Pulse Width Modulation (PWM) Fan Control VS Relay Control

Pulse Width Modulation (PWM)
VS Relay Control Radiator Fan Control

Well we decided to explain it all in layman's terms so you can make a good choice on what type of fan control you need.
I'm sure you've heard a lot about electronic or solid state versus electro mechanical relay fan control systems. 
I think most of us know how a relay works. Current through a coil pulls a contact closed or open to operate a device. Relays are simple a technology that has been in use for decades.

The problem with mechanical relay control is that they draw huge amounts of current when the contacts close when starting DC fan motors from a dead stop! This star up causes contact arcing and this arcing can result in welded or blown out relay contacts and possibly radiator fan failure over time.  Some relay failures results in the contacts welded together and then the fan or fans go to full 100 percent RPM. This could cause early fan failure and battery drain.

Let’s start with some of the terms in pulse width modulation.

It’s a method to save energy and have positive radiator temperature control by switching the fan motor ON and OFF at a high rate.  With transistors switching full ON & full OFF, this generates much less heat. Plus there is no arcing of contacts like found in mechanical relays. Most modern solid state fan controllers will also have Soft Start to eliminate high current spikes when starting fan motors from a dead stop.

The percentage of ON time versus OFF time is pulse width modulation. So if the cycle is 10% ON and 90% OFF, that is considered LOW fan RPM.  In the high fan RPM, the numbers would be reversed.
The PWM Controller has a special sensor attached to outflow radiator port. This temperature information automatically sets the fan RPM to control the radiator temperature. This is what you call a CLOSED LOOP. At freeway speeds, the fan or fans may be at or close to zero RPM – saving you energy. When you stop with the motor is running the fan will reach the RPM to hold the temperature set point.
Voltage spikes can happen when DC motors or magnet devices are turned off. It’s called the “collapsing field” from magnet energy stored in the motor or device. It will spike positive and then negative to zero volts. These voltage spikes can damage solid state devices, even diodes in alternators. This also happens when you stop cranking the starter on a high compression engine and on an AC clutch when it disengages.

If you have ever seen that little brown button attach to the AC clutch positive wire and the ground wire at the clutch connector. That's a negative spike suppression device (Diode).
Suppression is a way to absorb excess power so it doesn't blow back into your electrical system.  I'm sure many of you have seen when the head lights are on and the fan or AC is going then the fan stops and the headlight momentarily get brighter. That's a spike and it’s often mistaken for a surge. In good relays they have diodes that absorb that power so it decreases the spikes to a safe level.

AGSNO2, What the heck is that? Don't worry we're not getting all Heisenberg on you. AGSNO2 is simply Silver Tin Oxide. It is used on quality low voltage high current DC contacts to help eliminate the contact burnout or arc erosion as it’s technically called. The Beuler relays we use have these AGSNO2 contacts as well as the negative spike suppression. This is a must for fan relays otherwise they will fail quickly.

DC motor, I know you know what they are but here is some info on how they work.  DC motors can also be DC generators and that was how they were used in the early days of automobiles, the starter was also the generator. When a DC motor shaft is spun it can produce power. When DC voltage is applied to the motor windings it can spin the shaft. When you apply voltage to the windings and it starts to spin the shaft you get a small amp spike and as soon as its starts to spin it also start to produce some of its own power and that's when the spike drops off. This all occurs in milliseconds. Once the fan is up to speed the amp draw levels off. The spike is over before the meter displays the reading for you to see.

Wire and wire size. We all know the issues of using to light a gage wire but most of us are not aware of why they have strands and not solid copper in cars. One reason we NEVER use solid wire is that as it flexes it work hardens and can crack. It is also the reason you don't see copper brake lines or gas lines.

50 amps on a 10 gage wire you can only have a length of 5ft to maintain a 2% voltage drop to increase that load to 70 amps you would need an 8 gage wire and 100 amps a 6 gage wire.
Why is this important? With all the misinformation about Mark VIII fans spiking 100 amps or more and needing constant duty starter relays to work correctly you can see for yourself that's just not so. I have yet to see a M8 fan in good condition spike at 100 amps and I have never seen one push 4500 CFM. I have seen tests against a Spal that put it at 3200 max. To draw 100 amps you need a 6 gage wire. I have never seen a 6 gage in a Lincoln fan control or wiring and only seen it for the starter and battery cables. That alone should tell you 100 amp fan draws aren't happening.
So what's the better method for a fan control system? You decide!

Relays: The Good and the Bad!

The Good:

You can get them almost anywhere you can get car parts.
Relays are easy to understand.
Cost effective on a budget and fast replacement with sockets.
Relays can be used for any number of on off devices.
No signal back feed into the electrical system.  
Can be used independently of or in conjunction with a GM PCM to trigger fans

The Bad:

Relays eventually fail due to arcing contacts and generating heat.
Relays need to be over-rated or run in parallel for high current switching DC fans and motors.


The Good: 

Have positive radiator temperature control. This system holds a constant engine temperature by holding the radiator at a lower temperature. This lets the engine thermostat regulate engine temperate with coolant head room. No more temperature and pressure peaks with a constant fan RPM, not cycling fan on and off like found in relay control systems.
Soft start limits inrush current when the ignition is turned on. Transistors do not arc and should last for decades.  Solid state controllers always demonstrate dependability & reliable.  
A lower cost and much faster method over attempting to repair older car’s radiator control system. No need to search junk yards for OEM parts that have become obsolete.

The Bad:

PWM fan control system can have high pulse current when the fan(s) are at low RPM.  If pulse noise is getting into the electrical system, this is easy to correct or block by installing a big capacitor from the fan positive terminal to ground.  Capacitors can be found in surplus stores or electronic supplies houses around the country.

Cost is much higher but cheaper than replacing fan clutch systems, fan belts, and relays fan controllers over and over.   Install external fuses to give the first like of safety incase you have a short and not blow the internal controller fuses.  If it does fail the whole unit needs to be sent in for repair.  Internal fuses can not be replaced by the user.
Independent of a GM PCM and can't use inputs from the PCM to trigger fans. 

Which is the better system?  It really depends on what you can afford. Relays are tried and true.  However, switching inductive (fans) loads with relays needs way over rated relays that can withstand the high inrush DC switching currents over time and miles. If cost is not an object, you want quality and reliability; go with solid state Pulse Width Modulation radiator temperature control. If you’re concerned with cost go for the relays.  Both will handle the Mark VIII or Taurus fans with ease, they just do it differently and of course we do sell both PWM and Relay Fan kits.

Video's from the manufacturer of the PWM unit
AutoCool 85A

AutoCool III 125A

Vidoe on the Hollister road Company Relay kit