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Thermo Fan Sensor

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Guy Marshall

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Hi guys

I have a Kenlowe on my BN6 which works fine, but a quick google shows lots of opinions whether the thermostat sensor should be in the top or bottom hose. I have the relay connected to an always on fuse, so the fan runs on after the ignition is off, which I want, but one opinion suggested to use the bottom hose in this case as it will reduce the run-on time.

The bottom hose temp should be more regular, and it will look neater (though its harder to get at). Since the thermostat is adjustable we can make it come on earlier, so that should not be an issue.

It makes a lot of sense to me, but I see both sides (the top hose is a better measure of the temp of the engine). What is your opinion/experience?
 
Hi Guy,

I had a SPAL pusher fan on my BT7 which was controlled by a derale fan controller. I placed the thermal probe at the inlet (top) to the radiator ( I'm pretty sure the installation instructions specified that placement). My radiator has an uprated core. I retained the stock 4 blade fan on the car and everything else was left stock (no additional shroud/sealing).

For a low tech airflow test, I used a paper shop towel and placed it in front of the grill when the car was idling. I found that with the SPAL fan running, the towel was held firmly to the grill, but fell to the floor with the SPAL fan off. The original 4 blade mechanical fan doesn't appear to be effective.

I found the electric fan to be very useful when stuck in traffic, moving at low speeds and backing down the long driveway to my shop. I didn't notice an increased operating temperture when driving the car at speed.

As you indicate, the ability to dial in the temperature may make the placement of the temperature probe arbitrary. However, besides being a better indicator of engine temperature, I wonder if the top hose sensor placement will see a larger swing in operating temperature than the bottom hose and this would help the temperature sensor "detect". I don't have any data to prove/disprove this. I also wanted to solve the problem of my car heating up when idling at a stop light. The top hose sensor placement is going to trigger quicker than the bottom hose sensor placement. On the other hand, it would look better if the sensor probe was placed by the bottom hose.

I wasn't that impressed with using the fan controller to control the turn on and off of the fan as a function of temperature. I had wired the fan controller so that it was controlled by two switches. The first switch enabled/disabled the SPAL fan to be controlled by the fan controller/temperature sensor (this is how yours appears to be wired...but without a switch to turn the controller off). The second switch would turn the fan on and if in the off state the fan would revert back to being on/off as a function of the temperature sensor. I wired an LED light so that I would know when the fan was on. I found my Healey to have a pretty wide operating temperature range depending on the conditions - ambient temperature, load (hills), idling at stop light, traffic etc. I didn't want the fan to run when I had the car moving at speed (~> 30mph). I found that unless I set the temperature pretty high (say 200 degrees), then the fan might run on a long uphill on a hot day. If I set the temperature high, then when I was idling at a stop light, the engine would have to heat up too high before turning the fan on. When shutting the car off, the fan would also run quite a while before turning the fan off. As a result, I found myself manually controlling the fan for the most part.

It's possible that I didn't see the advantage of the fan controller because my engine seemed to heat up pretty quickly at idle and that was the problem I was trying to solve. I'm in the middle of restoring the car and haven't rebuilt the engine yet, but I know that the the block needs cleaning out. However, at this point (I'm wiring the car) I'm considering not installing the fan controller and just using the manual switch to control the fan.

joe
 
I have my thermo-switch in the upper hose. That's where the hot water from the engine is. It's also why the thermostat is there.
 
Just to wrap up on this I have had the sensor in the bottom hose for a few weeks now and I am v happy with it. Its set to come on when the temp gauge (top of the engine) is just over 190, and it seems much more stable; running on for less time and not coming on when driving.

It makes sense to me to use the bottom temp as a switch - after all the fan cools the water in the radiator and is not cooling the engine. Well not much anyway!

Anyhow +1 for bottom hose from me...
 
I would say; sensor at the bottom. The sensor checks the effect of the fan.
The thermostat is in the upper to keep the (cold) fluid there until the engine is warm
 
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