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Author Topic: Spring is in the air!  (Read 2357 times)
Anders Dinsen
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« on: March 01, 2007, 08:50:30 pm »

1st of March came today and with that the "calendar-spring".

Yesterday my Murena decided to play with me as the temperature gauge was all wrong and far too high. It started showing 80 degrees after having been left for about one-and-a-half hour in 5 degrees celcius, which I found odd. Quickly it climbed to 90 and approached 130. Something was wrong, and it was the gauge - not the coolant temperature.

Today she's been doing the same trick...

When I got home I pointed my infra-red thermometer at the thermostat housing and measured it to 77 degrees - gauge showed close to 130!

Then I checked the workshop manual for the resistances of the temperature sender which is described as a thermistor, and found the following values:

@ 49 deg: 73 Ohm +/- 14
@ 76.5 deg: 29.3 Ohm +/- 4
@ 110 deg: 12.6 Ohm +/- 1
@ 121 deg: 10 Ohm +/- 1

(Very odd numbers!)

These are quite low resistances compared to most sensors of this kind, I thought, but that must be how it should be.

So I have recorded the resistance of the sensor at different temperatures while the engine was cooling down. One thing puzzles me, however: The resistance reading is not stable; it slowly climbs towards a specific value. It's like I'm measuring the resistance of a resistor with a capacitor across. This makes no sense to me!

But alllowing the resistance to stabilize somewhat, I got the following measurements:

At 73 degrees: 200 Ohm
At 58 degrees: 490 Ohm
At 30 degrees: 630 Ohm

And now I'm twice as much puzzled - this makes no sense compared to the table above as they are off by about a factor of ten!

Measuring the internal resistance of the instrument in the other end of the wire, I get something like 50 Ohm. Now, that makes sense with the specified values, but again not with my measurements.

If someone can explain this to me, I'd be grateful!

I'll order a new temperature sensor and try to switch it over to see if it helps. I'll also keep my IR-thermometer in the car and do routine checks until I've got a working gauge again, but apparantly everything is working in good order!

- Anders  Huh
« Last Edit: March 01, 2007, 09:03:33 pm by Anders Dinsen » Logged

'82 Murena 2.2 prep 142
'01 Grand Espace 24v
'08 Smart Fortwo 0,8 cdi
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