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Re: a/c blower motor only operates on high.
The blower motor resistor could be the cause, if not then its a problem directly relted to the ecm. There is not other components that control this circuit. I would check to verify there are all powers and grounds needed. if not I would bring it to the dealer or a reputable shop that can check the functionality with the scan tool and multimeters and such.
Hope this helps
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The blower motor does not operate correctly - manual climate control
A/C blower motor resistor
A/C blower motor switch
Blower motor high speed relay
Go To Pinpoint Test O .
The blower motor does not operate correctly - EATC
A/C blower motor speed control
Go To Pinpoint Test Q .
Pinpoint Test Q: The Blower Motor Does Not Operate Correctly - EATC
Under normal operation, the blower motor control module receives a 1-2 volt signal from the EATC module through circuit CH218 (GN/VT) to determine the desired blower motor speed. The EATC module receives a feedback voltage from the blower motor control module through circuit VH219 (GY) to determine actual blower motor speed. When the highest blower motor setting is selected, the EATC module sends 12 volts to the blower control module through circuit CH217 (BN).
An open, short to voltage, ground or together in circuits CH218 (GN/VT), VH219 (GY) or circuit CH217 (BN)
Blower motor control module
If automatic HVAC system i would suspect Blower motor control module .
Yes,could be your blower resistor is bad,it's right next to the blower motor,it controls the blower motor speed,easy to replace. Duralast Blower Motor Resistor Just took a guess at the model type since you didn't state,wow,high dollar price,guess it comes with a Mercedes,paid about 18 bucks for mine on my 85 Nissan.
I'm going to start from the beginning. The blower motor gets power from the high blower relay and it is grounded thru the fan switch and the resistor. So you should have power on the pink white blower wire coming from the high relay to the blower motor. The orange black wire from the motor is ground going to the resistor and fan switch. The blue wires on the control panel are for the ac compressor. The high blower relay gets power from fuse number 5 in the box. The blower motor is grounded thru the resistor and then thru the fan switch. Photo courtesy of AutoZone.com
First: check the Front Blower Motor Relay located in the IPM. Its' location should be on the back side of the cover. There should be an available Spare Relay. Replace and Check operation.
If this does not solve it.
Try the Fan Speed on "high". If the Motor comes on only "high" and no other speed, then it is definitely the Blower (Fan) Motor Resistor and must be replaced. It is located behind the dash Glove Box. Remove the Glove Box. The resistor is located on the back fire wall, and has 3 to 4 wires going into it. It is approximately 3 1/2 inches wide by 1 1/2 inches high. Disconnect the electrical connection. There should be two bolts (I believe 8mm) holding the resistor in place. Remove the bolts and Remove and Replace. Check the operation.
If this does not solve it:
Pull the connection to the Blower Motor and check for Voltage with the Fan On "low". If correct voltage: the Blower Motor itself needs to be replaced.
Let me know if this helped; or if you have additional information or questions. Feel free to contact me at FixYa.com!
fan blower speed switch under hood usually in fire wall or in heater box under dash area if you do not change it you will keep losing all your speeds until nothing left! it is not!!!!the one in your dash that you hand operate! hope this helps.John