Ignition Module - New Design
Bulletin No. T 84-104 File In Group 6DNumber 20
Date Aug. '84
SUBJECT: NEW IGNITION MODULE FOR CADILLAC HT4100 WITH SWIRL PORT HEADS
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 P/N 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.
FIGURE 1 - IGNITION MODULE:
When relacing 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. The following page lists ignition module part numbers and applications for 1980 through 1985 Cadillacs.
IGNITION MODULE APPLICATIONS - 1980-1985 MODELS
MODEL 3-DIGIT CODE
YEAR APPLICATION MODULE P/N ON MODULE
1980 C 6.0L without ESS, 4.1L V-6 Four Terminal 1875990 990
1980 C,D,Z 6.0L V-8 Carb. with ESS Five Terminal 1894308 308
1980 E,K 6.0L V-8 DEFI Seven Terminal 1978617 617
1980 5.7L EFI (California) Five Terminal 1894308 308
1981 C,E,K 4.1L V-6 Seven Terminal 1978617 617
1981 C,D,E,K 6.0L V-8 DFI Seven Terminal 1976908 908
1982 C,E,K 4.1 L V-6 Seven Terminal 1976908 908
1982-1983 C,E,K 4.1L V-8 Seven Terminal 1976908 908
1982-1984 D Limousine 6.0L Seven Terminal 1976908 908
1980-1984 Z Comm. Chassis 6.0L Five Terminal 1894308 308
1984 D,E,K 4.1L V-8 Seven Terminal 1976908 908
1985 FWD "C" 4.1L V-8 Seven Terminal 1987744 744
1985 D,E,K 4.1L V-8 (Federal) Seven Terminal 1976908 908
1985 E Convertible 4.1L V-8 Seven Terminal 1976908 908
(California)
1985 D,E,K 4.1L V-8 Seven Terminal 1987744 744
(Calif. Excl. E Convertible)