Dual Alternator Upper Fault (Monitor)
The P1105 code indicates a fault in the upper alternator circuit of a dual alternator system, commonly 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, or a faulty voltage regulator.
01. Most Common Causes
- Failed upper alternator
- Damaged wiring or connectors in the alternator circuit
- Faulty voltage regulator
- Blown fuse or fusible link
- PCM software or hardware issue
02. Symptoms You Will Notice
- Battery warning light illuminated
- Dimming headlights or electrical issues
- Low battery voltage or dead battery
- Engine stalling or difficulty starting
- Reduced alternator output
03. Step-by-Step Diagnosis
Step 1: Use a scan tool to confirm the P1105 code and check for other related codes.
Step 2: Inspect the upper alternator wiring and connectors for damage, corrosion, or loose connections.
Step 3: Test the upper alternator output voltage with a multimeter (should be 13.5-14.5V at idle).
Step 4: Check the voltage regulator and replace if faulty.
Step 5: If wiring and alternator are good, test the PCM communication and update/replace if necessary.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive long distances as the battery may discharge, leading to a stall or inability to restart. The vehicle may still operate on the lower alternator, but electrical loads should be minimized.
How much does it cost to fix?▼
The cost typically ranges from $200 to $600, depending on whether the alternator needs replacement ($150-$400) or just wiring repairs ($50-$200). Labor adds $100-$200.