Oil Temperature Gauge Not Working

[rpiereck]

My C900 came with MotoMeter gauges, which are almost VDO copies, even the back side looks the same. I have engine oil temperature, oil pressure and voltage. The last two work fine, but the engine oil temperature doesn't. Even on the hottest days the highest reading I get is about 60 degrees Celsius. I have checked the wire going to the sending unit (in the engine oil drain plug...) and there is continuity. I am sure the engine oil gets a lot hotter than 60 degrees C, so what could be the problem? Fault sender? Short on the ground?

[euromobile900]

I've moved this away from my gauge tutorial thread and posted it here, where more people can comment. I think your best course of action is to test the gauge and sensor out. You can easily do the sensor with an ohmmeter, and you don't need anything to test the gauge.

All VDO style sensors work by varying their resistance. They increase in resistance as the thing they're sensing (pressure, temperature, etc) increases. The easiest way to test them is to test resistance from sensor to ground. Ohms are typically between 0 and 120, if I remember correctly. You kind of have to guess where the ohms will be, by scaling the 0-120 ohm range to whatever the teperature range of the sensor is, but typically a failed sensor will read as either a closed circuit (no resistance) or an open circuit (infinite resistance).

Following these same principles, the gauge can be tested very easily. If you connect 12v power to the gauge (leads should be marked "+" and "-"), and leave nothing connected to the sensor lead (usually marked "T"), the needle should rapidly move upscale until it hits its stop at the top. If you ground the sensor lead, it should read zero. If it doesn't do these things, it has failed internally somehow. I've attempted to repair VDO gauges internally, but I think it's impossible.

[rpiereck]

I'll test the sender that way. Do I have to remove it from the oil pan or can I test it installed? If I have to remove it I'll wait for the next oil change.

[euromobile900]

You can test it installed. Just test resistance from sensor contact pin to the bottom of the gearcase adjacent to the sensor. Should give you a good idea of whether it's faulty or not. Probably a good idea to disconnect the lead, though, just to isolate the sensor as the only thing you're testing.