Heater only blows on recirculation

[AMck]

The heating system on my 2000 R129 is playing up.

If I have it set on re-circulation everything works ok with plenty of air blowing through whatever vent I choose. On normal there is pretty much no air coming through any vents, although you can here the fan working.

I assume there is a control flap that is not functioning. Where should I start looking? Is it likely to be a broken/jammed flap or control solenoid/motor.

Would it be accessed through the engine bay or is this dash dismantling time?

Any help pointing me in the right direction would be appreciated.

 

[television]

I seem to remember that the evaporator sensor can cause this on a 129 with the symptoms the car gets colder the further you drive.

Sits just behind the radio

 

[Alex Crow]

presumably the pollen filter has been checked?

if it is heavily soiled it can cause these symptoms.

 

[television]

Only the 1 dust filter on these 105

 

[Shaun Roffe]

Start off by cleaning all the contact point of your fuses.

Over time, the copper/aluminum contacts become corroded and can cause a loss of proper voltage.

The fuse connections might "look" ok, but generally need to be cleaned. It's suggested that the fuses themselves be replaced every couple of years.

I am certain that another european owner will chime in now that we know what part of the world you are located in and help out with your problem. All US model W124's came equipped with automatic HVAC systems, so their function if different than yours.

 

[Alex Crow]

Start off by cleaning all the contact point of your fuses.

Over time, the copper/aluminum contacts become corroded and can cause a loss of proper voltage.

The fuse connections might "look" ok, but generally need to be cleaned. It's suggested that the fuses themselves be replaced every couple of years.

I am certain that another european owner will chime in now that we know what part of the world you are located in and help out with your problem. All US model W124's came equipped with automatic HVAC systems, so their function if different than yours.

good advice, but on this car the fuses will all be blade fuses, and not prone to this common problem.

 

[AMck]

A little more info on problem from using the car yesterday

If I put the temperature on LO it will blow cold air both on recirc and from fresh air. If I then change the temp setting to 12C it shuts off the air flow, although the fans continue to run at full power.

If I leave it on LO and then drive, after about 5-10 mins the air flow shuts off both on fresh and recirc. The fan is still running on full power.

I have found the information on how to access the diagnostic information after searching about the evap sensor from this post - http://forums.mercedesclub.org.uk/showthread.php?p=302316

so today before starting the car I read the values, which are

01: in car temp sensor - 26C
02: outside temp sensor - 22C
03: heater core temp sensor - 22C
05: evap temp sensor - 22C
06: ECT temp sensor - 21C
07: refrigerant pressure meassured in BAR - LO
08: refrigerant temp sensor - 21C

After running the heater for a few minutes it did as described above and the values were

01: in car temp sensor - 29C
02: outside temp sensor - 22C
03: heater core temp sensor - 25C
05: evap temp sensor - 26C
06: ECT temp sensor - 21C
07: refrigerant pressure meassured in BAR - LO
08: refrigerant temp sensor - 21

So from this it looks like the evap temp sensor rises the longer the system is switched on, as does the interior temperture sensor. I will change the Evap sensor, but might I also need an internal sensor as well? On the linked post it shows the location on LHD car. I assume it will be in the same place on RHD i.e. under passanger side and just needs removal of lower trim panel to access?

The low refigerant pressure I know about - basically the air con has not worked since I bought the car - diagnosed as a leak in the condensor. The other heater functions have always worked fine though and the lack of ac has never bothered me - as soon as it is warm enough the roof is down.

 

[Alex Crow]

please confirm, you have checked that dust/pollen filter haven't you?

only 2x screws and a few clips just to have a peek at it.

 

[AMck]

please confirm, you have checked that dust/pollen filter haven't you?

only 2x screws and a few clips just to have a peek at it.

OK, tail between legs and with a good mouthfull of humble pie.

Checked filter - looked pretty dirty so removed it and on initial testing the fan seemed quite happy to keep blowing even when changing temperature settings.

Will see how it goes when driving and order a new filter anyway.

I still think the internal temp sensor may be faulty as showing 22C now when outside temp is 17C. I believe there is a small fan that pulls in the air over the sensor and will remove this and check it's clean.

Thanks for help