I have a 2005 F250 that the heater fan will only blow on the 2 lowest speeds. I have changed the blower motor, the dash switch and the sensor but it sill only blows on low speed. Anybody got any suggestions what else might be the problem.

Thanks
Bandit 44

 

NEED MORE INFO. What model? Auto temp or manual?

 

Sorry the truck is a 2005 F250 diesel. The heater had manual control. I thing it must have something in the wiring not reconizing the sensor.

 

Take a look at the blower motor resistor. It is in the air intake plenum, accessed from the engine compartment. It will have a bunch of wires plugged into it, will come out of the plenum with two screws and will have about 3 heavy wire coil resistors and a thermal cutoff switch. Power is applied through the resistors in the right combination to apply the right amount of current to the motor for a particular speed.

 

Have replaced the resistor and it did not change anything.

 

Strange. The resistors are what allow the lower speeds to work. They add resistance which lowers the voltage to the motor. I'd suspect wiring from the switch. Not sure what sensor your talking about, I wouldn't think you'd need a sensor for fan speed.
You might try posting down in the electrical forum.
Good luck!

 

If i read this right, the low and medium speed works but u do not have high speed.

IF that is correct, its not a resistor problem. The blower motor relay provides power directly to the motor. The resistor varies the ground path to the motor through the resistor to give the various speeds. Except high speed. It goes straight to ground for high.

On the back of the heater control fan switch is an org black wire that is provided a direct path to ground when high is selected. It goes through a couple of connectors (d shaped 34 pin connector near batt junction box pin8 & 12 pin connector rectangle directly over blower motor near where refrigerant tube goes into firewall off of accumulator) into a splice around the blower motor resistor connector. The org and black wire that comes out of the resistor shows the splice to be between the resistor and the rectangle connector above motor. It remains org/blk the whole run. More than likely, you have a corroded terminal or melted connection in one of those connections. It is fairly common for the ground to melt on these b/c of running on hign all the time.

Good luck as it is gonna take some patience and a voltmeter to find. If u have a voltmeter and a roll of wire, u can check the continuity between the org black wire at the motor and at the back of the switch to verify bad wire.

 

Thanks for the suggestions, I will try tracing all the wires and see if I can find the problem. Things like this is why I have all but given up working on my own stuff. I never have a problem that has a simple solution.