Electric Fan Control

[buening]

Good info.  Thanks! :5

[09Frank65]

If the cooling system is up to the task, the water temperature just past the ‘stat should be at or only slightly above the opening temp of the ‘stat.  The ‘stat keeps water in the engine and only lets it out when the minimum temperature has been reached.  If the incoming water is cool enough, the ‘stat will regulate flow and never go to wide open – it is constantly modulating.  As the water temperature increases beyond that point, the stat goes wide open, and it is clear that the cooling system is struggling to keep up.  The engine is putting more heat into the water than the radiator can take out.

With electric fans, you get to help the radiator by increasing airflow and bring the coolant temps down.  Either the radiator can handle it by itself, or it needs some help from the fans.  Of course, the fan benefit depends mostly on how fast the car is moving.  Slow = high benefit.  Fast = no benefit.

So, by measuring the temp at the ‘stat, you can tell when the radiator needs some help.  When the temp there reaches a point significantly above the ‘stat opening, the radiator is clearly not doing enough – you need the fans to come on.

My ‘stat is a 180.  My switch is a 195.  So, if the water coming out of the engine hits 195, it’s clear that the ‘stat is wide open, the radiator is not keeping up, and the fans need to come on.

When sitting still, the difference between inlet and outlet temps on the radiator will be small.  With vehicle speed, the difference will increase.  What this means is that fans switched via ‘stat temps will come on sooner than those switched by outlet temps (assuming you use the same temperature setting).  And the delay will increase with vehicle speed.

If you use the same temp switch at the outlet, then you’re letting the engine get hotter before energizing the fans.  So, you have to use a lower setting if measuring at the output to equally protect against overheating.  Now you have to estimate what the temperature drop across your radiator is.  That varies with ambient temperature and vehicle speed.  So, how are you supposed to pick a setting when there are so many variables at play?

I’m sure that some manufacturers tell you to put their probes/switches in the outlet.  Spal tells you to put theirs in the cylinder head or intake manifold.

Based on my understanding of how the cooling system works (outlined above), I still think that the ‘stat housing is the ideal location for the temperature switch.  If the temperature there is too high, the cooling system is underperforming, and the reasonable action is to turn on the fans.

Now, if the fans are already on and the temp isn’t dropping, then you have other problems (like me) such as stuck ‘stat, collapsed hose, or clogged/blocked radiator.

I could be wrong – I haven’t thought this through very well  :)

A little late to this party, but the only difference between Johnpro's argument and yours is the setpoint of the thermostat.  After the thermostat and before the radiator you are looking at 195-200 degrees to energize the fan.  After the radiator and before the water pump you are looking at 16o-170 to energize the fan.

Both are valid, my preference is to monitor the temperature of the coolant to the engine.  My gauge tells me if the fan is not doing an adequate job.  That's what it's there for!

The one place you really DON'T want it is before the thermostat in the intake.  The thermostat controls coolant to 180, the switch resets at 175.  Fan never shuts off!

Glad to hear your problem was easily solved!