Cooling fan running slow all the time what to do to fix?

There's a lot more to this problem than meets the eye. The Maxima's cooling fan is electronically controlled and should be shutting off at times. Those times are when the car is running at highway speed or when moving along at any cruising speed. At cruising speed, air is flowing through the radiator naturally and your car doesn't need the fan to keep the coolant at or below the proper temperature. When the engine temperature rises, as when you are stuck in traffic, the fan comes on to keep things cool. Sometimes, if the engine compartment is very hot, say on a day with temperatures in the 90s, the thermostatic sensor can trip and turn on the fan, as well.

Yes, you can measure the voltage across the fan terminals and look to see if if is within range and you can also see -- if the voltages are listed -- whether there are any voltage signals that might be missing as in the higher voltages that might be needed when the fan shifts to high. The problem with this is that unless you know where and when the fan is supposed to upshift, and whether the upshift points are temperature-triggered via the thermostatic relay and sensor within the engine or cooling system or whether an underhood temperature sensor is supposed to assist, then there's really no way of knowing what the voltages should be for the higher speeds.

Suffice it to say, since the fan is running at low speed, you will find at least 14.8 Vdc across the terminals. If the fan were off you would find it open or 0 Vdc.

Here's the scoop on this, it is apparent that one of more temperature-related sensors or a relay associated with the sensors has failed and is stuck open so that the fan will neither upshift nor downshift nor turn off. It is possible the air temperature sensor is gone or the coolant temperature sensor and/or the thermostat are in bad shape. If you are planning to troubleshoot this, it's best to start right there and, with the help of the shop manual, trace the voltage chain so you can see where the voltage either drops out or rises. By the way, there are at least three sensors involved with engine temperature readings (engine temperature, exhaust temperature -- manifold -- and coolant).

Disconnect the fan and apply 12volts to it and see if runs on high or measure the voltage on it with a meter. If it has low voltage you need to replace the fan module