Defender and Discovery 48V Mild Hybrid Faults: BISG and Capacitor Issues

The Ingenium-engined Defender and Discovery models from 2020 onwards use a 48V mild hybrid (MHEV) system to improve fuel efficiency and reduce emissions. This system centres on two critical components: the BISG (Belt-Integrated Starter Generator) and the 48V lithium-ion capacitor module. Both are failing in the field with increasing frequency, and the symptoms are often misdiagnosed.

Short Answer

The 48V MHEV system uses the BISG — a combined starter motor and generator driven by the accessory belt — to provide engine start/stop, energy recovery during deceleration, and electric torque assist during acceleration. Energy is stored in a 48V lithium-ion capacitor module mounted under the vehicle floor. When the BISG fails, the engine loses its primary starting method and must fall back to a conventional 12V starter (if equipped) or will not start at all. When the capacitor module fails or loses charge, the BISG cannot operate, disabling stop-start, torque assist, and energy recovery. The vehicle defaults to running on the 12V system alone, resulting in increased fuel consumption, reduced performance, and multiple dashboard warnings. Both components require manufacturer-level diagnostics for accurate fault identification.

What You'll Learn

• How the 48V MHEV system operates in Defender and Discovery Ingenium engines
• The function of the BISG and how it differs from a conventional alternator
• What the 48V capacitor module does and where it is located
• Common failure modes for both the BISG and capacitor
• How to differentiate MHEV faults from standard 12V electrical issues
• The DC-DC converter's role in bridging the 48V and 12V systems
• Safety considerations when working on 48V systems

Real-World Scenarios

Case 1: 2021 Defender 110 P300 — Intermittent No-Start

A 2021 Defender 110 P300 presented with an intermittent no-start condition. The engine would crank briefly then stop, display "Hybrid System Fault," and require multiple attempts before eventually starting. A previous garage had replaced the 12V battery (£220) and the starter motor (£480) — neither resolved the issue. Using Pathfinder, we identified fault codes in the BISG control module indicating an internal inverter fault within the BISG unit itself. The BISG was failing to generate the initial crank torque needed for engine start. On attempts where it partially functioned, the engine would fire. On complete failure cycles, the conventional 12V backup start procedure was also compromised because the BISG was loading down the belt system. We replaced the BISG unit, programmed it via Pathfinder, and calibrated the belt tensioner adaptation. The no-start condition was permanently resolved.

Case 2: 2022 Discovery D300 — "Battery Charging System Fault" with Full 12V Battery

A Discovery D300 displayed persistent "Battery Charging System Fault" despite a fully charged and tested 12V battery. The vehicle also showed increased fuel consumption (from 32 mpg to 24 mpg) and disabled stop-start. The 48V capacitor module had developed an internal cell imbalance. While the module was not completely dead, its state of charge was insufficient for the BISG to operate as a generator during deceleration. Without regenerative charging through the BISG, the 12V system was relying entirely on the DC-DC converter — which was being flagged as insufficient by the BEM (Battery Energy Management) module. The 48V capacitor module was replaced, the BEM was recalibrated, and the full MHEV function was restored. Fuel consumption returned to the expected 31-33 mpg range.

Case 3: 2020 Defender 90 — Belt Squeal and Reduced Power

A Defender 90 D200 came in with a belt squeal under acceleration, a "Reduced Engine Performance" warning, and noticeably sluggish throttle response. The accessory belt was in good condition. The issue was the BISG's electromagnetic clutch, which engages and disengages the BISG from the belt drive. The clutch was slipping intermittently, causing the belt to squeal under load. When slipping, the BISG could not provide its torque assist function (up to 15 Nm of electric boost during acceleration), which the engine management system expects and compensates for in its torque model. The ECM was reducing fuelling to protect the drivetrain because the expected torque assist was not being delivered. Replacing the BISG unit and performing the drive cycle adaptation resolved both the belt noise and the performance restriction.

Why Inspection and Diagnostics Matter

The 48V MHEV system sits between the traditional 12V electrical system and full high-voltage hybrid/EV systems. It is low enough voltage to not require high-voltage safety protocols (the 48V system is classified as non-hazardous under current regulations), but it is complex enough to require specialist diagnostic tools and knowledge. The BISG, capacitor module, DC-DC converter, and MHEV control module all communicate over the vehicle's CAN bus network and are integrated into the engine management, transmission control, and battery management systems.

At Nine Torque, we diagnose 48V MHEV faults using JLR Pathfinder to interrogate every module in the MHEV system chain. We can perform active tests on the BISG (commanding torque assist and regeneration modes), monitor the 48V capacitor's state of charge and cell balance in real-time, and verify the DC-DC converter's output voltage and current draw. This level of diagnosis is not possible with generic diagnostic tools, which cannot access the MHEV-specific control modules.

If your Defender or Discovery is displaying hybrid system warnings, experiencing reduced performance, or showing unexplained increases in fuel consumption, the 48V system should be investigated as a priority. Contact us for a diagnostic assessment. Early diagnosis of BISG or capacitor faults prevents secondary damage to the belt drive system and the 12V electrical network.

Frequently Asked Questions

Is the 48V system dangerous to work on?

No. 48V is classified as a Safety Extra-Low Voltage (SELV) system under IEC standards. It does not present an electric shock hazard under normal conditions. However, the BISG and capacitor module can deliver high currents (200A+), so short-circuit protection and correct disconnection procedures are still essential. It is not comparable to the high-voltage systems (400V+) found in full EVs like the I-Pace.

Can I disable the 48V system and run on 12V only?

The vehicle will default to 12V-only operation when the MHEV system detects a fault, but this is a limp-home mode, not a permanent solution. The engine management system is calibrated to operate with the BISG's torque assist and regenerative braking. Running without it long-term results in increased fuel consumption, increased wear on the conventional starter motor and alternator circuit, and persistent warning messages.

How much does a BISG replacement cost?

A new BISG unit from JLR costs between £1,200 and £1,800 depending on the engine variant. Fitting and programming adds approximately 3-4 hours of labour. Used BISG units can be sourced and programmed to your vehicle, but availability is currently limited due to the relative newness of these vehicles in the used parts market. We can advise on the best option for your specific situation.

Does the 48V system affect the vehicle's off-road capability?

When functioning correctly, the 48V system enhances off-road capability by providing instant electric torque assist at low engine speeds — exactly where you need it for rock crawling and low-speed trail work. When the system is in fault mode, this assist is lost, and the vehicle relies solely on engine torque. The difference is noticeable, particularly in the Defender's low-range mode.

How do I know if my Defender has the 48V system?

All Defender and Discovery models with Ingenium petrol (P300, P400) and diesel (D200, D250, D300) engines from 2020 onwards are equipped with 48V MHEV as standard. There is no non-MHEV option on these powertrains. The V8 supercharged Defender (V8 Carpathian Edition) uses a different architecture without the 48V BISG system.