The brushless fan uses a pulsed ground (negative type, sinking PWM signal.) The PWM controllers of old were designed to pulse the power wires to the fan. So you can’t really use the old PWM controllers with the brushless fans. But you could use a grounding temperature switch to turn on/off the new brushless fans, but this way you would only get on/off function. You wouldn’t get the variable speed without using a PWM sensor, or a brushless fan PWM controller.
To directly override the brushless fan two conditions need to be met:
- The white/PWM wire needs to be switched/connected to ground. When the White/PWM wire is grounded this is sending the fan 0% PWM speed request, but it causes the motor to wake up.
- Once the motor is awake, and asked to run at 0% PWM, the motor will look to the yellow wire for 2-10V analog voltage.
Red/Black should be run directly to the battery positive and negative terminals, you put a fuse on the red wire. No relays are required. The brushless fan was designed to basically ‘replace’ the relay. Then you just ground the white and power the yellow and the fan should ramp up to full speed.
The easiest test you can do would be the voltage pull down test. When the sensor is ‘off’ on a cold engine, you can use a DC voltmeter to measure the voltage on the white wire while it is connected to the fan. You should see the same voltage on the white wire between the fan and sensor that you see on the red wire going into the fan, ~12.5V. When the sensor powers on, this white/PWM wire voltage will pull down to 0V.
This test makes sure that the sensor is powering up and is a strong indicator that the sensor is functional. It’s not a 100% test as the override function is a separate part of the circuit board, but if the sensor pulls down the PWM voltage on startup it usually works with coolant temperature.
The most common issue we encounter with these systems are on vehicles where the battery is remote mounted. The fans need a dedicated connection directly to the positive/negative terminals of the battery for the best reliability. We find that on vehicles where the power/ground are shared with other electrical devices the communication between the sensor/fan becomes unreliable. The fan and sensor use a digital PWM communication so the voltages (power sources, and grounds,) should be connected/shared as much as possible. On vehicles where the fan positive wire is connected to a fuse block, alternator stud, or a starter, the voltage will fluctuate as other electrical devices turn on/off on the vehicle and this can lead to intermittent function of the fan. The same is true for the grounds, if there are several electrical devices grounded at the same location, they can interfere with the voltages the fan references as ground. Ideally the red/black wires for the fan would be connected directly to the terminals of the battery, with a fuse on the positive wire. The sensor power/ground is not nearly as touchy because the current draw is much lower, but the sensor does need to reference the same voltage/ground for them to communicate properly. For testing I would recommend connecting everything directly to the battery, then see if the override function works correctly.