I received the GM oil level/temp sensor (p/n 12603781), stuck it in some olive oil and logged its resistance vs temp, just as I did with both the SU112 and VDO 323-092. I can confirm that it does indeed have a thermistor inside, and the behavior is very similar to the other two sensors. Resistance decreases with increasing temperature--> At 300F it's 47 ohms and at 160F it's 498 ohms.
However, it took the resistance a loooooooong time to stabilize after a temperature change....approximately 2-3 minutes, in fact. For example, after the olive oil had cooled from 310F to 300F, the resistance value didn't settle for about 3 minutes. This is in stark contrast to both the SU112 and VDO sensors which both had near-instantaneous reaction times. The major difference that I could see is that the GM 12603781 has some sort of plastic housing, which seems to be really really bad at conducting heat...a lot can happen to your oil in 2-3 minutes.
To see how this works in the car, I did the following test. I hooked up ohm-meters to both the SU112 & GM oil level sensor, turned on the car, and let it idle for about 40 minutes. Late in the test, I shut the engine off for a few minutes, and then turned it back on. Every minute, I logged the resistance of each sensor, along with the ECM's calculated Oil Temp read from OBDII via the Interceptor. Ambient temp was 61F. The results are as follows:
The above picture is pretty interesting. There are 3 lines:
- Green triangles are from the ECM calculated Oil Temp read from the OBDII port using the Interceptor. (I forgot to write down the ECM oil temp in the first few minutes, which is why there are some missing data points)
- Red Squares are from the GM oil level/temp sensor (12603781) at the oil pan
- Blue Diamonds are from the SU112 sensor mounted at the Mocal adapter
Some observations:
- At Idle, the ECM calculated oil temp is always about 20 deg F higher than actual
- Notice the spots circled in Yellow. This is where the GM oil level/temp sensor fails to react quickly to the changing oil temperature.
- The SU112 & GM oil level/temp sensor are actually pretty close with measured temp once the GM oil level sensor catches up. Looking at the raw data, they are within 1-2 degrees of each other, outside of the Yellow circled areas.
Let's zoom-in and take a closer look at the Engine Off/On cycle, as seen below:
When the engine is turned off after 31 minutes, notice how the GM oil level/temp sensor keeps getting hotter while the SU112 temp is decreasing. When the engine is turned on after 37 minutes, notice how the GM oil level/temp sensor keeps cooling while the SU112 temp is increasing. It takes between 2-3 minutes for the GM oil level/temp sensor to 'see' the changing temp, as was the case with the stove test.
What does this all mean? Here is what I'm thinking: the GM oil level/temp sensor is a poor choice for use in a motorsport application because it's too slow to react. Second, measuring oil temp at the Mocal adapter is definitely a suitable location since the temp is almost the same when measured in the pan (within 1-2 degrees). Third, the ECM's calculated oil temp is very conservative at 20 deg over actual while idling (note that the oil cooler was not really a factor in this Idling experiment).
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This has been a fun little odyssey. In the end, I guess the message is that the air-to-oil coolers described in this thread are working, but make sure you pick a new spot for measuring oil temp because both the ECM's calculated temp and the sensor in the pan are wildly inaccurate. Oh, and remember that the temp sensing function of the 'oil level/temp sensor' in the pan is essentially unused and seems to be in no way related to the ECM's calculated oil temp.
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Originally Posted by Greg@PacePerformance
