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I have a 2017 Ford explorer which has an intermittent issue with the blower motor. About 95% of the time when starting up the car, the blower motor will not start at all and will not respond to any changes in speed control from the front panel.

About 5% of the time the motor will start when the accessory power is enabled. However, this lasts only as long as I can get to the controls and change their settings. After turning the climate off and on from the front panel, the motor will not start up again.

I've replaced both the blower motor and the blower motor resistor, neither of which fix the issue. Measuring the input voltage of the blower motor with multimeter gives 4.1 volts while the motor is connected and 12.1 volts with the motor disconnected. I am suspecting there's a wiring issue at fault

I've checked all fuses and all relays and have verified that everything related to the HVAC blower motor is and working condition and is working as expected

The voltage across the relay contacts is 12.1 volts at all times.

One thing that I find strange is that the voltage across the resistors 4.1 volts, which might indicate that there is some faulty wiring somewhere. I'm not quite sure how to diagnose this, as all the wiring for the HVAC system is behind a bunch of finishing. While taking the center control panel off is easy following the blower motor wiring up behind the console is rather difficult.

With these things in mind, what are the possible different options for finding the root cause of this issue? I feel like I'm close to fubar as the wiring being behind a lot of clutter makes it really hard to trace and I don't have multimeter probes long enough to measure the resistance between the relay and the blower resistor.

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    Try swapping the blower relay with another one, like the windshield wiper relay. If the problem persists, move on to wiring issues; power to blower and blower ground. With blower set on high speed, connect a 12v load like a bulb and ground the other end. If the bulb lights brightly, then suspect a ground connection issue. Examine the two wires going to the blower as one may exhibit discoloration as a sign of overheating from a loose termination or actual ground stud issue. Examine all terminals related to the blower for discoloration as a clue.
    – F Dryer
    Aug 1, 2023 at 14:10

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