I want to test my O2 sensor. Clarks garage says"1.Start the car and run until engine is at normal operating temperature.
2.Disconnect the O2 sensor and connect a voltmeter to the sensor plug."

there are thee pins on the sensor plug. which ones do I test on? I searched and found it is possible to blow the fuse for the fuel pump if I use the wrong pin but couldn't find which are the correct ones.

 

Oops.. nevermind

 

The signal is on pin 1 of the plug, which is the one by itself in one half of the plug.

 

What do I do with the other probe of the meter?

 

It goes to ground. Maybe start by trying to test the voltage of a flashlight battery, just to get the hang of it. Red goes to positive, black to ground.

 

Hi, how come you are testing your O2 sensor? Still chasing the lean condition? If so, the crude way to determine if your O2 sensor is either causing or masking your issue is quite simple on an older Motronic car like the 951: wait till the engine is at full operating temperature, simply unplug the O2 sensor. If the idle changes drastically, then your O2 sensor is either masking something (more likely) or is the cause of the problem itself (less likely). If there is little to no change in idle, move on to something else.

Sure, you could attempt to check the that the O2 sensor is generating voltage by testing it with a voltmeter, but your average voltmeter will not give you much information. You might see a reading that oscillates between about 0.200V and 0.800V, which would be normal, but you might only see an average voltage of 0.450V or so (which a lot of voltmeters do when a switching signal is present). Either way, it won't tell you much. You really need an oscilloscope to accurately check O2 sensor voltage.

Chris

 

And for posterity's sake, the O2 sensor that that Clark's Garage prescribes is completely useless: when the O2 sensor is unplugged to test the voltage, all you are seeing is open-circuit voltage, which tells you next to nothing. The O2 sensor needs to remain plugged in to complete the circuit to the DME, and the signal wire needs to be pierced or back-probed with the voltmeter positive lead, with the negative lead connected to a good ground.

An even better test is to check the voltage at the DME connector itself!