ECU Sensors, Inputs and Outputs

Inputs used by the VR4 ECU

The VR4 ECU has a number of sensors it uses to allow safe operation of the engine.

Sensor/Input	Acronym	Purpose	Type	Location	Notes
Crank Angle Sensor	CAS	Measures rotational speed of the engine	3-wire 5V Pulse	Behind Crank Pulley	This is not a notched sensor. TDC can only be detected via combination of this signal and the cam pulse signal. Chopper wheel has a rising edge every 120 degrees for a duration of 45 degrees.
CAM Position Sensor	CPS	Triggers Top Dead Center of cylinders	3-wire 5V Pulse	Behind Front exhaust cam sprocket	Measures the rotation pulses of the front intake camshaft. This is used in combination with the pulses from the Crank Position Sensor to determine at what exact position the engine is in its rotation. Chopper wheel has a rising edge every 90 degrees with a duration of 30 degrees, with the exception of TDC which has a duration of 60 degrees.
Mass Airflow Sensor	MAF	Measures airflow into the engine	5V PWM on 5-wire connector	Just past airbox	Measures mass of air entering the engine. Sensor is karmen vortex type. Details of the sensor are details in patent: https://patents.google.com/patent/US5728946
Intake Air Temperature Sensor	IAT	Measures intake air temperature (before turbos)	5V analogue on 5-wire connector	Inside MAF assembly	Used as part of airflow calculations for air mass, but also used for timing trims.
Barometric Air Sensor	BARO	Measures intake air pressure	5V analogue on 5-wire connector	Inside MAF Assembly	Used as part of airflow calculations for air mass, but also used for timing trims.
Engine Coolant Temperature Sensor	ECT	Measures engine coolant temperature	2-wire 5V analogue	Screwed into thermostat housing	This sensor is not used to drive the dashboard temperature gauge. That is handled via a separate 1-wire sensor mounted next to the ECT sensor.
Knock Sensor	Knock	Detects engine detonation (knock/ping/pre-ignition)	One-wire piezo-electric microphone	Screwed into top of block, between "V" of engine (underneath intake runners)	Critical to avoid engine damage. If the ECU detects knock, it will retard the ignition and add fuel based on how much knock it detects. Knock Detection is fairly complex and uses an algorithm to convert the anologue sound signal into a digital rating between 1 and 35.
Throttle Position Sensor	TPS	Used to measure the position of the throttle plate in a percentage of how open or closed the throttle is	3-wire 5V analogue	Mounted on the side of the throttle plate	The TPS can be used as a backup airflow metering system (aka "Alpha-N" airflow measurement) if the MAF fails, however this is essentially a limp mode. The TPS is also used to detect the rate of change of the throttle position, so that extra fuel can be added to the engine to avoid transient lean conditions and therefore provide better throttle response. This process is analagous to an accelerator pump on a carburetor.
Speed Sensor	VSS	Measures vehicle speed	3-wire 5V Pulse	Screwed into the top of the gearbox (connected to the gearbox output shaft)	Vehicle speed is data seen by the ECU, with the pulses also sent to the speedometer in the dashboard cluster. Vehicle speed is used in the custom ROM files that use launch control (two-step), available from Transformance Mechanical Engineering. 4 pulses per revolution of sensor as square wave output.
Narrowband Lambda Sensor	O2 sensor	Measures if the air/fuel mixture is rich, lean, or stoichiometric (14.7:1 AFR/1.0 Lambda)	4-Wire 1 volt analogue	Screwed into the exhaust just past the catalytic converter	The stock O2 sensor is narrowband, which means it can only be used to identify if the air fuel ratio (AFR) is close to stoichiometric. It is only used when cruising, which puts the ECU into "closed loop mode", whereby it adjusts the injector pulses more/less to hit the target number. Once accelerating, the ECU goes into "open loop" mode and ignores the readings from the O2 sensor, relying on its volumetric efficiency tables which are pre-defined in the ECU based on a given airflow/rpm combination.
Ignition Failure Sensor	IGF	Monitors ignition coil primary winding pulses and detects when a coil does not fire	4-wire 12v analogue	Mounted on top of front head valve cover.	The Ignition Failure Sensor is often considered "three lies for the price of one" as coil related misfires have not been seen to raise trouble codes in the ECU. Rather, the IFS itself is known to commonly fail. The IFS is in-circuit for power to the coils and is the source of the tachometer signal, hence why failures in this module can stop the tachometer, the coils, or both, from functioning. IFS Failures result in one of two failure states: Tachometer stops working, engine keeps running Tachometer stops, followed by the engine stopping and unable to be restarted. Refer to the article on byassing or removing the IFS:Ignition Failure Sensor
Air conditioner compressor request		Signal from HVAC system to enable the AC compressor	1-wire 12v	A/C ECU	HVAC can request the ECU to turn the A/C Compressor on
Alternator					Smart alternator, ECU can control mechanical load by varying alternator output current.
Power Steering load switch		Signal from powersteering pump to allow ECU to increase idle	Powersteering Pump
MAF Reset

Sensors NOT used on the VR4

Whilst being a turbocharged car, the VR4 does not use a Manifold Absolute Pressure (MAP) sensor. Boost control is performed via a "target airflow" table in the ECU map. This map is used to dictate control over the wastegate control solenoid which controls the wastegates on the front and rear turbos together.

