drevenak, if you are getting 15 VDC and 0 Hz, then you have a solid DC signal coming out of the computer to tell it to drive the fans. I believe this corresponds to 100% drive. Because this is above a 95% drive threshold, this should cause the fans to run non-stop. Granted, I would be asking why the ECU is telling the fans to run non-stop during your current condition. It may be possible that this is the 0% drive signal (ie, the more the signal is grounded, the more the controller will be able to drive the fan, therefore the lower the voltage that will be seen). But, again, the 0% drive should also cause the fans to run non-stop (below the 3% drive threshold indicating failed circuitry to power the fans).

When measuring the PWM signal, you may find it easier to use an analog meter as it will tend to average out the signal, giving you more of a steady signal that is representative of the percent drive (ie, at 25% drive, you should get about 3 VDC, at 50% drive get about 6.5 VDC, at 100% drive 13 VDC). You can do this with some digital meters, but you need to make sure that they are set up to average the signal. May want to try putting the meter on VAC. This should also give you an indication of the changing voltage. But, if you are getting 0VAC, then you have no pulses occurring.

It would be better if you can put an O-Scope on it, but you should be able to get an idea of what your computer is telling your fan controllers to do. Now, on the subject of the fan controllers, it is possible that you have 2 bad controllers. These are a common failure item. So, unless you can prove them good in another vehicle, I would question their operability.