Dual Alternator Upper Fault (Monitor)
The P1105 code indicates a fault in the upper alternator circuit of a dual alternator system, typically found on heavy-duty trucks or vehicles with high electrical demands. The powertrain control module (PCM) monitors the voltage output from the upper alternator and detects a discrepancy, such as low voltage, no output, or a communication error. This can be caused by a failed alternator, wiring issues, a faulty voltage regulator, or a blown fuse. The code may also trigger if the alternator is not charging properly due to belt slippage or a bad connection.
1. Symptoms You Will Notice
- Battery warning light illuminated
- Dimming headlights or electrical issues
- Battery drains quickly or fails to charge
- Engine stalling or hard starting
- Reduced electrical system performance
2. Most Common Causes
- Failed upper alternator
- Damaged wiring or connectors in the alternator circuit
- Faulty voltage regulator
- Blown fuse or fusible link
- Loose or worn serpentine belt
- PCM or communication error
3. Step-by-Step Diagnosis
Step 1: Scan for additional codes and check freeze frame data.
Step 2: Visually inspect the upper alternator, wiring, and belt for damage or looseness.
Step 3: Test battery voltage with engine off (should be ~12.6V) and running (should be ~13.5-14.5V).
Step 4: Use a multimeter to check voltage at the alternator output terminal and compare to battery voltage.
Step 5: Check fuses and fusible links related to the alternator circuit.
Step 6: Perform a load test on the alternator to verify output under load.
Step 7: If wiring and fuses are good, replace the upper alternator.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive with this code, as the vehicle may lose charging capacity, leading to a dead battery and potential stalling. If the lower alternator is still functioning, you may have limited electrical power, but it's best to address the issue promptly.
How much does it cost to fix?▼
The cost to fix P1105 typically ranges from $200 to $600, including parts and labor. The alternator itself can cost $150-$400, and labor adds $50-$200 depending on the vehicle and shop rates.
5. Commonly Affected Makes
This is a manufacturer-specific code. The exact manufacturer is not confirmed in our current database.