I don't have room for a mechanical fan, as that would be my first choice.

I like the simplicity of a relay, and the fact that they tend to be more economical.

Also, this fan may run more due to it being in a slow moving vehicle.. Cruise speed is 35-45 MPH on level ground or downhill.

I know next to nothing about PWM controllers, other than the have a soft start. Are they reliable enough to justify in a slow moving lumbering truck?

What are your thoughts, what do I need to ask myself to make a good decision here?

I use 2 lighting relays on my Toyota truck.

Simple and cheap.

They're in parallel for redundancy (just in case)

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Thanks, that confirms for me what I was leaning towards.

Older electric fans with the big bulky motors use brushed DC motors. Newer slim motor'd fans use BLDC (BrushLess DC) motors. BLCD motors require a controller to operate properly. Many incorporate the controller within the motor, so all you have to connect is power and ground. BLDC is way more efficient than the older brushed motors.

That said, brushed motors respond well to PWM control. BLDC usually does, but not always. If you are looking for "on when hot, off when not", then the relay approach is the simplest solution. The only advantage to PWM is the ability to vary speed. PWM would offer slow speed when only slightly over-warm, higher speed when a bit more over-warm, ... full speed when outright hot. This requires some sort of temperature sensing (Analog to Digital Converter, or ADC) for a processor-based controller, or something like a LM555 timer that varies PWM based on a voltage input from an analog temp sensor.

With all that said, are your goals any more sophisticated than simply "turn the dang fan on before I pop a head gasket"? If not, use the relay and a thermo-switch to activate it. If you want the superior control of PWM, let me know. I may be able to help.

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Recognized by US Federal Courts as a Fuel Economy Expert!