I have the trusty old Nelson fan harness with a couple universal fans. T used to work correctly. The PCM would turn the fans on low, and the ac clutch would turn them on high. Or just one on high if the engine was cool. But now, for some reason, the PCM will only turn one fan on high when the engine is hot, and the ac will trigger the other fan on high. There is no low setting no matter what combination. What could be the problem?