Think about the last time one of your light bulbs burned out in your home, it was probably immediately after you turned on the light, right? The bulb was fine the last time it was on, but when you flipped the switch, a moment of bright light, a slight pop, and the bulb finally crossed over to the dark side. The constant on and off switching creates additional heat to your wiring and to the filament, and finally one day it was all it could take.

Like in your home, when a switch is flipped on in your car, current from your battery will travel through the component using as much as it needs. If you remember anything about Ohm’s Law, the one constant on any car’s electrical system is the voltage (if being charged properly) which means the current and resistance will vary depending on the component.

The more power a component requires, the more current it will require, and that current is delivered in amperage. We’ve all heard the saying, “it’s not the voltage that gets you – it’s the amps,” and that amperage is what can take its toll on your electrical system. Derale Performance has a product that can help take that initial shock out of your cooling fan’s electrical load with its PWM Electric Fan Controller.

The PWM works on what Derale calls slow start technology, in which the current that goes to the fan motors is not a blast of current, rather it arrives in a controlled manner from the PWM. And because the current is lower, so is the heat, and the wires stay cooler. We talked with Mike Murphy at Derale, and this is what he had to say, “Pulse width modulation eliminates amperage spikes in an electrical system. High amperage causes heat, and when fans are controlled by relays those amperage spikes cause high heat in a short amount of time.”

Top: All connections are labeled, including an override connection for A/C.
Bottom: Depending on the type of radiator - stock or aftermarket aluminum - you can use a probe or high-temp adhesive sensor.

These on-and-off spikes create hot and cool temperature effects on your wires, including the windings of your fan, and it can lead to premature breakdown. “It also effects the contacts in your relays, which will overheat and build up a tarnish layer on their surface that will eventually not allow power to transfer through,” he continued.

Pulse width modulation is controlled by the PWM controller, and the fans begin to spin at a much lower rate of speed when they are first turned on. As the temperature rises, the fans increase their speed based on the adjustments, and as the temperature cools down, the fans begin to slow down again. With relays, it’s typical that your fans turn on at one temperature, and off at another, lower temperature. Your temperatures are therefore varying up to 15 degrees and your fans are constantly being switch on at full power, and then off again.

Both sensors need to be mounted close to the lower radiator hose (outlet). When doing the wiring, be sure to use a quality crimper to make sure all connections are secure.

The PWM helps keep the temperatures more consistent. “A big misconception of pulse width modulation is that it varies the voltage to control what is being powered,” Murphy said. “This is untrue, whatever voltage you feed into the control will be on the output side, it’s just turning on and off at a very high rate of speed. In our case, we sense the temperature of the coolant feeding your engine, and with our internal algorithm we know what percentage the fan should be on to maintain that desired temperature.”

If it sounds like a lot of smoke and mirrors, it kind of is. The control uses MOSFETs (metal–oxide–semiconductor field-effect transistor) to manage the power to the fans. A MOSFET is a type of transistor used for amplifying or switching electronic signals. The advantage of a MOSFET is that it can take very little current to turn them on, but can deliver a much higher current to a load. While the temperature sensor is delivering a small amount of current to the PWM, it, in turn, can deliver the full 12 volts to the fans and allow them to eventually spin at 100 percent.

Measuring the voltage will give you an idea of where to set the controller. The best bet is to get to operating temperature and adjust the PWM so the fans will come on.

Inside the PWM is a variable resistor that allows you to set the sensing temperature for when the fans begin to turn on. As the fans start to spin, they begin pulling air through the radiator and cooling it. As temperature increases, the signal to the PWM increases the amperage to the fans and the speed increases until the temperature begins to drop. As the temperature drops, so does the current.

Power Off, Fans Still Spin

During our testing of the PWM, we noticed that the fans stayed on when we cut the ignition off. Initially, you might think that’s a sure sign that you’re going to drain your battery because, after all, cooling fans have a high current draw. One might think that adding a relay to the PWM to switch it on and off with the ignition might be necessary. But Murphy is quick to alert us that this is not going to be a good idea.

He explained, “Say you’re on a cruise on a hot day, and you stop to get a cold drink. You stop the car and when the PWM is detecting that the fans should be at 80-100 percent, you switch power off to the control. While you’re in the store, the temperature will actually rise a little due to heat soak from the engine.”

“When you come back out to the car you switch on the ignition and provide power to the PWM and immediately it senses that the fans need to be at near 100 percent, and a huge amperage spike hits the PWM. The PWM is very durable managing big power, but not amperage spikes, so when this spike hits the MOSFET it can cause it to overheat and burn out.” Therefor, Murphy highly recommends against adding a relay, noting that the fans typically don’t stay on long enough to actually drain down the battery.

Additionally, Murphy told us there have been times where a customer says they got out of their car and the fans were barely turning, and that was a concern. He simply asked the customer, “Was your vehicle overheating?” Of course, it wasn’t so what seemed like it might have been a problem really wasn’t in the first place: the fans were spinning slower because the temperature was not high enough to warrant 100 percent fan speed.

Adjusting the controller is done via a variable resistor, as mentioned earlier. The adjustments are based on the voltage reading that coincides with a temperature rating, from the chart and it can be adjusted accordingly as soon as the coolant temperature rises. The included chart is merely a guide to get you set up, but as systems vary you’ll find that you might have to make adjustments until you’re satisfied with the setting.

There are currently two types of sensors: the first one is a probe that will work better with a conventional factory radiator, and an adhesive sensor that works well with aftermarket aluminum radiators. The wiring is simple and straightforward, with the sensor connected to the the PWM and the PWM is connected to the battery (hot and ground) and to the cooling fan(s). For remote batteries, Murphy stated that a connection to the main battery wire and a solid, engine ground will be sufficient.

We mounted our PWM close to the radiator, but away from direct sources of heat. It's a compact unit that packs a big punch.

You may have to make some adjustments, however, once you’ve made them you should be able to secure the lid and basically forget all about it. If there’s anything that we can add to this it’s that we don’t often have our dual cooling fans spin at a full 100 percent, which we have noticed puts less strain on our alternator. The voltage gauge used to drop to 13 volts when both fans kicked on, and now it stays above 14 volts most of the time because the fans begin keeping the temperature down without the full draw.

You can find the PWM and many other cooling components such as fluid coolers and cooling pans on the Derale Performance website. And, as always, Murphy suggests that customers contact Derale for any of their cooling needs so that the experienced staff can be sure to pair you up with the proper components. As Murphy said, “We live cooling, so let us help you make the right decisions.”