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Let's attempt to fix the air-cooled Honda ATV excessive oil temp problem that has plagued us all for these last 30+ years, ehh?

This project will be a joint effort by @wheelsquad and myself. We are gonna attempt to build a better FCU gizmo for his Rancher 400AT. We are beginning this project with a new oil temp sensor (#37750-HC4-751) in hand that wheelsquad purchased and then graciously sent to me... so's I could get my meters busy on it.

Our ideas consist of making a gizmo that will be fully adjustable from end to end. We'd like to precisely control the cooling fan kick on temp and precisely control the cooling fan kick off temp, independently. That means we'll need to provide a wide range of hysteresis adjustability in our design. The circuit will need to be very stable as well, not be overly sensititive yet retain very fine-grained tuning abilities. The circuit basis must be expandable too, as we'd like to be able to put the over-temp idiot-light feature to better use. Our resulting gizmo design and build (if we are successful) should provide a suitable FCU replacement for all Honda ATVs that have a factory oil temp sensor in the sump.

1st step today was taking some baseline resistance measurements at various oil sensor temperatures. The oil sensor was suspended in a small pan of vegetable oil and slowly heated. Each temp point where measurements were taken were held steady for five minutes to allow sensor resistance to completely stabilize.

As you can see in the pics, we will need a circuit design that works primarily within a resistance range of under 3.5k ohms. The oil temp sensor resistance (vs temp) is not linear at all either, and it responds to temperature change lazily. We'll have to tolerate between 100-150 ohms of that laziness between rising and falling oil temp trends.

I learned that the thermo-couple and the infrared gun are running neck & neck, both display the same accuracies all of the way up to 240 degrees F. So from now on I'll just use the thermo-couple for all future temp measurements.

Pic dump...
 

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This is a simple circuit that I put together many years ago which utilizes an LM741 Op-Amp reconfigured to serve as a voltage comparator. This schematic was originally drawn with reversed LM741 pins for heating purposes. It was used with a 10k ohms thermistor, which controlled a heater and circulating fan for my homebuilt egg incubator/hatchery. In this drawing the circuit has been corrected to be used for cooling purposes.



As you've seen in the pics above in the 1st post, we will need to work with oil temp sensor resistances less than 3.5k ohms. So its likely that our basic voltage comparator circuit will change right away. I'm thinking that those 10k resistors on the left-hand side of the schematic might get swapped out for 5k resistors... we'll soon see whether I'm right or not.

Half of our problems may be solved by choosing this simple LM741 Op-Amp and transistor circuit. Our remaining needs/ideas can be added on to this... more about those ideas later.

I'll mock this up on a perf board this week an' see what can be learned. Any suggestions? Requests? Critiques? :)
 

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I am no ele wizard at all, correct me if I am wrong here !, but isn't veggy oil thinner than atv motor oil ?, wont this temp reading be off a tad once you dip that temp sensor into atv oil because its thicker ?.
 

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Yeah, veggie oil is thinner than motor oil. But temp transfer/properties are the same in all common liquids regardless of viscosity. If the sensor were suspended in heated water the resistances would measure the same as identically heated oil, until the boiling point of water is reached.
 

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Yeah, veggie oil is thinner than motor oil. But temp transfer/properties are the same in all common liquids regardless of viscosity. If the sensor were suspended in heated water the resistances would measure the same as identically heated oil, until the boiling point of water is reached.
Part of my problem while testing my sensors was measuring the lower resistance values with water above 200F. My sensors seem to read just a bit too high, so I don’t know if they would trip the fan until after the critical temp threshold has been crossed—i was it to trip before.

Retro I don’t know how the Op-Amp works and barely remember how to read a circuit.

I see a diode in there and many resistors. Do you have rheostats in there or is that handled by the OpAmp?

I often thot one could trick the fan relay into thinking the sensor circuit resistance was lower by wiring a smaller value resistor (or variable resistor) in parallel with the sensor. However a parallel circuit seems unlikely due to construction of the sensor itself.

So are you left with manipulating a series circuit?

Great job BTW i enjoy innovations like this.
 

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Yeah, veggie oil is thinner than motor oil. But temp transfer/properties are the same in all common liquids regardless of viscosity. If the sensor were suspended in heated water the resistances would measure the same as identically heated oil, until the boiling point of water is reached.
but isn't the boiling point on atv oil higher than veggie oil ?. seems to me if your seeking a certain resistance to the sensor for the correct diode..or whatever ?, you maybe should be getting readings with the sensor in atv oil ?, again..i have nooooooo clue what your doing !!..i'm just learning here is all !..lol.
 

