Looking into the Carter BBD carburetor you can see
the topmost of the two metering pins that adjust the fuel mixture.
They are controlled by the electric stepper motor on the rear of
the carb, which is in turn controlled by the computer. In a perfect
world the pins will be more-or-less centered and continuously
moving fore and aft of that fixed point. There is no "perfect"
position; the pins will be moving all the time if everything is
working properly. If this is the case, you can safely assume that
the computer, O2 sensor, and stepper motor are all
working.
The stepper motor sticks out from the rear of the carburetor
and has a squarish electrical connector plugged into it. This
connector comes directly from the computer, which moves the
metering pins in response to the signal coming from the oxygen (O2)
sensor in the exhaust manifold.
Screwed into the exhaust manifold just above the exhaust pipe
is the O2 Sensor, which looks like a spark plug with a wire coming
out of it. It develops a voltage relative to the oxygen content of
the exhaust, ranging from .1 volt for a lean
condition (oxygen percentage too high) to .9 volt
for a rich condition (oxygen percentage too low). The computer
watches this voltage, and adjusts the metering pins continuously,
always trying to achieve the optimum 14.7 to 1 air/gas ratio for
the most complete (cleanest) combustion.DETAILSOddly, the voltage output from the O2 sensor is not available at the diagnostic connectors, and the only way to check it with a voltmeter is to disconnect it, which can be difficult to do on an older unit without breaking it. Or you can tap into the (grey) wire higher up on the engine. In point of fact, knowing the exact voltage is not really necessary anyway. These units generally either work or they don't. Besides, the only thing the computer cares about is whether the voltage at the sensor is above or below a .6 volt trigger point. If (with the engine up to operating temperature) you simply look down the carburetor at the metering pins and they are moving back and forth, then the O2 sensor is working, as well as the computer and carburetor stepper motor for that matter. If the metering pins are all the way forward, in the rich position, and not moving, this could mean a non-functioning O2 sensor. But it could also indicate anything that would create a lean condition, such as an air or vacuum leak somewhere. To find out, raise the rpms and close the choke plate until the engine starts to gag. This forces a rich condition and within a few seconds the metering pins should start to move toward the back. This tells you that the sensor works. If the opposite condition exists, and the pins are all the way to the rear and stationary, the engine is running too rich and the computer is unable to lean it out. To test the O2 sensor, create an air leak somewhere by unplugging a vacuum line. This will force the fuel mixture lean and the metering pins should begin to move forward to compensate almost immediately. Again, this would mean the O2 sensor and the whole "closed-loop" system is working. Obviously, if these simple tests don't work, the computer could be dead, the carburetor could be broken, or there could be a wiring problem somewhere.
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