Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSCADILLAC1991SEVILLE V8-300 4.9LREPAIR AND DIAGNOSISHEATING AND AIR CONDITIONINGTESTING AND INSPECTIONSYMPTOM RELATED DIAGNOSTIC PROCEDURESSYMPTOM DIAGNOSTIC CHARTSCHART 9 - DEFOGGER INOPERATIVE
1991 Cadillac Seville V8-300 4.9L
Chart 9 - Defogger Inoperative
1991 Cadillac Seville V8-300 4.9LSECTION Chart 9 - Defogger Inoperative
CHART 9 DEFOGGER INOPERATIVE
ECC SYSTEM CHECK MUST BE PERFORMED FIRST
CIRCUIT DESCRIPTION
The programmer provides a ground for the defogger relay when it receives a command from the Electronic Climate Control Panel (ECCP) via the data line. The BCM times the operation initially for 10 minutes, then commands the programmer to deactivate the relay ground. Additional requests for rear defogger are for 5 minute intervals, thereafter until a new ignition cycle occurs.
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
1. This step checks to see if the rear defogger relay is receiving power.
2. This step checks to see if the circuit breaker is receiving power.
3. This step checks to see if the programmer is grounding the rear defogger relay coil.
4. This step bypasses the rear defogger relay to see if the grid will operate.
ECC SYSTEM CHECK MUST BE PERFORMED FIRST
CIRCUIT DESCRIPTION
The programmer provides a ground for the defogger relay when it receives a command from the Electronic Climate Control Panel (ECCP) via the data line. The BCM times the operation initially for 10 minutes, then commands the programmer to deactivate the relay ground. Additional requests for rear defogger are for 5 minute intervals, thereafter until a new ignition cycle occurs.
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
1. This step checks to see if the rear defogger relay is receiving power.
2. This step checks to see if the circuit breaker is receiving power.
3. This step checks to see if the programmer is grounding the rear defogger relay coil.
4. This step bypasses the rear defogger relay to see if the grid will operate.
RENDER: 1.0x
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.