M4500DT math problem for water temp sensor

[kdpearson]

I am rebuilding an M4500DT and I left the water temp sensor in the head when I sent it to the machine shop. Well, the head came back without one... No problem, I think, I'll just get a replacement. Well, I just found out a replacement sensor is $70 plus shipping!

What I'm hoping is to find an aftermarket replacement and I am looking for some clued-up electrical people on this forum who may be able to tell me how to work out the sensor specs, I have the gauge (can measure gauge resistance) and know and know the cooling system has a pressure cap rated at 13 psi so the coolant temp should reach max 120 deg C 245 deg F at that pressure, so that might be the red line on the gauge? Does anyone know the rest of the formula and/or have any suggestions?

 

[Roadworthy]

A tractor manual should give the operating temp of the tractor as well as the point at which it is over temp. My Mitsubishi hits the limit aat 111 C or about 230 F I think. In any case if the tractor gets up to the point at which your pressure cap begins to release it is WAY too hot!! I think you're asking about calibrating your temperature gauge. If you get the resistance of the sensor in a pot of boiling water you'll know what it's reading at about 100 C or 212 F. You can also determine where it's at in ambient air if you can measure the temperature. That gives two points to work from. I don't know if the sensor will be linear, I'm sure the gauge isn't. You can start by assuming the sensor is relatively linear and go from there. You may wish to check to see what voltage is supplied to your meter. Many instruments do not get twelve volts. You certainly don't want to burn out your meter.

 

[kdpearson]

Thanks Roadworthy,
What I am looking for is an aftermarket replacement temperature sending unit. I'm hoping is that someone versed in these sorts of things can point me toward an aftermarket unit that is equivalent to the original.

 

[Dave_eng]

You can expect Kubota to follow the same temp and resistance outputs from the sensor on most if not all of their tractors.

I looked at two completely different machines and both were the same.

Dave

 

[Roadworthy]

You could try your local NAPA dealer. They sell a lot of parts for farm equipment. It is likely a standard type sensor, not something Kubota had custom made for certain models.

 

[kdpearson]

Thanks guys. I will try Napa now that I have an idea of the resistance values needed.

 

[200mph]

Out of curiosity, checked the values from the L4740-3 and they are different. Keep in mind though the Grand L's use an electronic panel and not a conventional gauge with electro/mechanical movement.

 

[kdpearson]

Out of curiosity, checked the values from the L4740-3 and they are different. Keep in mind though the Grand L's use an electronic panel and not a conventional gauge with electro/mechanical movement.

Wow thats considerably different.

I found out that "american standard" universal sending units are 33 to 240 ohms and could not find any listed as low as the Kubota unit. I reached out to my machine shop and they didn't have anything left on the bench so I broke out the Visa and ordered one from Kubota. I don't like giving up but sometimes it makes sense...

Thanks everyone for the help.

BTW, NAPA is no help these days. If its not on the computer it appears tehy can't find it. The guy I called said the only thing he could sell me for an M4500DT was a fan belt... He had it in stock though! Just sad.

OK, done ranting

Ken

 

[GeoHorn]

Even if a generic sending unit could have been found.... one would also face the possibility the threads would not match. It could be NPT, BPT, or...???

There’s always merit in purchasing the genuine OEM part.

 

[dochsml]

Even if a generic sending unit could have been found.... one would also face the possibility the threads would not match. It could be NPT, BPT, or...???

There’s always merit in purchasing the genuine OEM part.

This is a good point. I also don't see anyone mentioning the type of temperature sensor (ie: PT100, J, K, T type). This would be something that would need to be known before trying to find a suitable replacement.

 

[JohnDB]

I also don't see anyone mentioning the type of temperature sensor (ie: PT100, J, K, T type). This would be something that would need to be known before trying to find a suitable replacement.

Having the same tractor, I'm curious about the reference to the temp sensor type - dochsml why does the type matter, as long as the sensor creates the same or similar resistance at each temperature point?

 

[dochsml]

Having the same tractor, I'm curious about the reference to the temp sensor type - dochsml why does the type matter, as long as the sensor creates the same or similar resistance at each temperature point?

Someone on here posted a chart showing resistances corresponding to temperatures. Assuming this chart is correct, the sensor is going to be some sort of NTC (thermistor) as opposed to the other types that I mentioned. The types I mentioned work like a PTC. This means that the resistance goes up as temperature goes up. This sensor being an NTC, the resistance goes down as the temperature goes up. The first problem is that this resistance doesn't necessarily change linearly. Most likely it follows a curve. There are additional coefficients needed to know what exactly this curve is. Also, once again assuming this chart is correct, it conveniently doesn't tell us what the nominal resistance at 25C is (usually the standard reference point). This tells me that the chart is only for checking the correct part and not for finding a suitable replacement part. Finally, we don't know what the tolerance is. Thermistors can have a tolerance as low as 1% up to 20%.

 

[JohnDB]

Someone on here posted a chart showing resistances corresponding to temperatures. Assuming this chart is correct, the sensor is going to be some sort of NTC (thermistor) as opposed to the other types that I mentioned. The types I mentioned work like a PTC. This means that the resistance goes up as temperature goes up. This sensor being an NTC, the resistance goes down as the temperature goes up. The first problem is that this resistance doesn't necessarily change linearly. Most likely it follows a curve. There are additional coefficients needed to know what exactly this curve is. Also, once again assuming this chart is correct, it conveniently doesn't tell us what the nominal resistance at 25C is (usually the standard reference point). This tells me that the chart is only for checking the correct part and not for finding a suitable replacement part. Finally, we don't know what the tolerance is. Thermistors can have a tolerance as low as 1% up to 20%.

Thanks for answering, I had no idea about the different types of sensor, I appreciate you taking the time to explain it.

 

[JerryMT]

Someone on here posted a chart showing resistances corresponding to temperatures. Assuming this chart is correct, the sensor is going to be some sort of NTC (thermistor) as opposed to the other types that I mentioned. The types I mentioned work like a PTC. This means that the resistance goes up as temperature goes up. This sensor being an NTC, the resistance goes down as the temperature goes up. The first problem is that this resistance doesn't necessarily change linearly. Most likely it follows a curve. There are additional coefficients needed to know what exactly this curve is. Also, once again assuming this chart is correct, it conveniently doesn't tell us what the nominal resistance at 25C is (usually the standard reference point). This tells me that the chart is only for checking the correct part and not for finding a suitable replacement part. Finally, we don't know what the tolerance is. Thermistors can have a tolerance as low as 1% up to 20%.

You can plot the resistance vs temperature rom the chart( 3 points) fair a curve through those three points and then interpolate to get an idea of what the resistance is at 25°C.
The curve is inverse in resistance vs Temp because these electro-mechanical gauges use a heating wire to heat a bi-metal to swing the needle. The max current occurs at min resistance which occurs at high coolant temperature. There is also generally a voltage limiter on the circuit to limit the voltage to 5V from the normal system voltage of 12.6 V.

 

[GeoHorn]

Heres a video on how to troubleshoot/repair gauges: https://www.youtube.com/watch?v=puNKAW5Nurc

NAPA also carries Echlin brand electronic gauges and sending units. Their catalog gives the exact voltage-resistance-values of various gauges and sending units. Ask to look at their catalog when at their store.