The computer
The electrically controlled carburetor
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The computer |
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The electrically controlled carburetor |
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The Oxygen (O2) sensor in the exhaust stream |
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 they are, you can safely assume that the computer, O2 sensor, and stepper motor are all working. |
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Here is a close-up view of
the BBD stepper motor connector, seen from the bottom, showing all five harness wires. The
wire colors are:
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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. |
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The computer, or MCU, is located under the dash on top of the heater housing, on the passenger side. It's held in place by only one Phillips screw and is very easy to remove with the wire harness attached. The large harness connector on one end is held tight by a bolt in the center, which must be removed before the harness can be disconnected. |
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- Details -
Oddly, 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 door 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, the O2 sensor could be bad, or there could be a wiring problem somewhere.
Many of the wires, switches, and vacuum lines that you see on the engine only do what they do while the engine is cold and warming up. Once it's close to operating temperature, most of these extra gizmos have done their jobs, and drop out of the picture, leaving just the three major players still actively functioning.