P1108
GMDual Alternator Battery Lamp Circuit Malfunction
Medium SeverityThis code indicates a malfunction in the circuit responsible for illuminating the battery warning light when a vehicle is equipped with a dual alternator system. This can result in the battery warning light not functioning correctly, potentially leaving the driver unaware of a critical charging system problem.
⚠️ Before You Begin (Safety First)
Electrical system work can be hazardous. Always disconnect the battery before attempting any repairs involving electrical components to prevent accidental short circuits or electric shock. Ensure the vehicle is off and cooled down before working on any engine bay components.
Common Symptoms
- Battery warning light illuminated on the dashboard
- Battery warning light not illuminating when it should (e.g., with key on, engine off)
- Intermittent illumination of the battery warning light
- Dimming headlights or interior lights
- Slow cranking or no-start condition
Common Causes
- Damaged or corroded wiring in the battery lamp circuit
- Loose or corroded electrical connections at the alternator, battery, or fuse box
- Blown fuse related to the dual alternator or charging system circuit
- Faulty instrument cluster or battery warning light (less common)
- Malfunctioning Powertrain Control Module (PCM) or Body Control Module (BCM) affecting the circuit
- Faulty alternators (though this code specifically targets the lamp circuit, a failing alternator could sometimes indirectly cause related circuit issues)
Frequently reported on: Chevrolet Silverado 2500HD (2007-2014), GMC Sierra 3500 (2007-2014), Chevrolet Suburban 2500 (2007-2014)
Diagnostic Steps
- Visually inspect the battery terminals and cables for corrosion or looseness. Clean and tighten as necessary.
- Inspect the wiring harnesses and connectors leading to both alternators and the battery for any signs of damage, fraying, or loose connections. Secure or repair as needed.
- Locate and check all fuses related to the charging system and alternators in both the underhood and interior fuse boxes. Replace any blown fuses.
- Using a multimeter, test the battery voltage with the engine off (should be around 12.6V) and with the engine running (should be between 13.5V and 14.7V). Ensure both alternators are contributing to charging.
- Check for any signs of physical damage to the alternators themselves or their mounting hardware.
👨🔧 When to Call a Professional
If basic checks of fuses, wiring, and connections do not reveal the issue, or if you are uncomfortable with advanced electrical diagnostic tools such as multimeters and wiring diagrams, it is best to consult a professional mechanic. This code often requires detailed circuit testing.
Commonly Asked Questions
Can I drive my GM vehicle with a P1108 code?
It is not recommended to drive for extended periods with this code. While it indicates a circuit malfunction for the warning light, it can mask a real charging system issue that could leave you stranded if the battery runs flat.
What is a dual alternator system?
A dual alternator system uses two alternators to provide increased electrical output, typically found in heavy-duty trucks or vehicles with high electrical demands like ambulances or those with extensive aftermarket accessories.
Does P1108 mean my alternators are bad?
Not necessarily. P1108 specifically points to a problem with the circuit that operates the battery warning lamp, not directly the alternators themselves, though a bad alternator could sometimes indirectly lead to related circuit issues. It's often a wiring or fuse problem.
Can a bad battery cause a P1108 code?
A bad battery itself is unlikely to directly cause a P1108 code, as this code relates to the charging system's warning light circuit. However, a failing battery might exacerbate or highlight underlying charging system problems.