Chapter 53 · Boost Control & Thermal Management

Cooling Fans & Thermostat

Coolant sensing, the fan module, and mapped thermostats with torque reserve.

Calibration symbols

G186 J285 J293 BTS FAW FST G62

ECUs covered

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Size

7 symbols · 1 diagram · ~788 words

Coolant sensing, the fan module, and mapped thermostats with torque reserve.

Maintaining optimal cylinder head and oil temperatures is essential for balancing volumetric efficiency, anti-knock threshold, and mechanical friction. On the transverse VAG 1.8T platform, engine thermal management involves a tightly coupled interaction between dual negative temperature coefficient (NTC) sensors, the Electronic Fan Control Module (J293), and the Bosch ME7.5 operating system.

In addition to basic electric fan switching, ME7.5 executes a continuous thermal simulation to scale transient fuel wall-wetting, idle torque reserves, and ignition advance. On specialized longitudinal and high-output variants, the ECU controls an active **Electronically Mapped Thermostat (Kennfeldthermostat - F265)**, which dynamically alters operating coolant temperature between economy cruise (105\text{°C}) and full-load power delivery (85\text{°C}).

┌──────────────────────────────────────────────────────────────────────────────────────────────────┐
│                   ENGINE THERMAL MANAGEMENT & COOLING FAN CONTROL SCHEMATIC                      │
├──────────────────────────────────────────────────────────────────────────────────────────────────┤
│                                                                                                  │
│   [ Dual NTC Coolant Sensor: G2 / G62 ]                                                          │
│     ├─ Pins 1 & 2: Analog Output ──► Instrument Cluster Temp Gauge (J285)                        │
│     └─ Pins 3 & 4: Analog Signal ──► Bosch ME7.5 ECU Pin 93 (0V - 5V ADC Input)                  │
│                                           │                                                      │
│                                           ▼                                                      │
│   ┌─────────────────────────────────────────────────────────────────────────┐                    │
│   │ Thermal Compensation Pipeline:                                          │                    │
│   │ • Cold Enrichment: FRLFKT (Multiplies base injection pulsewidth)        │                    │
│   │ • Wall-Wetting Decay: KFWT (Accelerates puddle evaporation calculation) │                    │
│   │ • Knock Timing Sensitivity: KFZWW (Retards timing if T_coolant > 102°C) │                    │
│   └──────────────────────────────────────┬──────────────────────────────────┘                    │
│                                          │                                                       │
│                                          ▼                                                       │
│   ┌─────────────────────────────────────────────────────────────────────────┐                    │
│   │ Radiator Cooling Fan Logic (J293 Module):                               │                    │
│   │ • Stage 1 (Low Speed / 97°C): Triggered via Rad Thermo-Switch / A/C ON  │                    │
│   │ • Stage 2 (High Speed / 105°C): Triggered via High Temp / A/C p > 16bar │                    │
│   │ • CAN Climatronic Pre-Emptive Torque Reserve (CAN Frame 0x288 / 0x388)  │                    │
│   └─────────────────────────────────────────────────────────────────────────┘                    │
└──────────────────────────────────────────────────────────────────────────────────────────────────┘

53.1. Coolant Temperature Sensor Architecture (`G2` / `G62`)#

The 1.8T 20V engine utilizes a single physical sensor housing containing two isolated NTC thermistor elements located in the plastic coolant distribution flange on the transmission side of the cylinder head:

G2 (Cluster Element - Pins 1 & 2): Connects directly to the instrument cluster (J285). Designed with a wide plateau buffer in cluster firmware that displays an artificial 90\text{°C} (194\text{°F}) whenever actual coolant temperature is anywhere between 75\text{°C} and 107\text{°C} to avoid alarming the driver.
G62 (Engine Management Element - Pins 3 & 4): Connects to ECU Pin 93 (signal ground on Pin 108). Delivers an unbuffered, unadulterated analog voltage to an internal 10-bit ADC inside ME7.5.

Voltage to Temperature Steinhart-Hart Transfer Function#

The NTC resistor drops in resistance as temperature rises:

Temperature (T_{\text{coolant}})	Resistance (R_{G62})	Voltage at Pin 93 (V_{\text{sig}})	ECU Thermal Mode
-20\text{°C} (-4\text{°F})	15,000\ \Omega	4.45\text{ V}	Extreme Cold-Start Enrichment (`FAW` active)
+20\text{°C} (+68\text{°F})	2,500\ \Omega	3.05\text{ V}	Ambient Cold Start (`FST` active)
+80\text{°C} (+176\text{°F})	320\ \Omega	0.85\text{ V}	Warm-Up Transition Complete
+90\text{°C} (+194\text{°F})	240\ \Omega	0.65\text{ V}	Normal Operating Thermal Equilibrium
+110\text{°C} (+230\text{°F})	140\ \Omega	0.40\text{ V}	Over-Temperature Emergency Retard (`KFZWW`)

53.2. Fan Control Module (`J293`) & Auxiliary Air Conditioning Integration#

On transverse platforms (VW Golf MK4, Jetta, Audi A3), cooling fan speeds are managed by the solid-state J293 Fan Control Module mounted under the vehicle battery tray:

Low Speed (Stage 1): Both the 345\text{ mm} primary radiator fan and the 290\text{ mm} A/C condenser fan run in series/low-power mode when:
- Coolant temperature exceeds 105\text{°C} (221\text{°F}).
- The A/C refrigerant high-side pressure thrust sensor (G65) detects pressure exceeding 16\text{ bar} (232\text{ psi}).
High Speed (Stage 2): Fans switch to full parallel 12V high-power mode (450\text{W} total electrical load) when:

Climatronic Pre-Emptive Torque Reserve Feedforward#

When the air conditioning compressor engages, it imposes an immediate 15\dots 25\text{ Nm} parasitic mechanical torque load on the serpentine belt:

If uncompensated, engine speed would stumble by 200\text{ rpm} or stall at idle.
Via drivetrain CAN bus frame 0x288, the Climatronic module informs ME7.5 before commanding the clutch relay.
ME7.5 opens the electronic throttle plate (G186) by +1.2\text{°} to pre-charge the intake manifold with air, while temporarily holding idle spark timing retarded.
The instant compressor clutch N25 engages, ME7.5 advances ignition timing back to nominal, absorbing the mechanical drag with zero RPM drop!

53.3. Electronically Mapped Thermostats (`F265` / `KFK`)#

Certain late-model 1.8T engines (such as the 190 PS BEX in the Audi A4 B6) incorporate an Electronically Controlled Thermostat (F265):

Features an internal electrical heating resistor embedded inside the wax expansion element.
Part-Load / Cruise Mode: The ECU leaves the heater unpowered. The thermostat opens mechanically at 105\text{°C}. The higher operating temperature reduces engine oil viscosity, lowers hydrodynamic piston skirt friction, and improves fuel economy by 2\dots 3\%.
High-Load / WOT Mode: The ECU pulses the heater via PWM on ECU Pin 105 based on the calibration map KFK (Kennfeld Kennfeldkühlung). The artificial heat forces the thermostat to open wide, dropping cylinder head temperature to 85\text{°C}.
This cooler cylinder head temperature increases the detonation threshold, allowing +3.0\text{°}\dots +4.5\text{° KW} of additional ignition timing advance without knock!