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Part of my problem while testing my sensors was measuring the lower resistance values with water above 200F. My sensors seem to read just a bit too high, so I don’t know if they would trip the fan until after the critical temp threshold has been crossed—i was it to trip before.
Water is plenty mysterious me thinks. It boils at 212 degrees (atmospheric pressure at sea level) and it won't get any hotter than 212 degrees once it reaches that boiling point. I'm no expert in any of this stuff, but it seems like water begins to vaporize before it reaches the full boiling point, as you mentioned that at about 200 degrees its latent heat and vapor pressure properties seem to begin to shift a great deal. We observe heat energy that was being absorbed (heating the water), then begins to be consumed by the water (and then removed by the gas), in its' liquid to gas conversion process. The nearer to boiling point the water gets, the more heat energy it consumes, rather than absorbs...? Observationally thats what seems to happen but I'm sure that I don't understand water very well. So yeah, I agree with ya... for some reason ya can't trust a thermister in heated water as it nears its boiling point. If you understand why/what happens, please poke me with a sharp stick until I understand too. :)

Retro I don’t know how the Op-Amp works and barely remember how to read a circuit.

I see a diode in there and many resistors. Do you have rheostats in there or is that handled by the OpAmp?
The LM741 Op-amp is configured to act as a voltage comparator device. Its no longer functioning as an Operational Amplifier in its strictest sense, in other words. It works by comparing two voltages at its two input pins (battery voltage passing thru a resistor @pin 2, versus battery voltage passing thru the thermistor/oil temp sensor @pin 3) & outputs a voltage signal to the transistor (which energizes the fan relay) when those two input voltages differ.



For instance: a 10k ohms @25c degrees thermistor @pin 3 requires a 10k ohms resistor load @pin 2 to balance the circuit @25c degrees. Whenever resistance drops below a target threshold thru the thermistor @pin 3 as it becomes heated, the LM741 responds to those two differing voltages at its' inputs by sending a voltage thru its output pin.

So we must choose our resistor load for pin 2, to meet or exceed the resistance load pin 3 will see thru the oil temp sensor in order to keep the fan turned off until the oil reaches our target temp. We put a resistor on the LM741 output pin to insure that we maintain a fully saturated transistor while the output pin is live. The three trimpots (VR1, VR2, VR3) provide us with adjustment/calibration for the circuit.

I often thot one could trick the fan relay into thinking the sensor circuit resistance was lower by wiring a smaller value resistor (or variable resistor) in parallel with the sensor. However a parallel circuit seems unlikely due to construction of the sensor itself.
Yes, I have a 200k ohms trimpot connected in parallel with the oil temp sensor on my Rancher, in fact. It works to a limited extent. My fan kicks on @205 degrees F and kicks off @150 degrees F. I don't like it because I cannot adjust for hysteresis and its temp range is static... and very limited. Can't complain really... it only cost about 50 cents for the Bournes trimpot.

So are you left with manipulating a series circuit?

Great job BTW i enjoy innovations like this.
I got no idea how this might end yet... but I'm having fun with it too, regardless. Thanks for joining in! :)
 

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Ladies and gentlemen, be careful what you ask for when saying "hey retro would it be possible to _______?"

Fun for me too, and learning a lot also.
@shadetree, great questions. And yes the RATE of heat transfer can vary between liquids, but retro is only taking measurements after the temperature has completely stabilized. So the resistance reading at said temperature will be exactly the same regardless of the liquid it is in, provided it has stabilized.

And yes the boiling/burning point of vegetable oil is lower than motor oil, but we should be well below those temperatures for this. If we were at those temps with the sensor installed I'd be having much bigger motor problems.
 

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but isn't the boiling point on atv oil higher than veggie oil ?. seems to me if your seeking a certain resistance to the sensor for the correct diode..or whatever ?, you maybe should be getting readings with the sensor in atv oil ?, again..i have nooooooo clue what your doing !!..i'm just learning here is all !..lol.
Well...? Ya got me in a bit over my head I think! I'm not sure that anything I might be thinking about might be correct nowadays. :)

I know the veggie oil remains well below its boiling point up to 240 degrees, so I use it for testing sensors all the time. I have lots of it too (10+ years old gallon jug I think?), because I don't ever put that stuff in my food. And its cheaper than motor oil & less risky to have out in the kitchen where I gotta cook and eat. It seems to work OK as a cheap, disposable oil... :)

But getting back to science, you might be onto something that could become a factor for error if this oil temp sensor were a high quality, very sensitive device. But this bugger is lazier than a millennial, I assure ya! LOL

I watched resistance vs temp measurements vary from 100-150 ohms apart, depending on whether the temp trend was a rising one or a falling one. It almost stabilizes itself eventually, but that takes patience... about 5 minutes or so, but it still stops short of its previous resistance at that same temp.

