Changed the duovalve...but still only hot air!

[IMD]

Hi all,

My 1990 W124 has been blowing only hot air from the heater for the last couple of months. I recently bought a secondhand Duovalve from a Merc parts specialist on Ebay, which was said to be working.

I changed the valve today and its still only blowing hot air. Hot water is going past the valve, as the pipes that connect to the heater matrix are hot.

What else could be the problem. I took off the light panel (by the sunroof) an the pipe for the sensor is connected, so are the two electronic sensors behind the heater controls, as I took the switch panel off to check.

What next?

I want to sell the car on Ebay, but a faulty heater isn't going to get the buyers flooding in this weather!

Thanks

Ian

 

[IMD]

Any thoughts anyone?

 

[Phil Thomas]

Hi all,

My 1990 W124 has been blowing only hot air from the heater for the last couple of months. I recently bought a secondhand Duovalve from a Merc parts specialist on Ebay, which was said to be working.

I changed the valve today and its still only blowing hot air. Hot water is going past the valve, as the pipes that connect to the heater matrix are hot.

What else could be the problem. I took off the light panel (by the sunroof) an the pipe for the sensor is connected, so are the two electronic sensors behind the heater controls, as I took the switch panel off to check.

What next?

I want to sell the car on Ebay, but a faulty heater isn't going to get the buyers flooding in this weather!

Thanks

Ian

There is a thread on servicing the duo valves. It looks straightforward. Good luck!

 

[IMD]

Thanks Phil. I didn't intend to service the valve, as its supposed to be a working model from a secondhand parts specialist. I wonder whether there is anything else to check before I take it apart?

How do I check if power is getting to the valve? Do I need a special meter?

 

[nick mercedes]

Any thoughts anyone?

New heater panel?

 

[ecossebev]

I had similar problem as yourself (only hot air blowing out both vents), then one by one the lights on the heater control panel began to fail. Acting on Nicks advice, I removed the heater control panel, only to find the multiplug at the back had worked itself loose. Clicked it back in, now have hot/cold air blowing and all the lights are working again.
Not saying it will be as simple as that, but I would begin the search there.

Thanks Nick.

 

[the1beard]

There are three sensors one in the roof area and two in the fan enclosure one LH and one RH. if either one of the LH or RH become disconnected then that side will only give out full heat.

I reconnect the RH matrix sensor cable, which had somehow became disconnected!

My intention was to check that is was working correctly but when I looked it wasn't even connected!

If you want to check yours it's behind the control unit you will need to remove the control unit and the switch panel above (this is quite awkward I did it by removing the LH switch and the rear cable cover which you will need to remove by flipping over) then push the switch panel back until it unclips then move it to the left behind the clip and it will the come forward and hang to the left.

The sensor is almost directly behind the RH heater control dial "the dial which directs the air up or down etc."

It is inserted into the plastic matrix housing and the connector is a push fit there is also one for the LH side.

If mine had been connected I would have checked to see if it gave the correct readings i.e. compared the resistance with the LH sensor.

Also if the connection to the duo valve is removed the heater will blow hot.

Hope this helps.

If all else fails Merc dealers will find the fault easily though also expensively.

 

[the1beard]

If heating only working on one side a simple test of the duo valve to rule it out is to switch the wiring this will then tell you if it the duo valve or the control unit with the dials.
Just had this problem on anothe w124 e220 with A/C.

 

[jpskiller]

what about the aux pump!

 

[alexander patie]

hello imd, being as ebay is not always the best place to buy s/h items i recommend you test the resistance of the duovalves at the pins. they should both read 12-16 ohms, any lower than 8 ohms is a definate cause of problems.

 

[Donbronski]

Hi all,

My 1990 W124 has been blowing only hot air from the heater for the last couple of months. I recently bought a secondhand Duovalve from a Merc parts specialist on Ebay, which was said to be working.

I changed the valve today and its still only blowing hot air. Hot water is going past the valve, as the pipes that connect to the heater matrix are hot.

What else could be the problem. I took off the light panel (by the sunroof) an the pipe for the sensor is connected, so are the two electronic sensors behind the heater controls, as I took the switch panel off to check.

What next?

I want to sell the car on Ebay, but a faulty heater isn't going to get the buyers flooding in this weather!

Thanks

Ian

You got any pictures?? how much you looking to get for her??

 

[the1beard]

what about the aux pump!

The Aux pump will not affect the lh or rh temp.

If one side is not working then it's

1.Duo
or
2.sensor
or
3.controler
or
4.wire between one of the above.