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Joined: Jan 16 2016
Posts: 97

I had Ashcrofts fit a HP24 gearbox to my diesel way back in 2012. At their recommendation I had a boss welded on to the gearbox sump to accept a temperature probe.
At the time I bought a cheap digital temperature gauge on Ebay and cannibalised the guts. I fitted the digital panel bit into a spare space in the instrument binicle. The temperature sensor that came with the kit was a two wire thermistor type.
It has worked well for 6 years but has just failed. The temp shows 01 deg C.
I have measured the old sensor resistance and it has gone open circuit. I have now plugged a new two wire sensor in (not yet screwed into the gearbox) and the digital panel now does its startup cycle which it didn't before but then shows 130 deg C. at just ambient temp.
The new sensor resistance is about 50 ohms which does alter with temp. I tried it in boiling water. It is an NTC thermistor. Resistance drops as the temp rises. It looks physically exactly the same as the old one.
I thought that all cheap car temperature probes would be standardised, use the same thermistor and have the same range. However I am now suspecting that there are different resistances despite the ranges all saying 0-150 deg C.. Anybody shed some light on this?
I know there are single wire and twin wire types. There is no technical data included with the sensor other than it is 0-150 deg C.
I could buy another complete gauge kit that comes with a sensor so they are a matched pair but the dash will have to come out again to fit it.
One other thing: I measured the voltage to the sensor plug and it is only 5 or 6 volts. Not sure what voltage it should be from the digital head unit. It has a 12v supply to it behind the dash.

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Joined: Dec 30 2015
Posts: 917

It probably is a 5v system to the sensor, I wouldn't worry about that too much.

What will make the difference is what value NTC sensor it is. Most of them are a 10k, but it will depend on what the sensor head unit is expecting, as you can get different variations.

I'm on my phone at the moment, but will get my laptop out a bit later and will find a couple of web pages which show different temp values vs resistances for NTC sensors and might be able to match up to what the system is expecting..

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Joined: Dec 30 2015
Posts: 2394

I'm with Marty on this, 50 Ohms is way too low where 10k when at their lowest temperature is normal dropping as it gets hot.

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Joined: Sep 14 2016
Posts: 199

50 Ω at ambient sounds very low. Judging by this list : https://www.omega.com/temperature/Z/pdf/z256-257.pdf a reading of 130°C on the meter for that resistance would be pretty typical if an "ordinary" Negative Temperature Coefficient thermistor were used.

As Marty says you generally expect around 10 K Ω somewhere in the normal(ish) ambient temperature range, say 0°C to 30°C for an ordinary NTC thermistor. Although the shape of the response curve is pretty much the same for most types there is considerable variation in actual values as the detail construction of the device is altered to put the most linear part of the resistance variation in the temperature range where its most wanted.

But some, like the one you have are very different. See 1DA500J and 1DA500K about half way down this page : https://www.ametherm.com/thermistor/disc-and-chip-ntc-thermistors for one specified as 50 Ω at 25°C needing a 6 volt supply. Probably not what you bought but similar. I'd have thought that would be pretty specialised device needing bit of hunting to find. Not something you'd just buy off the shelf.

Clive

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Joined: Dec 30 2015
Posts: 917

You can get a 47ohm NTC sensor - the resistance they are labelled as is for an ambient temp of 25 deg C

You could have a play about with it and temporarily wire a 100K pot in place of the thermistor.... then varying that should make the temperature vary on the display. when you hit 25 degrees... measure the resistance with a multimeter and you should have a reasonable punt at what the normal NTC sensor should be...

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Joined: Jan 16 2016
Posts: 97

I knew this was the place to come. You are all clever buggers on here.

Done some more delving this morning. It appears the small multimeter I keep in the car is a mile out.
I have now measured the new thermistor using my other bench top digital multimeter and got the following results:

deg C kΩ

13 3.23
24 1.595
52 0.544
90 0.156

So at ambient temp under the car it was about 1.6kΩ. The digital display showing 130 deg C.

The head unit should therefore be paired with a thermistor that measures 1.6kΩ when actually at 130 deg C when it will be correct.

In the meantime, I have ordered another twin wire and also a single wire thermistor to try out. They are cheap enough.

Dave

ps: how do you do columns?

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Joined: Dec 30 2015
Posts: 917

hmmmm... a 10K NTC sensor shows that the temp at 1.6K should be between 70-75 degrees C.

So I'm not quite sure what range sensor you should need.... Let us know what the replacement sensor does!!

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Joined: Dec 30 2015
Posts: 2394

If it's showing 3.23k at 13 degrees, that would suggest a 5k sensor.

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Joined: Nov 16 2016
Posts: 369

I wonder if an LPG thermistor would be any good? They usually have the negative temp coefficient and are made in many various specs, 4.7K is very common, usually very cheap.