Is a 1 Ohm reading normal for a cold 12 volt thermistor?

[Kriis]

TL;DR Summary

Attemting to discover current or possible working parameters of an Automotive Thermistor

I've just joined the Forum and not sure if I'm in the right section (or possibly Forum even).

The 12 volt thermistor in question is extremely common and is fitted to many makes of car and motorbike. They do fail from time to time however and so I set about determining if one is functioning correctly or not by simply setting about measuring it's resistance when subjected to boiling or cold water. The hot reading was approaching zero Ohms when I stopped which I had already assumed as many dashboard instruments I've tested before will show a full deflection when 12 volts applied. The cold reading is the bit I need some help with:- The reading was approximately 1 Ohm. Electronics is not my primary subject and I assumed this was perfectly normal but have been ridiculed for thinking this. Am I doing something wrong or is this possible? Do I need to use a different meter for this test? It's a brand new unit but could it be faulty even?
I would be very grateful for any help here.

 

[Dullard]

'Thermistor' is a specific device, but some people (incorrectly) call any temperature sensor a 'thermistor.' Thermistors are available with positive or negative temperature coefficients. I've never seen one with as little apparent range as you describe. It seems possible that what you have is something other than a thermistor. How many connections are there to the device? What is the specific application/vehicle?

 

[Dullard]

I'm guessing you have 1 of these:

https://www.samarins.com/glossary/airflow_sensor.html

 

[jrmichler]

If it really is a thermistor, the factory service manual (FSM) should have some temperature vs resistance information. The FSM for my truck shows this for the coolant temperature sensor:

So, if it really is a thermistor temperature sensor, and if it is working properly, the resistance will be in a range that a normal multimeter can easily measure. Your reading of 0 to 1 ohm means that it is either not a thermistor or it has failed.

 

[Kriis]

Thank you jrmichler, that is helpful. I have 3 other units that I will go and measure now and publish the readings. The reason I hesitate to think that the described reading is incorrect (apart from it being new) is that the resistance values measured throughout it's temperature range are repeatable and consistent.

EDIT I've just measured all 3 of the secondhand units fitted to vehicles, all cold/ambient and they measured:- 2.5 ohms, 2.76 ohms and 1.42 ohms. Considering the temperature outside compared to the indoor temp I measured the new unit at, these are close enough for me to trust my first reading. The term "thermistor" has always been used in the car trade for an oil or water temperature sender, but I always suspected that it was probably a misuse of the true electrical definition. It does seem an incredibly small resistance range but the mystery continues. As said in my original post, are there any different ways of measuring resistance? what am I missing here?

 

[Dullard]

is your meter in 'Kohms?'

 

[Kriis]

If it really is a thermistor, the factory service manual (FSM) should have some temperature vs resistance information. The FSM for my truck shows this for the coolant temperature sensor:
View attachment 264323
So, if it really is a thermistor temperature sensor, and if it is working properly, the resistance will be in a range that a normal multimeter can easily measure. Your reading of 0 to 1 ohm means that it is either not a thermistor or it has failed.

I've determined that this is a thermistor, of the NTC type, the only thing I haven't done is obtained resistance readings at lower temperatures, which I might try today. With the four readings now fairly similar, and knowing that 3 are definitely working - apart from a faulty or wrong type of meter, it looks as though the low results are correct for this item unless someone can help further.

 

[Kriis]

At close to freezing, new thermistor (as mentioned above) reads 6 ohms. Any further thoughts would be appreciated.

 

[jrmichler]

When obvious methods fail, it's time to try a different approach. Search four point resistance measurement, then measure the resistance at four or five different temperatures. Plot the results and show us the plot. The four point technique is much more accurate for measuring low resistances because it reduces the effects of probe contact resistance. And it will rule out a meter problem with the ohmmeter.

2.5 ohms, 2.76 ohms and 1.42 ohms

I'd like to see a photo of your test because my (expensive) Fluke 89 DMM has a resolution of only 0.1 ohm.

 

[Kriis]

When obvious methods fail, it's time to try a different approach. Search four point resistance measurement, then measure the resistance at four or five different temperatures. Plot the results and show us the plot. The four point technique is much more accurate for measuring low resistances because it reduces the effects of probe contact resistance. And it will rule out a meter problem with the ohmmeter. I'd like to see a photo of your test because my (expensive) Fluke 89 DMM has a resolution of only 0.1 ohm.

Thank you for the reply but I don't understand the request for a photo? - on the 200k range I can read to 3 decimal places, on the 20k range, 2 dec. places and on the 2k range, 1 dec. place. On all 3 scales, the result is similar, and the resultant measured figure is in OHMS.

Re: chart, when I checked for resistance reading in boiling water I did it twice and (as said) the results were consistent, and the increments stable without unusual fluctuations as the readings dropped toward zero. To date, before joining the Forum I have taken over 2 dozen readings now, I take your point about meter sensitivity, and I have checked for zero reading across various low resistance surfaces. There's nothing happened to make me suspect meter until someone educates me, I used to calibrate Mk 8 AVOmeters while an apprentice so I don't think it's rocket science.

 

[Dullard]

What I tried to tell you in post #6 and jrmichler is trying to tell you now is:

You are probably (95% confidence) mis-reading your meter. You need to post a picture of it (or give the make/model) if you want help sorting it out. "On all 3 scales... figure is in ohms" isn't correct for any meter that I've seen (or can imagine).

 

[Averagesupernova]

If the range is 200K, 20K, 2K the reading will likely be read out in K. So which scale are you in?

 

[Kriis]

ah, so despite the "ohm" symbol in the resistance sector, the reading could be in kilo ohms or ohms, depending on selection. So my "1 ohm" is actually 1000 ohms?

 

[Dullard]

exactly.

 

[Kriis]

Just confirmed as in Megaohm* range, it just about reads .001. I am very rusty when it comes to meter use obviously. If someone had got me to try this* I would have got here sooner, Thanks to all who answered.