P1105
GMP1105 GM indicates a fault in the dual alternator control circuit or a discrepancy between the upper (or primary) alternator's output and the expected values. This can involve the voltage regulator or the communication between the alternators and the Engine Control Module (ECM).
Medium SeverityThis code signifies an issue within your vehicle's dual alternator system, specifically indicating a fault with the primary or 'upper' alternator's performance or its control circuit. This can lead to an inefficient charging system, potentially resulting in a drained battery and various electrical problems if not addressed.
⚠️ Before You Begin (Safety First)
Always disconnect the negative battery cable before performing any work on the electrical system to prevent accidental shorts or electrocution. Allow the engine to cool completely before handling hot components.
Common Symptoms
- Check Engine Light illuminated
- Battery warning light on
- Dimming headlights or interior lights
- Slow cranking or no-start condition
- Malfunctioning electrical accessories
- Burning smell (in severe cases of alternator failure)
Common Causes
- Faulty upper/primary alternator
- Damaged or corroded wiring to the upper alternator or its control circuit
- Malfunctioning voltage regulator within the upper alternator
- Loose or corroded battery terminals or ground connections
- Blown fuse in the charging system circuit
- Faulty Engine Control Module (ECM) (less common)
Frequently reported on: Chevrolet Silverado 2500HD (2001-2010), GMC Sierra 2500HD (2001-2010), Chevrolet Suburban 2500 (2001-2013), GMC Yukon XL 2500 (2001-2013)
Diagnostic Steps
- Visually inspect the upper alternator and its wiring for any signs of damage, fraying, or loose connections.
- Check the battery terminals for corrosion or looseness and clean/tighten them as necessary.
- Test the battery voltage with the engine off and running to ensure proper charging.
- Use a multimeter to check the voltage output of both alternators (if accessible and safe to do so) to identify discrepancies.
- Inspect all fuses related to the charging system in the under-hood and cabin fuse boxes.
- If comfortable, disconnect and reconnect the electrical connectors on the upper alternator to ensure good contact.
👨🔧 When to Call a Professional
If you've checked the basic connections and tested the alternators, but the code persists, or if you're uncomfortable diagnosing complex electrical systems, it's best to consult a certified mechanic. Specialized tools and expertise may be required for accurate diagnosis.
Commonly Asked Questions
What is a dual alternator system?
A dual alternator system uses two alternators to provide increased electrical power, typically found in heavy-duty trucks or vehicles with high electrical demands, like those used for towing or with many accessories.
Can I drive with a P1105 code?
While your vehicle might still operate on one alternator, it is not recommended for extended periods. The charging system is compromised, and you risk a dead battery and potential damage to other electrical components.
How can I test my alternators?
You can use a multimeter to check the voltage output at the battery with the engine running. A healthy system should read around 13.5-14.7 volts. For individual alternator testing, specialized diagnostic tools or a visit to a mechanic might be necessary.
Is this an expensive repair?
The cost varies greatly depending on the cause. A simple wiring fix might be inexpensive, while replacing an alternator can be a significant expense, especially if it's one of the higher-output units.
What's the difference between 'upper' and 'lower' alternators?
In dual alternator systems, they are often referred to by their position. 'Upper' typically refers to the primary alternator or the one mounted higher on the engine, while 'lower' refers to the secondary. The P1105 specifically points to an issue with the 'upper' one.