Oil temp, best place?

[OldTee]

Where is the best place to get an accurate reading of oil temp using a laser pointed thermometer. Purpose is to calibrate the temp gage. Oil filter? Oil line from the oil cooler? Chain tensioner oil line?

Thanks

[Steve@Rennsport]

Well sir,...the truth is that you cannot do this accurately using an infared thermometer.

There are several ways to properly do this job:

1) Install stand-alone oil temp sender and gauge, (this is what we usually do)

2) Remove gauge and sender and use 212F water to calibrate gauge with accurate thermometer in water.

Infared's are very useful for many things but this (like tire temps) isn't one of them.

[DW SD]

I bought an accurate +/- 1/2 degree digital lab thermometer which you can drop in the oil tank. I think it has a 12" probe. I don't use it for calibration, necessarily, but it is nice to know an accurate temp for the oil in the tank.

The reality is, oil is at different temps at different points in the system.

Calibrating with boiling water is your best bet. I used 10' extension wires with alligator clips on both ends to extend the sensor lead and ground so as to drop the sender in water on a bench.

Doug

[al lkosmal]

I am not sure regarding the Bosch sender, but often the sender is a thermistor (changes resistance with temp change) and the gauge is matched/calibrated to this resistance change. I think that the addition of long wires may change the resistance enough to effect your calibration. I.E. your bench cal will match the thermometer readings, but once you remove the long wires from the circuit, it may shift the zero and span somewhat.

I'm not an electrical engineer, so perhaps a EE will jump in ...and shed some light on this. wrong.

Regards,

Al

[DW SD]

Al,
good thought. I used 16 or 18 gauge wires and believe the effects are neglibible. They register as effectively zero ohms resistance at ten feet. The current flow here is extremely low.

Doug

[DW SD]

For high current flow such as headlights or starter motor circuit resistance in the wires is an important consideration.

Doug

[al lkosmal]

Originally Posted by DW SD

Al,
good thought. I used 16 or 18 gauge wires and believe the effects are neglibible. They register as effectively zero ohms resistance at ten feet. The current flow here is extremely low.

Doug

Good explanation Doug. Very low load with almost no change in resistance. No problems. Thanks.

Regards,

Al

[OldTee]

You guys are overthinking the problem. All I did was change the instrument face and in doing so moved the needle. I don't think the instruments in these cars are that accurate anyway. All most of us care about is "cold, kinda warm, warm , operating temp, kinda hot, hot, real hot."

I have a two SC's and on the other one I bought an instrument from an older car and put in the sender called for. At the filter it was almost dead on. Just want to know if there is a better place to test.

[Grady Clay]

The measured temperature depends on where you measure it. The factory gauge measures it in the crankcase in the flow to the 4-5-6 bank cam oil flow. The temperature is different (usually higher) elsewhere.

This location is in the pressurized oil and after the engine oil cooler (and after any front cooler). This is cooler than the oil in the tank (and filter) and much cooler than the oil coming out of the engine.


Measuring the oil temperature can be very accurate. The best device is the inexpensive Type-K thermocouple and measuring instrument.

Thermocouples can be clamped to the oil pipe coming out of the engine (under #3 cylinder), the return from the front cooler, the oil filter and at the Factory temperature sender location. You can correlate all these temperatures to your dash instrument.

You will find the oil coming out of the engine varying wildly depending on driving, somewhat like head temperature but less so. The temperature at the Factory sensor will be the most stable having two thermostat controlled coolers upstream.

Best,
Grady