Electric Fan Control

[PJ Moran]

I'm going to jump in with both feet and agree wholeheartedly with JohnPro on the placement of the fan temp sensor.

I'm sorry, you haven't thought this through....

The purpose of the fans is NOT to keep coolant within the RADIATOR at or below the desired temperature setting.... their purpose IS to keep coolant entering the ENGINE at or below the desired temperature setting.

Placing the fan temp sensor switch either within the lower (radiator outlet) hose or within the radiators tank at a location just prior to coolant entering the outlet hose is the 100% perfect location.

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

 

[buening]

I can see both sides of the argument. The temperature gradient will vary depending on the motor size and cooling system. I'd be curious to see how much different inlet and outlet coolant temps are.

Let say that the coolant at the thermostat is 195° but your radiator is huge and you have very high flow e-fans. E-fan sensor is set up to kick the fans on at 195° and is placed in the thermostat. With the fans running and the car in traffic, the temperature of the coolant exiting the radiator could be 170° or so (for example). Inlet temps are lower than recommended operating temps, but as the fluid passes through the motor it is heated up until it reaches the end of the system where it is again 195°. Average engine temp would be 178° approximately in a perfect world, but doesn't take into account fans running to the point of their turn-off temperature.

The reverse can happen if the fan sensor is in the lower hose. Coolant exiting the radiator is 195° (based on fan kick-on setting) and as it passes through the engine it is heated up. Assuming the same temperature gradient, you'd be up to 210° at the thermostat. Average engine temp would be 203°.

In my opinion, one needs to adjust when their fans kick on depending on the location of the sensor. Any flaws in my rationale? If the temp difference of outlet and inlet coolant is small, then all of this location of the sensor talk doesn't matter.

 

[PJ Moran]

I pulled my 'stat (a Mr.Gasket high-performance model). I tested it in a pot of boiling water and could see nothing happen. I got a new Stant 'stat and performed the same test and saw it promptly open wide.

I installed the new 'stat and tried it out. The engine warms up, fans come on, engine cools, fans go off. Lather, rinse, repeat.

On a road test, the gage stays at 180 except when at a light. While cruising, it sat on 180 the whole time. It would rise a coupla degrees when I got on it really hard, but...

So, I guess my old (new) 'stat was on it's way out. Everything looks fine, now.

 

[AzPete]

Good deal.

All that talk for a r&r of a t-stat. Amazing what one little part can bring out for a discussion around here. All is good.

 

[apollard]

I can see both sides of the argument. The temperature gradient will vary depending on the motor size and cooling system. I'd be curious to see how much different inlet and outlet coolant temps are.

Heh. I've actually done this one. I put efans on my Dodge tow vehicle. Obviously, towing 5000# (11,500 GVW) through the mountains, I was very concerned about the ability to cool the beast.

Before I put them on, I had a temp differential (engine outlet vs inlet) of 40 degrees (ambient was around 80F). This was with a 24"!!!!! engine driven fan (clutched, of course). When that clutch kicked in, it sounded like a freakin' jet taking off

I installed the efans; the fans are controlled based on radiator exit temp. I adjusted to have them maintain about the same temp differential at idle as the stock fan - so they will only run when the temp of the radiator exit rises above 160 or so. This gave me an average maintained temp differential of ~45F.

After efans, I got about 65 degree differential right when the fans cut off- so, better cooling at idle. In other words, they cut on at 160 engine inlet temp, and off at 135. Having no way to test under load, that idle test gave me the confidence to try it out. Slow speed towing was OK, high speed was OK, so off to the mountains I went - with the old fan in the bed, just in case.

While towing (outside of the mountains) I run about 195 on the engine, no matter what - and the fans never cut on. Sitting in traffic, it hits ~205, then the fans cut in, and it cools down. Pulling long hills in the mountains, the fan stays constantly, then will cut off going down the next hill. Engine temp rises to 215 pulling hills. Pretty much what it did with the stock fan & clutch. No issues so far.

When not towing, they only cut on when idling for long periods.

Based on the fact that the fans stay off when towing on "flat" land, I'd say it's running somewhere around that 50-60F differential at speed.

 

[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!