Testing PC Computer Fans, Cooler Master, with Multimeter or Oscilloscope (A12025012CB-3BN-F1)

Summary

Cooler Master towers have usually been my computer case of choice for several years. The latest tower used four removable SSD's in the top bay using an Icy Doc adaptor with a separate four Western Digital red drives in RAID for mass storage. Needless to say on occasions the drives in the tower generated some heat which required the three Cooler Master internal fans for some airflow!

N300 Cooler Master Mid Tower Case

Cooler Master Fan Type
Shipped with the Cooler Master tower were three 120mm fans - A12025012CB-3BN-F1. Spinning at 1200RPM the fans were not particularly noisy so when one or two failed for different reasons this was initially not recognised. For those running Linux (Ubuntu in this box) lm-sensors is strongly recommended to prevent such issues.

Fan Testing Digital Multimeter
After removing the fans the 3 pin connector was verified using site All Pinouts. 

Cooler Master Fans

As described, on the All Pinouts site, the connections were:

1. 0VDC
2. 12VDC
3. Tacho

Power was connected to both fans pin 1 and 2. A Digital Multimeter 'DMM' with a frequency function was used to measure the tacho output. This measurement was performed between the tacho output and 0VDC with a 220k pullup resistor fitted between the tacho output and 12VDC. 

DMM Measuring Fan Tacho

The second fan did not register any frequency.

Fan Testing Oscilloscope

Using an Oscillscope whether top of the range Yokogowa, or a pocket SeedStudio device, to sight a waveform, regardless of the novelty of the signal, can yield useful information that a traditional DMM cannot provide.

Below are the two captures of the tacho outputs for each fan.

Cooler Master Fan 1

Cooler Master Fan 2

From the two captures shown above, fan 1 was running below its rated speed and the tacho output on fan 2 appeared to be damaged. Both fans were replaced with a newer model, as shown below.

Cooler Master Replacement Fan

As a reference, the same pullup resistor was fitted to the tacho output of the new fan, with expected results shown below.

Cooler Master Replacement Fan Tacho Output

Summary
Diagnosing a faulty fan can be achieved with the requisite hardware support on a PC motherboard and a suitable application, a DMM with frequency input or an oscilloscope of some variety. All methods provide diagnostic methods for fans with tacho feedback.