controller lights keep flashing which is getting annoying now.

 

[wrdcc01]

Ok so looks like the controller is good so far (the flashing is a mystery for now). Not a lot of room to fiddle about with the duo valve but things are pointing towards dismantling in situ. Have a look on YouTube as there are a couple of "how to" videos. Either the gasket has had its day or it's just stuck with sludge. Fingers crossed it will just need a clean though.

 

[ash_2009]

Just to update in case this helps someone in the future that the above issues were due to bad contacts on the back on the heater control unit. The unit was removed cleaned and put back together and all seems to be working fine now.

 

[Celicasaur]

Good work Ash, thanks for posting the solution. I also recently had an issue with my blower cooking fuses, but it turned out that it was the blower unit itself.

P.S. you're not Ash from Harrow by any chance are you?