ASUS G14 GA401QM No CPU Fan RPM

[ResoluteHawk]

I have an ASUS G14 that does not read out the CPU Fan RPM. When I power the laptop on and get into the BIOS, the CPU fan reads 0 RPM even though it is spinning. If I wait about 2 minutes, the CPU fan will climb and reach max RPM.

The same result happens if I get into Windows and try to go into Armory Crate. There is a GPU fan RPM readout but no CPU fan readout, even when I know it is spinning.

The GPU fan does not exhibit this issue.

I've tried replacing the CPU fan, flashing the BIOS, resetting the EC (holding power button for 30 seconds), and even swapping the BIOS chip out for another one on a laptop with properly working fans.

I've probed around the fan circuit and everything seems to measure properly, but I don't really know where to look next. It seems like the culprit could be a faulty CPU temperature sensor but I'm not sure.

Boardview and schematic are here: https://www.badcaps.net/forum/showth...96#post1246796

[mon2]

Re: ASUS G14 GA401QM No CPU Fan RPM

Measure the voltage to ground of fan0_tach. Repeat for fan1_tach. Each should output a voltage when it is spinning.

[ResoluteHawk]

Re: ASUS G14 GA401QM No CPU Fan RPM

FAN0_TACH => 3.35V from the time that the laptop is first powered on. FAN0_PWM slowly climbs to 3.3V.

FAN1_TACH => 3.35V when powered on but about 3 seconds later it drops to 1.7V

I do notice that on my properly working system, this reading starts at 3.3V but then drops to 1.7V shortly before POSTing (on both FAN0_TACH and FAN1_TACH).

[mon2]

Re: ASUS G14 GA401QM No CPU Fan RPM

Is FAN0 the one with the issue? With the pwm at 3v3, the fan is in full on mode.

[ResoluteHawk]

Re: ASUS G14 GA401QM No CPU Fan RPM

Yes FAN0 (CPU fan) is the issue here.

TACH voltage should drop to 1.7V shortly after turning the laptop on but it doesn't drop to 1.7V on FAN0_TACH. It just stays at 3.3V and FAN0_PWM slowly climbs from 0V to 3.3V (max speed).

[mon2]

Re: ASUS G14 GA401QM No CPU Fan RPM

This fan was replaced? Is the pinout of this replacement fan the same as the original?

[ResoluteHawk]

Re: ASUS G14 GA401QM No CPU Fan RPM

Yes it's the exact same fan.
Even if the fan is disconnected, the voltage on the FAN0_TACH pin exhibits the same behavior.

Why is the TACH voltage locked though? Just like the GPU fan, TACH reading should drop from 3.3V to 1.7V when the laptop POSTs.

[mon2]

Re: ASUS G14 GA401QM No CPU Fan RPM

Remove all power. Meter in resistance mode. Measure the resistance of R5002 which is the local pull-up resistor on this FAN0_TACH line.

Is the resistance ~10K across R5002?

[ResoluteHawk]

Re: ASUS G14 GA401QM No CPU Fan RPM

Yes it measures 9.93K.

[mon2]

Re: ASUS G14 GA401QM No CPU Fan RPM

Confirm that this same CPU fan is able to communicate the tachometer output to the host in the known good and working system.

Do measure the voltage to ground of the other fan pins on the known good logic board.

The temperature sensor will be a separate leg of this circuit as compared to the CPU fan driver and CPU fan tachometer. The tachometer is an output from a hall effect switch that will open and close as the fan rotates.

[ResoluteHawk]

Re: ASUS G14 GA401QM No CPU Fan RPM

Ok so in my working system, both of the fans turn on after the system POSTs.

If I unplug the CPU fan in the working system, the voltage on Pin #4 (TACH) never changes from 3.3V and voltage on Pin #2 (PWM) starts climbing.

With the working laptop already on, when I plug the CPU fan in, the voltage on Pin #4 (TACH) drops to 1.7V and the fan starts spinning and voltage on Pin #2 (PWM) drops respectively as the CPU is cooled.

In the problematic laptop, I swapped in a known working fan that behaves normally and with the known good fan plugged in, the voltage on Pin #4 stays a 3.3V and Pin #2 voltage slowly climbs to 3.3V. Swapping the fan back into the other system yields normal results.

[mon2]

Re: ASUS G14 GA401QM No CPU Fan RPM

I see your logic on the thermal sensor. What is the temp reported for the CPU?

[ResoluteHawk]

Re: ASUS G14 GA401QM No CPU Fan RPM

CPU temperature is 49C just after getting into the BIOS and the CPU fan is already starting to ramp up. CPU temp climbs to around 60C and the CPU fan in the BIOS is reading out at 0RPM even though I can see the fan spinning.

