Fan control knob on a/c and heater will only run on high speed. 2003 dodge mini van

fan motor relay is for fan speeds
heater and AC

blower motor has a resistor
did the motor bench test ok
if fan motorand resistor are ok
i suspect
a faulty on /off switch in the dash panel

i think that system has electronic directional flap actuators not vacuum ..if vacuum might be a leak ..

May have broken a connection/shorting in the blower resistor pack which controls motor speed. Can get fragile with older vehicles.

Usually a bad resistor. The high speed circuit bypasses the resistor and is powered by a relay.
The resistor is mounted in the HVAC box, either on the firewall or under the glove box, and has 4 or 5 wires.

THE HEATER WILL ONLY RUN ON HIGH SPEED

Check for vacuum at the control switch, and replace the blower motor speed control resistor for the high speed only problem.

usually it is the fan speed resistor in the air box, I had the same problem with my wife's 2003 stratus sxt. I found that the resistor, that is what I call it, had broken a soldering point. The fan will only run on high if this piece is broken or burned out. It is in the air box usually near the blower motor behind the glove box. Generally has two or three small screws holdin it in

Check for a blown heater motor supply fuse,
Check operation of heater motor control relay,
Check operation of fan speed control switch,
Check heater motor speed control resistor pack (usually located in the heater ducting - in the air flow),
Check the heater motor runs if power directly applied to the motor- if not motor may be faulty.

Sounds like the fan control knob has shorted out. You can check the fan "blower" relay and fuse in the engine compartment but you need to make sure the control knob is good or you will keep blowing fuses and relays. If your blower fuse and relay are good, then just have the control knob checked and replaced if needed.

if you have checked the switch as recommended, it is most likely to be the heater resistor which contols the diffrerent speeds. it is
normally fited near to the motor and is a squarish block with multiplug on it and coils like springs in the centre of it, look at the coils they may be burnt out.hope this helps

there is no computer control unit, what u need to replace is the fan speed control resistor module.