Here's a breakdown of how to approach diagnostics:
1. Check Engine Light (CEL) and Other Warning Lights:
* Meaning: The check engine light's primary function is to indicate a problem detected within the engine's computer system (PCM/ECM). Other lights, such as the battery light, alternator light, etc., might also be illuminating, providing additional clues.
* Action: Note which light(s) are illuminated. This gives a *general* area for trouble-shooting. However, the CEL itself won't directly tell you *what* is wrong; you'll need further investigation. The other lights are usually more straightforward in pointing to specific systems.
2. Built-in Test Modes (Possibly):
* Limited Availability: Some 1991 Cadillacs might have a rudimentary built-in self-test that would cause the check engine light to flash a code sequence, indicating specific trouble areas. This is *not guaranteed* and the procedure, if it exists, is *not standardized*. You'll need to consult a Cadillac repair manual specific to that year and model to find out if this exists for your vehicle and how to activate it. Often, this would involve specific key-on/key-off sequences.
3. Using a Multimeter:
* Essential Tool: To diagnose problems more deeply, you'll need a multimeter (voltmeter/ohmmeter). This will allow you to test the voltage and resistance in various circuits related to the issues suggested by the warning lights.
* Repair Manual Needed: A repair manual (Haynes, Chilton, or a factory service manual) is absolutely essential here. It contains wiring diagrams, component locations, and detailed diagnostic procedures for various systems. You'll use the manual to find the correct test points and interpret the multimeter readings.
4. Common Problems in 1991 Cadillacs:
* Sensor Issues: Oxygen sensors, mass airflow sensors, coolant temperature sensors, and crankshaft position sensors are common points of failure and often trigger the CEL.
* Ignition System: Issues with the ignition coil, distributor, spark plugs, or wires can cause misfires and illuminate the CEL.
* Fuel System: Problems with the fuel pump, fuel injectors, or fuel pressure regulator can cause drivability issues and trigger the CEL.
* Electrical System: Problems with the battery, alternator, or other electrical components can also trigger warning lights.
In Summary:
Diagnosing a 1991 Cadillac DeVille is not a simple "press this button" affair. It's a much more involved process requiring a repair manual, a multimeter, and some mechanical aptitude. If you're not comfortable working on cars, it's strongly recommended you take it to a qualified mechanic. Attempting diagnostics without the right knowledge and tools can lead to further damage or incorrect repairs.
