Jeeper  Adventures Under the Hood  Jeeper

The Three Major Players

Simply, the O2 sensor reports the condition of the exhaust,
and the computer tells the carburetor what to do about it.

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.
Metering Pins
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.

- Stepper Motor Initialization -

Watch the metering pins as a helper starts the engine. The pins should move all the way forward, all the way back, then return to center as the computer locates their exact position.
Stepper Motor


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 14.7:1 air/gas ratio for the most complete (cleanest) combustion.
Oxygen Sensor

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.

- Details -

Oddly, the voltage output from the O2 sensor is not available at the diagnostic connectors. The only way to check it with a voltmeter is to tap into the (grey) wire higher up on the engine. The only thing the computer cares about is whether the voltage at the sensor is above or below a .6 volt trigger point. You should see the voltage popping up and down every few seconds.

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.

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 test for a lean condition, 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, although there is a problem, the O2 sensor and system are working.

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 for a rich condition, create an air leak somewhere by unplugging a vacuum line. This will force the fuel mixture leaner 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 are working.

If these simple tests don't work, the computer could be dead (unlikely), 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.

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