So I think the oil type/viscosity that it gets dunked into, won't ever become a problem for us unless/until Honda redesigns their sensors to be better. Ain't gonna see that outta china... :)
 

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Ok now Shade and i are gonna crack a cold one while Retro and Wheelsquad discuss Fourier’s and Fick’s Laws of momentum transfer:D
 

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The cold ones sound interesting, Fouriers & Ficks sounds like an upscale NY dept. store to me tho? :)

I mocked up the gizmo today using 3.65k ohms resistors (can't find any 5k in my bins) on the left side, following my hunch. It works good with those 3.65k's in there, but I think it will work even better with 10k ohms resistors put back in there.

It is overly sensitive during calibration right now IMO, using those 3.65k ohms resistors in there. I'm gonna swap the 500k ohms Hysteresis trimpot out for a 1 Meg ohm trimpot too, to help relieve some of the over-sensitivity on that function of the circuit. So I'll test again tomorrow and hopefully, get started on adding a coupla' mo' ideas to it.

No pics taken today, sorry. I ran out of time. I'll get a coupla' pics of the mockup tomorrow.
 

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I'll mock this up on a perf board this week an' see what can be learned. Any suggestions? Requests? Critiques? :)
Don't forget the fail-safe feature where the fan will actually run when the sensor goes bad.... vs fan never coming on, how Honda has it.
 

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Yeah, matter of fact I was thinking about that feature today so I'm glad that you mentioned it. We have the idiot light in the display that will likely never, ever come on. So, if we can design the gizmo so that the fan starts if/when the sensor fails high out of range... AND we can start the fan if the oil sensor wire ever gets unplugged/ripped in half by a stick or something...? Should we tie that over-temp LED into that half of the device, so that the idiot light comes on whenever the sensor fails high, or it gets unplugged?

We can take advantage of that light somewhere else besides an overheat condition I feel, because the primary function of the gizmo should already eliminate any overheating by its sensitivity and precision.

Of course, the oil cooler could get plugged up with mud or the cooling fan motor could fail or blow a fuse, causing an overheat. Unless we add a thermister onto the cooler to monitor cooler temps and add a hall-effect tachometer circuit to the fan motor... :)

Think about it... lets bounce some ideas.

So far I think we are gonna have to monitor the ambient air temperature, because sub-zero air temps would false-trigger our sensor failing high feature. Complexities are imminent. :)

EDIT:
Actually we might be able to turn that over-temp LED on if the motor overheated too, so don't rule that possibility out just yet.
 

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I promised a pic of the mockup so here it is. As you can see I am using a Red LED (its lit in the pic) to indicate to me when the transistor turns on/off, rather than tripping a fan relay at this point. I'm also using a 5k ohms pot in place of the oil temp sensor to speed up testing. Since we've already learned our operating resistance range from the veggie oil tests we can quickly dial in our targets using the pot.

I didn't get to spend any time on it today cause universe made other plans. I'll try again tomorrow. I am putting together a short list of parts that we might need, probably order those up this weekend.
 

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We have the idiot light in the display that will likely never, ever come on. So, if we can design the gizmo so that the fan starts if/when the sensor fails high out of range... AND we can start the fan if the oil sensor wire ever gets unplugged/ripped in half by a stick or something...?
Yep exactly what happened to mine, sensor wire came unplugged, my oil cooler may have been restricted some, it was really hot out and I was in slow going in old pasture bogs, but since the default fail from mode from being unplugged was to not run the fan, and oil temp light also doesn't come on then, mine got REALLY hot before I realized it.
 

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"A" cold one is a teaser -------- I don't drink during the week , unless it is a special occasion , but tomorrow is Saturday and I will be on a mission to destroy a 20 pack of Coors Lite
Can’t drink all day unless you start before breakfast.
 

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@retro, I'll try and think of ideas to best utilize the idiot light. But I'm thinking I like your idea most for having it come on if there is an error in the fan or fan circuit itself. Since it should be obvious with the gizmo in place of a temp sensor or wire issue with the fan running continuously.
 
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