Wave Form Generator for large PWM fans.

Thread Starter


Joined Jul 24, 2016
I have an issue trying operate larger PWM fans using my Wave Form Generator. I have two different models of radiator fans from Mercedes Benz vehicles. The smaller one is 600W fan which I am successfully able to operate using my cheep $65. two channel wave form generator. The larger one is 850W and I am unable to get that one to run. Both fans use the same logic.

0% on/off ratio or duty cycle is fail safe mode where the fan operates at 100% speed
10% on/off is fan stopped normally
20% to 90% on/off ratio is the normal operating range

The fans are both 4 wire units meaning 2 large 8 or 6 ga wires are directly connected to the battery at all times. The 3rd 18ga wire is connected to the ignition key to turn the fan on. The forth 18ga wire is the PWM signal wire.

From what I have read both require a 3.5V 10Hz. PWM signal.

For the smaller fan I was able to successfully run it by setting the function generator to 5V at 10Hz square wave. But it took 4V to get 3.7V while it was running and being measured with the oscilloscope. The larger fan I was not able to run even when setting the output to the 15V max it would drag down to 1.2V when I turned on the fan and consequently the fan would run in safe mode to max speed.

The two screen shots show the output with the wave form generator set to 4V at 10Hz. and 20% duty cycle.

Picture 1 shows the fan running with voltage output reading 3.7V also note the floor of the wave moves above zero compered to the second picture.

Picture 2 shows the fan disconnected and only reading the wave form generator output at 4V with the floor just above zero where it was set.

My theory is that the cheep wave form generator just cannot put out the current necessary to drive the PWM signal and while it was marginal for the smaller fan I was able to overcome it by turning up the voltage a little. But for the big fan I couldent get the voltage high enough while it was running.

So that being the case I thought I would look up the specs and find a waveform generator with more power capability. Except I couldn't find any power specifications for the one I have or any of the other ones available.

So my question is, does my theory sound reasonable and if so How do I specify a better wave form generator? And don't forget the larger fan may just be broken?

20% V4.0 Fan On.jpg

20% V4.0 Fan Disconnected.jpg

Thread Starter


Joined Jul 24, 2016
OK I sort of answered my own question at least about how to get the fan to work. It turn's out that smaller fan uses a blue wire with a green tracer for the PWM signal and a black wire with a white tracer for the power to the module. The large fan had only a solid blue wire and a solid white wire. So I ASSUMED that the blue wire was the PWM signal and the white wire was power to the module. But after procuring some wiring diagrams I confirmed that the larger fan used blue for power and white for the PWM signal. And once I switched those all worked well. See the video.

I still have the question of how do I know how much power these wave form generators are capable of. While it works in this case there has to be some kind of current limit before it can no longer create the required signal. The industrial units I used at work typically weighed a couple of pounds and were manufacured by Hewlet Packard or Agilent. The one I'm using in the video weighs 1lb 3oz.