The electric engine cooling fan won't come on in my Mazda 3. The first thing I did (a mistake, in retrospect) is replace the original fan.
When the new fan didn't run, I ran through the electrical diagnostic procedure in the service manual using a multimeter. Power and ground check out, so does continuity on the signal line across the wiring harness (from the "plug" to the PCM.) There are also no unexpected grounding issues. Also getting good readings from the engine temp sensor on my OBD-II device and dashboard gauge.
I should mention that the fan ran briefly, on three occasions over a few days, on the last occasion it ran when the car was started and AC turned on, then after the car had warmed up the fan no longer came on.
After the last occasion I unplugged the fan, hooked my multimeter up to the signal line and ground, set it to DC volts, and thought I'd see if there was a signal which would look like this on an oscilloscope (my model corresponds to 1R):
Assuming the signal was good, would I even be able to see the oscillations on the multimeter readings? I assume by the oscilloscope settings in that image that I should expect a 5V signal for a few milliseconds? All I saw were variations of 5-10 millivolts when I expected the fan to be running. If readings on the multimeter can be expected, perhaps something is causing a lot of resistance on the signal line? How much resistance would be too much on a 5V line? The plug looked entirely free of corrosion. I'll try again if I can find an actual oscilloscope.
One last mystery is a small green wire which is not referenced in the manual, and which seems to be a mystery even to the internet:
As far as I can tell by the "250 / 0.47" markings, it's a capacitor. But there are three leads - the yellow connects to a tab on the electric motor itself, there's a metal tab behind the black casing that connects to a tab on the fan control module, and the white lead connects to the wiring harness and goes to ground. Is it just for electrical noise suppression? Is it necessary for the signal? It's very corroded, the new fan did not include a replacement.
I'm at a total loss here. As luck would have it, I've got another new fan I can try out, and that's about all I can think to do. The last step in the electrical diagnostics is replacing the PCM which I'd rather avoid!
Additional info: I found out that the green wire is listed in Mazda parts as a "harness" or "condenser", part #LF50-66-991A and is an astonishing $30-$40 to replace.