NEW IGNITION MODULE PARTS LIST

MODELS AFFECTED: 1985 FWD DEVILLE/FLEETWOOD WITH HT4100 ENGINE

During normal Electronic Spark Timing (EST) system operation, the ECM provides spark timing information to the distributor. If an ECM or EST system failure occurs, the distributor can use a back-up spark advance curve built into the ignition module.

The 1985 FWD "C" with swirl port heads requires less spark advance due to the fast-burn combustion chamber design. To prevent detonation in cases of EST failure (distributor using back-up spark curve built into module), the P/N 1987744 ignition module was created. The new ignition module includes much less spark advance in back-up mode only. If a P/N 1976908 module (used for other DFI engines) is " incorrectly applied to a 1985 FWD "C", severe and possibly damaging detonation may occur if the vehicle is operated with an EST failure.

If a PIN 1987744 module is incorrectly applied to a non-swirl port head engine (V-6, 6.0L and HT4100 longitudinal), the vehicle may not be driveable in back-up spark mode. Since some future model longitudinal HT4100 engines will use swirl port heads, be certain to check the Parts Book for correct ignition modules.

When replacing an HEI module in an HT4100, check the three-digit part number on the module (see Figure 1). The three digits are the last three digits of the module part number. Figure 2, on the following page, lists ignition module part numbers and applications for 1980 through 1985 Cadillacs.

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.