12V Electric Fan Speed Control

[daveward]

I've got another car improvement project on my plate today.

I am trying to add a device that will moderate the speed of my electric auxillary fan so it will run a bit more slowly and, more importantly, much more quietly.  I've found at just a tiny bit less rpm, the fan is still very functional but much more qiuet.  Separately, I'll have it wired to run at 100% when necessary.   The fan in question has two power wires and a ground wire, one power wire that runs through a ballast resistor, and it's this wire I used for the testing below.

To test this approach I hooked the fan up to an extra headlight switch that had a dash instrument lights dimmer/brightner.  My theory worked just fine while I had it wired from 12v to dimmer switch to fan...I could change the fan speed at will...except:

1.  When I wired it up with the fan's Bosch relay, the fan ran at 100% speed regardless of the dimmer switch setting.  (Relay Output Terminal 87----> dimmer switch-----> fan = 100% at any dimmer setting)  I don't get this.
2.  I'd like a suggestion on a better device than my dimmer switch idea for moderating the fan's speed.

Thanks again for your help while I tinker here and tinker there,

Dave

 

[MacGyverS2000]

I'm willing to bet you placed the dimmer in series with the relay coil rather than the fan input/output, and that's why you saw no change.

 

[curious125]

Can you PWM the speed of the fan relative to the temperature? Then at lower temp. the fan will be slower/quieter and when the temp is elevated the fan would run at 100%

 

[daveward]

To MacGyverS2000 and curious125, thanks for the responses.

I've got follow-ups for both of you:

MacGyver: re the wiring...

Here the dimmer modulated the fan speed:

+12v----><----dimmer switch----><----fan---->ground

Next, substituting a relay, the dimmer had no effect:

+12v via Relay Term 87----><----dimmer switch----><----fan---->ground

I have a mental block I guess...I don't see another way to wire it up.

curious125: re the temperature sensed speed control...

I like that notion quite a bit but the triggering device (at least within the oem air conditioning system is an ON (When the temp is too high) or OFF (when the temp is OK switch. I don't know how I'd modulate from this starting point. I assume the PWM suggestion may be the best clue of all, but speaking of mental blocks, what would PVM mean in this case?

Thanks again, Dave

 

[curious125]

Sorry Dave.

PWM is pulse width modulate.
By pwm you can vary the on vs. off time of the motor and giving you the ability to vary the speed directly, and quite accurately.
Some engines have a thermocouple in the block as well as the temperature switch. You can use the thermocouple as your analog input to let you know how fast you need to drive the fan.

Just an Idea.