Merc Mtneer Auto Temp Control Breakdown

[RojoBlanco]

As I arrived at my destination yesterday a slight smell of what I believe was electrical in nature came from the dash area. It was late, so I parked and made a quick check to make sure a major electrical meltdown was not eminent. Today I started the Merc and had no front A/C blower speeds. The ATC is functioning flawlessly, except for the blower speeds: The compressor kicks on, the function buttons all work properly, the cold/hot temperature buttons work properly. The rear seat blower fan works properly, also. I believe I've checked all the necessary fuses, relays, etc. and not found anything amiss. I have normal 12 v power at the blower motor connector, but don't understand how the resistor and speed switch all integrate with the computer controls. I have isolated the blower motor and jumpered it to the battery and know the motor is perfectly good.

I don't know how to proceed. I am thinking it is the rotary speed dial switch but I don't understand how the computer control relates to it since the driver can override the computer or let the computer have its way.

This system has a fancy circuit board blower resistor and I don't know if it can be tested. Anyone experienced with this problem, and can you offer any advice. Thanks. David B.

 

[koda2000]

It sounds like your "fancy blower resistor" (AKA the blower control module) is burnt out. This is not uncommon. Some here have managed to take it apart, replace components and/or re-solder the circuit board, but I'd just replace it and make sure the electrical connector is clean. IDK of a way to test the resistor, other than the fact that you've already tested the motor and say it's fine.

 

[J_C]

Yeah, the circuit board transistor is probably blown. There's a topic somewhere with the replacement part if you're comfortable with a soldering iron.
http://www.explorerforum.com/forums/index.php?threads/aircon-fan-always-running.421358/#post-3382294

 

[RojoBlanco]

Yeah, the circuit board transistor is probably blown. There's a topic somewhere with the replacement part if you're comfortable with a soldering iron.
http://www.explorerforum.com/forums/index.php?threads/aircon-fan-always-running.421358/#post-3382294

Thanks to koda and JC for your input. I will plan to replace the blower module and hope for a cure.

 

[J_C]

Well, in the link there is testing advice, it could still be the head unit in the dash but it's less likely. A multimeter will tell you if the base voltage to the firewall transistor pack is being sent by the head unit.