R5002 is a Pull-Up Resistor. Is it possible that we have a blown/bad Pull-Down resistor somewhere? The voltage on Pin #4 (TACH) not dropping from 3.3V to 1.7V on POST when the fans start to spin makes it seem like this could be the case.

[mon2]

Re: ASUS G14 GA401QM No CPU Fan RPM

How are you reading this CPU temp? Is it through the bios / motherboard or an external IR temp gun? If the temp is changing the the sensor is likely ok but perhaps consider to validate the reading with another temp gun.

On the resistor check, remove all power -> meter in resistance mode. Measure the resistance to the pull-up voltage rail with your meter. Repeat with a check to ground for the tach pin. Check against the known good motherboard.

[ResoluteHawk]

Re: ASUS G14 GA401QM No CPU Fan RPM

CPU Temperature is read directly from the BIOS. When using a thermal camera the heatsink temperature right on the CPU is within 7-10C of the BIOS temperature on both boards.

(NO FAN RPM BOARD)
FAN0_TACH to +3VS on R5002:
9.88K Ohms (CPU Fan Plugged In)
9.88K Ohms (CPU Fan Unplugged)
FAN0_TACH to GND:
14.59K Ohms (CPU Fan Plugged In)
14.59K Ohms (CPU Fan Unplugged)
R5002 +3VS TO GND
1.88K Ohms (CPU Fan Plugged In)
1.88K Ohms (CPU Fan Unplugged)

(WORKING BOARD)
FAN0_TACH to +3VS on R5002:
7.843K Ohms (CPU Fan Plugged In)
9.92K Ohms (CPU Fan Unplugged)
FAN0_TACH to GND:
10.83K Ohms (CPU Fan Plugged In)
14.91K Ohms (CPU Fan Unplugged)
R5002 +3VS TO GND
2.1K Ohms (CPU Fan Plugged In)
2.13K Ohms (CPU Fan Unplugged)

The No Fan RPM Board seems like it's not even reading that the fan is present, but it is still sending voltage to it. Resistances don't change even when using a known working fan.

[ResoluteHawk]

Re: ASUS G14 GA401QM No CPU Fan RPM

I've looked at this a little bit more and this might be obvious from my previous posts, but in a working system, if the fans are off, the TACH pins of whichever fan is off measure 3.3V.

It's only when the fans turn on that the voltage of the TACH pin drops to 1.7V, but it's strange that the TACH pin for FAN0 is staying at 3.3V in the defective system even when the fan is spinning.

I have so far been unable to locate any other component that switches the voltage on the TACH pin from 3.3V to 1.7V. I'm wondering if the EC is just not communicating with the hall sensor in the CPU fan.

[mon2]

Re: ASUS G14 GA401QM No CPU Fan RPM

I believe that the tach output from the fan is open drain. The local pull-up resistor is parking the voltage to 3v3.

Remove all power. Meter in resistance mode. Measure the resistance from the pull-up resistor to the fan tach pin on the connector. It should read ~0 ohms. If this trace is broken, the EC will not be able to read the tach output.

[ResoluteHawk]

Re: ASUS G14 GA401QM No CPU Fan RPM

Measure the resistance from the pull-up resistor to the fan tach pin on the connector. It should read ~0 ohms.

From the FAN0_TACH end of the resistor to the FAN0_TACH Pin on the fan connector is 0 Ohms.

[mon2]

Re: ASUS G14 GA401QM No CPU Fan RPM

Confirm the details of the other fan connector pins to be sure that the fan can function like the known good board. Check for the continuity to ground on the fan connector pins, etc.

Also, consider to test in diode mode. Remove all power. Meter in diode mode.

Red meter probe to ground. Black meter probe to the FAN0_TACH pin. What is the reading?

Compare this to the known good board.

[ResoluteHawk]

Re: ASUS G14 GA401QM No CPU Fan RPM

Known Working Board
DIODE MODE
CPU FAN PLUGGED IN
PIN #1: 0.50V
PIN #2: 0.775V
PIN #3: GND
PIN #4: 0.68V
CPU FAN UNPLUGGED
PIN #1: 0.50V
PIN #2: 0.775V
PIN #3: GND
PIN #4: 0.746V

NO CPU FAN RPM BOARD
DIODE MODE
CPU FAN PLUGGED IN AND UNPLUGGED
PIN #1: 0.51V
PIN #2: 0.775V
PIN #3: GND
PIN #4: 0.82V