This is likely the same way that the Lancer Evolution of the same generation manages boost pressure, however as those cars have boost gauges, a MAP sensor is needed from factory.

The VR4 ECU also has no other boost-related measurement, and does not measure:

• Pre-Intercooler temperature
• Post-Intercooler temperature (aka Charge Air temperature)
• Turbine speed
• Exhaust Gas Temperature (EGT)

Outputs from the VR4 ECU

Based on data collected from sensors, the VR4 ECU controls the engine via a number of outputs.

ECU Outputs list

Output	Purpose	Type	Location	Notes
Injectors (x6)	Injects fuel into each cylinder	2-wire ground-switched 12V	Inserted into the intake runners	There are six individual injectors, one for each cylinder. The ECU uses sequential injection, only firing each injector in one pulse for each intake stroke of the relevant cylinder.
Coils (x3)	Connects to spark plugs and ignites the air/fuel mixture in each cylinder	3-wire 5V triggered	On top of the front cylinder bank	The stock coil setup on the VR4 is three coils with each coil driving two separate spark plugs/cylinders. This is a "wasted spark" or "batch fire" ignition setup where each spark plug fires on the compression stroke and on the exhaust stroke (e.g. "wasted"). Coil on Plug (CoP) kits add an additional three coils so that each cylinder has its own coil, but if using the stock ECU the setup still uses a wasted spark model.
Fuel Pump	Turns on the fuel pump and switches between low and high speed on the pump		Fuel pump located under driver's side of rear seat. Fuel pump relay and speed control resistor located on passenger side firewall in the	Fuel pump is dual speed. Power for fuel pump runs through resistor block at low load, and switches to full 12v power over approx. 80% load.
Idle Stepper Motor (ISC)	Controls the idle speed when the throttle is not depressed	4-wire 5V PWM	On the lower part of the throttle body, underneath the intake elbow	The ISC is a stepper motor with a number of positions which can be controlled via signal from the ECU. The ECU monitors the engine speed at idle and will adjust the ISC stepper motor position to obtain the desired idle speed. Note that if there are massive air leaks or other issues, the ISC may be unable to maintain idle and the car can still stall. It's capabilities are limited and not intended to overcome engine problems.
Cooling fans	Switches the cooling fans on and off, and between high and low speeds	4-wire relay-switched power control and 5V PWM speed control	On top of fan shroud, passenger side of the car	Based on engine temperature, vehicle speed, and the status of the air conditioning compressor (on or off), the ECU will switch on one or both of the radiator fans, and at high or low speed. Higher temperatures will result in the fans being on at higher speed. When driving, the fans will not come on unless temperatures in the engine reach high levels.
Fuel Pressure Regulator solenoid	Bumps up the idle fuel pressure	2-wire 12V ground switched	Bolted to passenger side of rear head, underneath the throttle body.	The ECU can artificially increase Fuel pressure by disabling the vacuum line between the regulator and the intake. This solenoid is commonly disabled as it has a high failure rate and does not materially improve idle performance.
Wastegate Control solenoid	Controls how much boost pressure is applied to the wastegates in the turbochargers	2-wire 12V ground switched PWM	Bolted to passenger side of rear head, underneath the throttle body (next to FPR solenoid).	Commonly replaced with a GM 3-port wastegate solenoid and valve which improves accuracy of wastegate control, particularly for higher boost levels.
Air Conditioner	Controls whether the AC compressor clutch is activated or disengaged.	Relay	AC relay is located in the main fuse/relay compartment in the engine bay, on passenger side.	The ECU recieves requests to turn the compressor from the HVAC system, but the ECU is what controls AC compressor clutch engagement. This is so the ECU can disengage the compressor based on engine load and TPS. When the AC compressor request is made and the clutch is activated, the ECU brings up the idle speed and, depending on vehicle speed, turns on one of the cooling fans. Use of the air conditioning at low speeds always results in one cooling fan being turned on, so as to provide a cooling airflow through the AC condensor (mounted in front of the radiator). Without the cooling fan active, the AC system can disengage due to high pressure in the AC coolant lines caused by overheating of the AC refrigerant gas. This is known as "high head pressure", and is a mechanical safety mechanism built into the compressor.

ECU pinout

The following image was taken from Merlin's VR4 tuning guide and will be replaced with a text table.

Note: This diagram does not show the IFS "IGNition Feedback" (IGF) signal wire.

VR4 ECU pinout