Blower motor resistor

The fan is variable speed and is electronically controlled by the AC Amplifier module using one signal wire. It does not use a power resistor as typically used with dc universal motors.

A better description of the problem would help. Is the fan speed steady if you manually change the fan speed, eg take it out of Auto?

Have you checked for a mouse nest or other clogs through the cabin filter access?

The blower motor has a built-in blower controller and is regulated using variable duty cycle control from the AC Amplifier module. There are several sensors involved in Auto mode and with temperature control.

 

You could start by hanging a voltmeter on the B terminal (where rjparker circled) and seeing whether you have a solid 12ish volts there, or if that's coming and going, corresponding to when the fan "randomly" kicks in. If the power supply is coming and going, that's what needs to be fixed.

If that 12 volt supply is solid, you could next hang the meter on the SI terminal. SI carries a pulse train, and it's the duty cycle of the pulses that controls the speed of the fan. Some multimeters will have a handy function to read off the duty cycle of a pulse train.

If you see the duty cycle jumping around, then the blower might just be correctly following orders, and getting weird orders. The getting weird orders part would have to be fixed.

But if the duty cycle being sent to the blower looks steady, and reasonably corresponds to the expected blower level shown on the display, so the blower is getting orders that make sense but its speed is jumping around, that'd point to a bad blower.

 

Have you checked for a mouse nest or other clogs through the cabin filter access?

 

Frayed wire? Broken connector?