Oil Temp Sensor Specs ?

[schosh]

As I come to "know" my new-to-me 914 I am discovering some unusual quirks. One that has me concerned is a fluctuating oil tempature gauge. It seems that as the car warms up the needle begins to move up in a rather linear fashion. However once it gets about 1/3 thru the scale it will leap to full "hot" --red zone"

I suspect either the sender or the gauge. I will remove the sender this weekend to test , but what I'm wondering is : Does any body know the specs for voltage output for one of these things? Is it 0 for cold and a full 12 volt when warm ?
Any help will be appreciated.

[lmcchesney]

We need to know what sender and what gauge. Many temp. gauges read the resistance in the sender. Some are positive coorelation, increased temp yeilds increased resistance, others are neg such as the CHT Temp sensor II.
Hope this helps.
L. McC

[jamcleod]

i think these things (sensors) are thermisters. if i read the wiring diagrams correctly, it seems that a 12 V signal is sent to the gauge (white/red stripe wire) and the sensor is in series with the ground connection, that is, the 12 V signal passes "thru" the sensor before reaching ground (electrical circuit operation "loosely" explained). you could remove the gauge and hook it up to various voltages to see the deflection of the needle. this would also allow you to calibrate the gauge if you put the sensor in known temperatures (ie, boiling water = 100C). as has been mentioned/stated in other posts, the accuracy of the gauges is not very good.

my guess is that at cold temp, the resistance of the sensor is high, dropping 12 V so the gauge gets 0 V and reads "low/0". when the sensor heats up, the resistance of the sensor decreases, dropping smaller V, gauge reads higher V so needle deflects more. basically a negative coefficient variable resistor.

(sorry about the high school physics circuit explanation, it's what i do to support my car habit )

hopefully DD will impart his wisdom

[Dave at Pelican Parts]

You've just described how the gauge works!

CHT gauges work on a different principle, but the above is exactly how the oil temp gauge works.

--DD

[lmcchesney]

Dave, what is your understanding of how the CHT guage works? The information is what Westach explained to me.
L. McC

[jamcleod]

CHT gauge uses a thermocouple under the spark plug to generate a voltage to power a gauge

[Dave at Pelican Parts]

Exactamente!

(Heh, you have the same avatars--made me thing you were answering your own question!)

--DD