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While this repeats what was posted above I thought I would post it as it adds "a little more" detail and applies to 1984-1994 engines.
3.8L and 3.8L Turbo Camshaft Sensor Operation:
The camshaft sensor provides one pulse at 25 degrees after top dead center (ATDC) of the number 1 cylinder for every two crankshaft revolutions. This camshaft sensor pulse is used to synchronize sequential fuel injection (SFI.)
Both the camshaft sensor pulse and the crankshaft sensor 3X pulse must be present for the ignition module to sequence the firing of the 3-6 coil and start the engine. The camshaft sensor "sync" pulse must also remain present to sequence SFI fuel delivery when the engine is running. If the camshaft signal is grounded or opened when the engine is running, the ECM stops SFI and goes to multiport fuel delivery, but the engine continues to run.
Chuggle, intermittent miss, high emmisions and stall symptoms can sometimes occur when the camshaft signal is lost. The Service Engine Soon light comes on and Code 41 is set if the signal is lost for more than one second.
Here's a little additional information pertaining to the 3.0L and 3300.
The 3.0L and 3300 engines will run without a camshaft or sync signal if the engine is running before the failure occurs, but will not start without it. However, if this circuit is pulled low while the engine is running, the module will re-sync and frequently stall the engine.
Note that some 3.0L engines may start if the sync signal is pulled low and the driver repeatedly tries to start the vehicle.
And finally for the 3800.
As with the crankshaft sensor, the ignition module sends a reference signal to a Hall-effect switch in the camshaft sensor. A permanent magnet, attached to the camshaft sprocket, passes close to the sensor once each camshaft revolution (the camshaft rotates once for every two crankshaft revolutions). As the magnet passes beside the sensor, the Hall-effect switch is activated by the magnetic field, and its transistor turns on to ground the ignition reference signal line and pulls its voltage low (0-0.1 volts). The camshaft sensor "sync" pulse is received by the ignition module as piston number 1 (and 4) reaches approximately 25 degrees ATDC on the power stroke. The camshaft signal out is also sent from the ignition module to the ECM.
The 3800 engine will not default to simultaneous (MFI) injection when the camshaft sensor signal is not received at any time, whether in bypass or Electronic Spark Timing (EST) mode. If the camshaft signal pulse is not present during cranking, ECM/PCM signal logic begins sequencing injectors after two crankshaft revolutions or six pulses from the 3X reference signal. Code 41 is stored on the seventh pulse from the 3X signal and sequential injection begins in a random pattern.
There is a one in six chance that fuel injection will coincide correctly with valve opening. If injection is off, driveability is affected only a small amount (slight hesitation).
Source: GM Powertrain V6 Introduction to Fuel Injection, Diagnosis and Service by Ron Harlow.
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