# Car Water Temperature Sensor

Discussion in 'Sensors and Actuators' started by duke37, Apr 13, 2013.

1. ### duke37

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Jan 9, 2011
My friend has a 1957 Hillman Minx car which has a water temperature sensor which is a little out of calibration.
TT/1200 by Smiths Industries

The supply is 12V which then goes through a thermal meter, (shows maximum at zero current), then to the sensor which is screwed into the engine with one connection.

The meter current pulses at about 1/sec and the amplitude (perhaps pulse width) decreases as the temperature rises.

Does anyone know how this pulsing sensor works?
Is there any way the meter deflection can be dropped by 10%?

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2. ### debe

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Oct 15, 2011
Generaly on thermal type meters there should be a mechanical calibration adjustment on the instrument. Pictures are from Mitsubishi Pajero temp gauge. This particular gauge has 2 adjustments indicated by arrows from end of biro.

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Last edited: Apr 14, 2013
3. ### Solidus

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4
Jun 19, 2011
It sounds like what the setup is to me (correct if wrong)

12V supply through a thermal sensor (most likely a form of resistor) to a meter.
Thermal sensor registers high when it allows zero current to pass.
Meter reads current and registers appropriate calibrated temperature.

From here there are a couple places you can go, depending on how sophisticated you want to solve the issue. First one is to place an ammeter in series with the sensor between the sensor and meter, as well as a variable trimming potentiometer also between. If you know the range of the meter (minimum to maximum deflection current) and you know your starting position and we assume that the motion is roughly linear based on current, then you work out what the approximate current should be and dial in the potentiometer to account for that.

I doubt the car would use PWM for a meter current reading, especially of a vehicle of that age. That would also require either a pretty complex sensor or an overkill controller (a controller that converts an analog current moderating temp sensor to a variable pulse width signal...now I know why my check engine light comes on randomly)

Use a variable current supply to determine the deflection range of the meter, and use the potentiometer to drop 10% of the feeder current moving south of that sensor and you should have a ballpark estimate.

Last edited: Apr 16, 2013
4. ### john monks

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1
Mar 9, 2012
I'm taking a wild guess but I think the object you posted is a thermistor and that the problem is in the gauge. There probably is a mechanical voltage limiter, that works by heat, that is not working right and that needs to be taken apart and readjusted.
Can you check the sensor with an ohmmeter?

5. ### duke37

5,330
750
Jan 9, 2011
Thanks for the replies.

The meter is a thermal meter, working on PWM with pulses at about one/second. Debe has posted a picture of a similar meter showing the heater wound round a bimetallic strip.

The 12V goes through the 25R meter and then through the 25R switching sensor to chassis.

The system works but the calibration is a littlr bit out. The meter could be adjusted as Debe says but the adjustments have been riveted and the owner is not willing to attack it with a drill. A 220R resistor across the sensor improves things somewhat and this is what will probably be done.

The on/off ratio will depend on the voltage supply and the water temperature so there seems to be negative feedback when a resistor is added.

Later systems (Morris Marina) had a separate pulse width regulator which fed both the water temperature and petrol gauges. I think it is remarkable that the switching sensor is still working after so many years.