Friday, November 24, 2006
AV - Oil Temps & Sensor Locations
In theory, you may install an oil temperature sensor in any location, so long as the sensor is bathed in an active flow of oil. This is to ensure the sensor reflects any change in the oil's temperature as soon as that change begins to take place. As the Pilot-in-Command your main interest is any anomalous change in oil temperature, rather than the temperature itself.
One reason for our lack of interest in precise quantified temperature data is cost. Calibrated mechanical instruments, certified accurate within a given degree of precision are expensive. When precision accuracy is combined with reliability you’re looking at a very expensive piece of goods. Instruments developed for land-based vehicles aren’t especially accurate but are sufficiently responsive for our needs.
The reason we are less interested in the magnitude of the temperature shown on the gauge is because the temperature of the lubricant varies throughout the engine. That is, you may see a wide variation in oil temps from the same engine, depending on where in the temperature is measured. When using low cost instruments, rather than rely on specific numerical readings we insert the sensor into the active oil flow and by reference to other, more critical parts of the engine that may not allow convenient temperature-sensing, we calibrate the meter to our particular engine, dividing the scale of our meter into colored arcs to show the safe operating range.
For example, the oil temperature gauges installed on some VW industrial engines placed the sensor at the inlet to the oil pump and divided the face of the meter into red, yellow and green arcs and provided no numerical information at all. By comparing the VW system against a 400 degree mercury thermometer borrowed from the chem lab at Modesto Junior College, I found the green arc covered (approximately) 170 to 220 degrees on the Fahrenheit scale, followed by a yellow arc extending up to about 250*F and a red arc beyond that. This seemed rather low until I learned that the oil temperature in the valve gallery was typically a hundred degrees higher than that being sensed at the inlet to the oil pump. Clearly, the intent was to warn the operator to reduce the load on the engine when the valve gallery oil temps exceed 350*F.
The How-To information for installing a temperature sensor at the inlet to the oil pump (ie, VW's factory-preferred method) (*) has been posted to the internet numerous times since 1994 and there are a couple of web sites that offer step-by-step photographs of the procedure.
(*) The VDO instrument cluster offered starting in 1970 (?) was a dealer-installed option and subject to numerous Service Notes over the years due to their often hilariously incorrect readings)