I drove my truck to the gym this morning and the heater blower was working, when I left for work the blower would not come on. I had heat coming out but not being blown out. I checked what I think is the relay (#3) and the fuse (# 51) and they seem to be OK. What else do I should I be looking at? Only had limited time to look at it as I have no garage and the sun and temp dropped quickly, but I really don't want to take it to the dealer.

 

Is the blower dead in all speeds?

What trim level and cab style is the truck and what type of climate control system? Manual or electronic, single or dual zone?

 

The power for the blower motor is traced from the battery as follows:
1. Fuse51 in the engine bay,
2. Relay3 (normally open) in the engine bay,
3. Six position plug inside the cabin via Yellow/Green wire.
4.Connection to Blower Motor
5.Connection to Motor Speed Controller
Relay 3 is energized when the climate control system is on.
The blower motor receives 12VDC power via the Yellow/Green wire. The motor speed is controlled with the return voltage on the Yellow/Blue wire. Voltage on that wire is controlled by the blower motor speed controller, part number 9L3Z19E624B or similar.

The blower motor speed controller is not a resistor array. The speed controller provides a pulse width modulation (PWM) of the voltage to the motor on the Yellow/Blue wire so that a variable voltage is obtained. Control of the motor speed controller is via a voltage signal on the White/Purple wire. Not having a schematic, this wire must be from the climate control module. The climate control module provides the user interface for the fan speed.

The blower motor trouble was first addressed by replacing the speed control module. This did not change the behavior of the motor. The change may have made the behavior a bitmore consistent; the motor would run continuously and it would stop only when under heavy air flow conditions. For example with half of the vents closed, the blower motor would run. But open one of the vents and the blower would stop. This behavior appeared to repeat leading to a conclusion the blower motor was drawing too much current. This turned out to be a false conclusion.

The actual cause of the trouble was a burned connector pin at the Black/White wire (ground) to the motor speedcontroller. Replacing the pin and reinstalling the speed controller saw the problem corrected. The connector part number is WPT-688, a 4-cavity plug, that sells for around $50. However a pin left over from a 2005 Jeep Wrangler multi-function switch connector replacement was soldered to the fresh end of the 14AWG Black/White wire and the connector reassembled. The system has functioned satisfactory since.

Finally, you can test the operation of it by unplugging the motor and applying 12V and ground to it. If it spins, you know it works.

Good luck!

 

It is an XLT supercrew cab with a manual single zone system. The blower seems to be working on low speed only.

 

Are you sure? That would be a most unusual failure mode.... There is some air movement that does take place through the system when the truck is in motion.

The really great post by Manualf150 above describes the automatic system (which we now know you don't have) so not much of it applies.

The schematic for your system is attached.