Thread: Electric fan control

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I bought a "185 degree on" electric fan sensor, but, according to the temp gauge, the fan cut on at about 165 and stays on prettymuch all the time once it cuts on. This is the second fan sensor I have bought because the other did the same. I just thought it was a bad sensor.

The temp sensor and the fan sensor are mounted about 6" apart on the intake. Which one should I trust and which one should be mounted near the thermostat?

I don't have anything scientific to back this up, but I've always felt the spot right at the thermostat housing was the most accurate reading of an engines temperature. My logic is that this is right before the water leaves the engine and should be at it's hottest. I know on my 350 chevy there was a 20-30 degree temp difference when I moved the sender from the back of the block to the front by the water neck.

Don

I use the Spal controllers, I find them to be the best.

Originally Posted by roadster32

I use the Spal controllers, I find them to be the best.

Dan just bought one for his rpu.........very impressive unit, heavy duty. Not cheap, but it seems worth it.

Don

I like spal or derail variable speed, i put the sensor in the radiator tank so you have the option of letting the fan run after shutting the engine off.

if the sensor is for the cooling system fan---why not put it in the radiator near the rad exit back to the engine---after all--isn't that where you want it to be cooler????The thermostat should control the temp of the coolant leaving the engine so you should use the fan to control the temp of the coolant leaving the radiator.

I did not have a port in the radiator so that was not an easy option for me. I'm using a SPAL controller which gives the flexibility to program your "fan on" point, and the "fan high speed" set point independently. I have three ports, one on each side of the intake on the front water jacket runner, and a third on my water neck downstream of the thermostat. My ECU temp sensor was pre-installed in one of the intake runner ports, and I used the opposite port for my SPAL fan controller sensor. My water neck port holds my dash indicator sensor. My thought is that I want to see the blended temp coming out of the thermostat, while the ports right at the head may be slightly hotter (not much, but maybe a few degrees). Running a 195 T-stat I have mine programmed for fan on at 200F and fan to high speed at 215F. SPAL recomends a 15F to 20F differential, so I used 15F.

On yours, you say you bought a 185F Fan On temp switch, and you're concerned that the fan runs all the time. You did not say what T-stat you're running, but if it is 185F or above (which is likely) then it makes sense your fan runs continuously - the T-stat is controlling to a temp at or above your temp switch setting. I like the idea of a controller vs hard switches that have approximate "on" set points and built in dead band for "off" (like maybe 185F on, but must drop to 175F to turn off).

The first pic below is the drivers side intake port, with the ECU sensor; and the second is the sensor in the water neck feeding the dash indicator.

In your second pic, I assume that that is teflon tape on the threads????

Originally Posted by jerry clayton

In your second pic, I assume that that is teflon tape on the threads????

Yep, it is teflon tape, but only on the top 1/3 of the threads to ensure that the bottom threads are clean and conducting to ground. I know this because the gauge initially did not work until I re-installed the sensor as described.

OK just checking--

Now--I want to point out that the thermostat controls the minimun temp in the engine

and---the fan controls the temp of the coolant leaving the radiator going back to the engine----so---put the sensor in the lower part of the radiator or possibly lower part of the engine block----

In my mind for a street vehicle if everything is sized right the thermostat will modulate to control to a fixed temperature - with a 195F stat you will run 195F once up to temperature, and it will not vary from 195F unless you lose air flow, your radiator is too small or your water pump drops flow. I like the idea of putting the fan control sensor at the exit of the engine or inlet to the radiator, telling you that the fluid is going to need additional cooling (feed forward control). If you put it on the cooled side (feed back control) then your temp is going swing before you take any action, and if the fan is sized right you will over correct, cooling more than ideal. With feed back control and any component undersized (or low speed operation, low air flow) your operating temp will tend to swing up and down on some form of sine wave. Feed forward will be more stable for the street - just my $0.02.

Look at it this way Roger---we're talking about the fan control--we're varying the amount of air flow thru the radiator --its the only variable that we have any control over--the thermostat will have the coolant coming out of the engine at 195 no matter which of the 3 holes you put your sensors in or even if you don't use any. You need to run the fan only if there isn't enough airflow thru the radiator to cool the liquid down enough to keep the engine under control of the thermostat--the key element here is the temp of the fluid leaving the radiator---so put the sensor at the outlet of the radiator and thru trial and error determine what the temp needs to be---set the controls so the fan comes on as the temp gets up to that temp and shuts off as it falls below that temp

If you care to really get a feel for what is best for power / economy, check the temps in and out of the raiator to find out how much the radiator will drop the temp and then adjust the sensors/thermostats, etc around that data.

I agree, but if you measure your temp at the exit of the radiator and it is too hot then that water is now going to the engine to pick up heat, so when it gets back to the inlet it's now even hotter. By the time your sensor sees cooler temps it will back off on cooling, and start the roller coaster again. If you measure your temp at the inlet to the radiator you start your fan quicker, add cooling to that hot water and minimize the added heat above set point. Feed forward control eliminates most of the trial and error of a feed back control system. Now if you want to really get fancy use both sensors, one on the inlet to the radiator and one at the outlet in a feed forward control with feed back trim. Anticipate the cooling needed and start your fan, then measure how it's doing on the outlet and adjust the speed up or down based on your feed back sensor. Only really effective with a variable speed fan though.... Throw in a little integral action on the feed back loop and you'll run cool as a cucumber.

I've never run an automatic controller, personally. I just have a toggle switch to turn the SPAL fan on when I see the gauge start to get over 180 at a light or something like entering a show. Since I constantly watch all my gauges I feel I have better control over it by doing it manually. Might be old fashioned, but it just works for me.

Don

When I installed one in my 66 Lincoln, I was able to buy one that was adjustable for the temp that I wanted. Remember that they run with the ground wire. So the ground wire is what controls it, not the other wire.