By Joe Leoni
If you think your oil temperature gauge is not giving you an accurate reading, most likely either the oil temperature gauge or sender has failed. However, there is a third case where the sender and gauge are not compatible. The indication would be that the oil temperature shows hot with a cold engine, then the temperature decreases as the engine warms. [Editor: Brad Ripley emphasizes that currently available VDO senders will not work with an original gauge. New senders from Porsche (made by Beru) that will work look different and cost plenty.]
First, some background on the sender location in the engine compartment. In mid 50's and later cars, it is located just behind the distributor. In earlier cars it's hidden behind the fan shroud. Brad Ripley writes: "On late 2-piece case engines, the sender is threaded in the engine case above the flywheel. On early 3-piece case engines, the sender is threaded in the case at the lower left corner. Starting with engine numbers: 67001 and 81201, the sender is mounted in the 'Tree" just to the right of the oil cooler (and continuing on thru the 912)." Once you've located the sender, try the following tests.
Start by testing the gauge to determine if it works and if it is original or modified. With the ignition on (no need to have the engine running), lift the green/black wire from the sender. The gauge will show full scale if it is original. This is true if the wire is off at the gauge or is broken. If you have a modified gauge, it will read minimum scale with the wire lifted. Now ground the green/black wire to the engine case. The original gauge will show minimum, and the modified gauge will show full scale (the gauge is confused!). [Editor: if that is the case you can have your gauge modified by North Hollywood Speedometer or Palo Alto Speedometer, but that will take weeks. If you want it right away, NLA sells rebuilt and restored combi gauges modified to work with the new senders.]
If the result of these tests are as described above, you probably have a bad sender. Now test the resistance of the sender: original senders (with an original, unmodified gauge) should read around 20 ohms at room temperature. A new sender will read about 200 ohms.
Next check to see if, with the ignition still on, the sender is seeing some voltage. Your voltmeter should show a positive voltage (not necessarily as much as 6V). If you're seeing a voltage, it is likely that there is power to the oil temperature gauge (combi instrument). If there is no voltage, check the back of the gauge for a loose wire. The gauge receives +6V from the blue/yellow wire.
Want to test your sender to see if it is reading temperatures correctly? Sure you can use a dipstick thermometer to check the oil temp in the sump, but that is before it goes through the cooler and it will be much hotter than what the gauge reads. Try this: remove the sender (you can leave the gauge in the car) and run a wire from the black/green wire in the engine compartment that was connected to the sender terminal, to the sender unit which is now outside the car. Place the sender probe in a pan of boiling water. Do not immerse the terminal and wire. The gauge should read about in the mid-point of the scale (water boils at 212F / 100C). If you are ambitious and have an oil bath you can test at higher temperatures, and use a cooking thermometer to check the temperature of the oil bath and compare to the gauge reading. The beginning of the narrow top green band is about 266F / 130C.
Remember, these are analog gauges that were not designed to be particularly accurate. We now live in a digital world and expect everything to be precise, but that was not the case in 1950's era sports cars.