INVESTIGATING GM'S COMPRESSION SENSE IGNITION

Motor,  Nov 2004  by Garrido, Jim

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When starting up the Tech 2 scan tool, be sure to choose the "Saturn" vehicle line choice, regardless of whether the vehicle is indeed a Saturn. Ii you don't, the CCO bit PID will not be displayed. If you do not own a Tech 2, you can use any other aftermarket scan tool that provides a current cylinder misfire PID. If a Tech 2 is not available, use the current cylinder misfire ID PID on your scanner and your dual-trace scope on the CAMOUT signal and UKP or REF Hi signals to provide all the CCO bit pattern information you'll need to use this chart.

The chart lists the expected duty cycle (D/C) of the CAMOUT signal, as well as the affected cylinder. The affected cylinder is identified via the CKP or RRF HI signal count and the rise and fall of the CAMOUT signal. For the sake of brevity, the 30 corresponding diagnostic charts are not reproduced here. Refer to your service information source for these charts.

Cylinder 1 is misfiring in Charts 1 and 2. The cylinder 1 on compression bit position will generate an abnormally low CAMOUT state, which results in a 25% positive duty cycle. The signal rises coincident with cylinder 3 firing on compression and falls coincident with cylinder 4 Bring on compression. This signal output combination is due to a defect causing the cylinder 1 compression firing event to occur before cylinder 4 fires on waste. The system will not have the required negative-to-positive CSI signal transition the CSTO needs to see in order to send the CAMOUT high signal.

If cylinder 1 were registering steady misfires at idle, use Chart 1 and inspect for defects such as extremely low compression, intake valvetrain issues resulting in a reduction in cylinder density, a badly arcing plug boot or a shorted/fouled spark plug gap. If cylinder 1 were misfiring randomly or only under load, we would use Chart 2 and inspect for leaking secondary components like a torn plug boot, a cracked spark plug or an externally arcing coil.

If cylinder 4 were registering steady misfires at idle, use Chart 3 and inspect for defects such as an open spark plug connector spring or exhaust valvetrain issues resulting in a reduction in flow out of that cylinder. An excessively large spark plug gap may also be responsible. If misfire is occurring randomly or only under load, use Chart 4 and inspect for defects such as an internally arcing or defective coil.

If both cylinders 1 and 4 are misfiring steadily at an even rate at all engine speeds and the CAMOUT signal displays 25% positive duty cycle, use Chart 5. In your waveforms you'd also notice that the normally characteristic bump in the CAMOUT waveform caused by the feedback of the coil firing is missing. This means that the primary circuit never charged and discharged any energy. Under these conditions, check for an open connection in the dual female interconnects between the EI module and the ignition cassette. Once the interconnect is removed, check for proper primary coil resistance. If both of those components are good, the EI module is bad. Lack of an EST signal from the PCM would not generate this 25% D/C CAMOUT. If both cylinders 1 and 4 are misfiring only under load, use Chart 6 to look for a defective ignition coil.