SERVICE MANUAL REVISION CHART A-3 AND C4 SECTION 6E

1987 BONNEVILLE MODELS WITH 3.8L (VIN 3) AND 1986-87 GRAND AM MODELS WITH 3.OL (VIN L)

The attached page has been revised. Please discard this page from bulletin 87-6(Gasoline)-75, dated 10/87, and replace it with the attached.

CHART C-4F-1 C31 MISFIRE AT IDLE 1986-1987 3.8L (VIN 3 & B) Circuit Description: ------------------- The C3I uses a waste spark method of spark distribution. In this type of ignition system the ignition module triggers the #1/4 coil pair resulting in both #1 and #4 spark plugs firing at the same time. #1 cylinder is on the compression stroke at the same time #4 is on the exhaust stroke, resulting in a lower energy requirement to fire #4 spark plug. This leaves the remaining high voltage to fire #1 spark plug.

Test Description: Numbers below refer to circled numbers on the diagnostic chart.

1. If the "misfire" complaint exists under load only, diagnostic CHART C-4F-2 must be used. Engine rpm should drop approximately equally on all plug leads.

2. A spark tester such as a ST-125 must be used because it is essential to verify adequate available secondary voltage at the spark plug. (25,000 volts.). Secondary voltage of at least (25,000 volts) must be present to jump the gap of a ST-125.

3. If ignition coils are carbon tracked, the coil tower spark plug wire nipples may be damaged.

4. By checking the secondary resistance, a coil with an open secondary may be located.

5. By switching a normally operating coil into the position of the malfunctioning one, a determination can be made as to fault being the coil or C3I module.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.