I'm not sure I get what you are saying. If, as I've assumed, the fans are indeed capable of removing the heat from the water faster than the engine can add it then turning them on will cool the water at the same rate (in equal conditions) no matter where the sensor is. IOW running the fans for 5 minutes will always remve the same amount of heat given equal ambient conditions and equal startup temperature.

While my method indeed results in a lag in startup time compared to direct thermostat sensing placement would cause it should stay on longer on average since it will be less likely to shut off as soon as the hot water mixes with cold radiator water in situations where the radiator weater happens to already be cold without running fans (like winter morning startup conditions for example). While the water emerging from the cooling circuit via thermostat will indeed always be hot and turn on the fan if the probe is up there the problem with that is I don't necessarily want the fan to turn on every time the thermostat dumps water back to the radiator such as during cold startup when I'm still trying to bring the radiator coolant up to up to reasonable operating temp. In my view the fan should never come on until the water isn't naturally being cooled to around 20 or so degrees cooler or lower than the thermostat cycling temp in my opinion.

After all the purpose of e-fans at least in my case is only add a fan load to the engine only when necessary as opposed to always with clutch fan therby freeing up horse power for putting down to the road. Up here in the pacific NW fan on time should only be while towing low speeds during the summer if everything is working right. Natural air currents are plenty to keep things nice and cool up here except in extreme circumstances. I don't ant those fans coming on except in those extremes.

 

You'll have no flow of coolant into the radiator at cold startup due to the thermostat not being open yet thus the cooling fan will not turn on. The thermostat keeps the coolant inside the engine via the bypass hose until it reaches a certain temperture and that depends on the degree thermostat you're using.

I have a MarkVIII fan on my RT and running a 180* stat. My controller is set to come on at 195* and goes off at 190*. The only time the fan comes on is in stop and go traffic. Out on the road, there's enough air being forced thru the radiator to do the necessary cooling.

If the only thing you're trying to eliminate is the current draw created on the engine by the fan, then IMHO you'd be better served by installing a 180* stat and a manual fan switch inside the cab so you can turn it on and off when you feel it's necessary.

 

I thought it would be clear I was referring to times when there has been mixing of cooler coolant with hotter coolant. For example: the first time the thermostat cycles after startup.

For a moment after thermostat opening water in the upper tube will purely consist of water superheated by the engine at the temp determined by the thermostat as the water pump is squirting out of the engine and back into the radiator. Presumably after the thermostat closes the mixing of the water due to convection currents and truck vibration would result in a rapid migration of the water temp found in this upper tube toward the radiator waters average temp.

During periods of water exchange as initiated by the thermostat the thermal sensor, if mounted in this upper tube, would be completely submerged in this hotter water which would by (my) design likely be somewhere above the thermal sensors trip point thus causing the fans to come on. As this water quickly gets cooled after the thermostat gets shut and the net temp is lowered to near ambient by completely cold readiator water the fan would shut off again.

I could see a particularly fast acting thermal sensor exhibiting this nonideal behavior very well.

If your goal is to quickly bypass inefficient warmup coolant operating temperatures as quickly as pssible (BTW this is one of my goals of implementing an efan) to a more optimal operating temp then clearly it would be best to not turn on the fans until this lower threshold of optimal operating temp range is entered. For this reason thermal sensor tracking of the thermostat would not result in the quickest warmup period and efficiency would suffer. If this effect were prnounced enough there is a risk the engine never gets up to ptimal temp. Not quite as bad as overheating obviously but still nonideal and I believe I have suggested a simple way to avoid it.

Don't ask me what an optimal delta operating temp would be. Unfortunately I only know half the answer...the stock thermostat is a 196 degree F. I'll probably pick around 180F for a thermal sensor trip point but would sure like to peruse some old design notes about this stuff. The good news is our engines are obviously fairly tolerant to deviation from optimal as evidenced by all the guys running 180's or even 160degree thermostats.

Not a bad idea but I have 2 problems with this: 1. I prefer running the 196 egree thermostat as my mechanic tells me that is where the engine was engineered to operate at peak efficiency. 2. I would ruin my battery in a matter of days no doubt forgetting to shut her down, or wrse yet ruin a motor since I forgot to turn it on and sieze up overheating it. Doh!

 

I understand what you're saying and also what you're trying to achive, but IMHO, I think you're really overthinking the situation. The bottom line is your results are going to be very mimimual at best but I can appreicate your style of 'thinking outside the box'. Good luck with the project.

 

Thanks man. I appreciate your thoughts on the matter. I think if it turns out to be not such a great idea hopefully I'll be aware enough to pick up on that fact in a hurry. I've got an IR probe for my multimeter and I certainly plan to be paying especially close attention until the system gets time to prove itself in the real world. Up here where I live we usually don't even need fans for most driving, especially during winter time. In these conditions overheatting usually occurs at a slow enough rate to be dealt with as long as the operator is aware it is happenning which I hopefully would be watching closely after a major functional alteration like this. I'm thinking at idle in my driveway I should be able to maintain sufficient control of the situation even if my efan setup can't. haha