Alternator & Battery Voltage

Vehicle electrical system voltage is never a static 12.0 Volts. During normal vehicle operation, battery terminal voltage (UB) dynamically fluctuates between 8.5V during freezing sub-zero engine cranking and 14.4V under full alternator charging.

Because electromagnetic actuators (fuel injectors, ignition coils, boost control solenoids, electronic throttle motors) rely on electrical current to generate mechanical force, changes in operating voltage fundamentally alter their physical response times. Bosch ME7.5 incorporates an active Electrical Energy Management Subsystem that continuously samples system voltage (UB) and monitors generator mechanical load via the Alternator Terminal DF (Dynamo Field) PWM interface to guarantee injection accuracy, spark energy stability, and idle speed smoothness.

code

┌──────────────────────────────────────────────────────────────────────────────────────────────────┐
│                   ALTERNATOR LOAD SENSING & VOLTAGE COMPENSATION ARCHITECTURE                   │
├──────────────────────────────────────────────────────────────────────────────────────────────────┤
│                                                                                                  │
│   [ Bosch 90A / 120A Alternator ]                                                                │
│     ├─ Terminal B+: 14.2V High-Current Output ──► Primary Battery Distribution Bus               │
│     └─ Terminal DF: Digital PWM Excitation Signal ──► Bosch ME7.5 ECU Pin 38                     │
│                                                            │                                     │
│                                                            ▼                                     │
│   ┌─────────────────────────────────────────────────────────────────────────┐                    │
│   │ Alternator Mechanical Drag Torque Calculation: MDDF                     │                    │
│   │ • Reads DF Duty Cycle (0% to 100% excitation)                           │                    │
│   │ • Calculates mechanical power absorbed from serpentine belt:            │                    │
│   │   P_alt = (U_batt * I_alt) / efficiency                                 │                    │
│   │ • Commands pre-emptive idle throttle opening before RPM sags!           │                    │
│   └─────────────────────────────────────────────────────────────────────────┘                    │
│                                                                                                  │
│   [ System Operating Voltage Sensing: U_B (ECU Pins 3 & 121) ]                                   │
│                                    │                                                             │
│         ┌──────────────────────────┼──────────────────────────┐                                  │
│         ▼                          ▼                          ▼                                  │
│   ┌────────────────────┐     ┌────────────────────┐     ┌────────────────────┐                   │
│   │ Fuel Injector Dead-│     │ Ignition Dwell     │     │ Throttle Motor     │                   │
│   │ Time: TVUB(U_B)    │     │ Charging: KFSZT    │     │ Drive: DVE         │                   │
│   │ • 14.0V: 0.85 ms   │     │ • 14.0V: 2.2 ms    │     │ Scales PWM duty to │                   │
│   │ • 10.0V: 1.45 ms   │     │ • 10.0V: 4.8 ms    │     │ hold blade position│                   │
│   │ •  8.5V: 2.10 ms   │     │ (Prevents spark    │     │ against spring pull│                   │
│   │ (Preserves lambda) │     │ blowout at WOT)    │     │ during voltage dips│                   │
│   └────────────────────┘     └────────────────────┘     └────────────────────┘                   │
└──────────────────────────────────────────────────────────────────────────────────────────────────┘

57.1. Alternator Terminal `DF` (Dynamo Field) Interface & Torque Compensation

On factory VAG 1.8T engines, the alternator voltage regulator communicates with the ECU via Terminal DF (ECU Pin 38):

Terminal DF outputs a square-wave PWM frequency (≈ 100 Hz).
The duty cycle of the signal reflects the percentage of time the internal rotor field coil is energized by the voltage regulator.
Electrical Load Surge: Turning on high-current accessories (rear window defroster 25A, heated seats 20A, dual cooling fans 35A, headlights 15A) causes total generator output to jump from 20A to over 95A.
Mechanical Engine Drag: Generating 95A at 14.0V consumes 1.33 kW of electrical power. Factoring in alternator mechanical efficiency (η ≈ 65%), the alternator absorbs: Pmech =1330 W0.65 = 2.05 kW ⟹ Mdrag =Pmechωcrank≈ 24.5 Nm of torque at 800 rpm Idle!
Pre-Emptive Torque Compensation: If the ECU waited for engine RPM to drop before reacting, the engine would violently stumble or stall. Instead, ME7.5 detects the rising duty cycle on Pin 38, looks up mechanical drag torque in MDDF, and immediately commands the electronic throttle plate (G186) open by +1.8°, maintaining an unwavering 800 rpm idle regardless of electrical loading.

57.2. The Physics of Battery Voltage Injection Dead-Time (`TVUB`)

Fuel injectors are electromagnetic solenoids. When the ECU grounds the injector pin, current does not rise instantly due to coil inductance (L):

i(t) =UBR · (1 - e-RL · t )

The injector pintle cannot lift off its mechanical seat until magnetic coil force overcomes fuel rail hydraulic pressure and the internal return spring. This mechanical latency is known as **Dead-Time (Spannungskorrektur der Einspritzzeit - TVUB)**.

Calibration Table: `TVUB` vs. Battery Voltage in `06A906032LP` (Flash `0x018742`)

Battery Voltage (UB) · Factory 317cc Injector (TVUB) · Upgraded 550cc Bosch EV14 · Upgraded 1000cc Injector Dynamics
Battery Voltage (UB)	Factory 317cc Injector (`TVUB`)	Upgraded 550cc Bosch EV14	Upgraded 1000cc Injector Dynamics
6.0 V	3.55 ms	3.20 ms	2.85 ms
8.0 V	2.15 ms	1.95 ms	1.72 ms
10.0 V	1.42 ms	1.32 ms	1.20 ms
12.0 V	1.05 ms	0.98 ms	0.92 ms
14.0 V (Nominal)	0.82 ms	0.76 ms	0.72 ms
16.0 V	0.68 ms	0.62 ms	0.58 ms

Why This Matters: If a tuner swaps to 1000cc injectors and changes KRKTE but neglects to update TVUB, cold-cranking fueling at 9.0V will be severely incorrect. Because effective pulsewidth at idle is only ≈ 1.2 ms, an error of 0.4 ms in TVUB produces a massive 33% error in delivered fuel mass, causing persistent idle misfires and rich/lean hunting whenever headlights or cooling fans cycle!

57.3. Ignition Coil Dwell Scaling (`KFSZT` / `DWELL`)

To deliver an energetic, high-intensity spark across the plug gap under 1.5 bar of dense cylinder boost pressure, the ignition coil must reach its saturation current (Ipeak≈ 7.5 A):

Espark =12 · Lprimary · Ipeak2≈ 45 mJ

The rate of current build-up is directly proportional to applied voltage (dI/dt = UB / L).
At 14.2V, charging the coil primary winding requires 2.2 milliseconds.
When battery voltage sags to 11.5V (heavy electrical draw at idle), charge time must expand to 3.6 milliseconds via the voltage compensation map KFSZT.
If KFSZT is miscalibrated or clipped, stored energy drops below 25 mJ, resulting in high-boost spark blowout and misfire codes (P0300–P0304).