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australia:car_electrics

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car electrical systems

see also:

Introduction

  • when you start the car, the switch activates the starter relay and the Engine Control Unit (ECU) which turn on the circuits from the car battery to drive 3 main systems initially (each of these usually has a separate fuse in the fuse box):
    • starter motor to crank the engine via the fly wheel
      • the starter relay is an electrical switch that controls power flow from the battery to the starter solenoid and motor when the ignition key is turned to the “Start” position
        • receives a low-current control signal when the ignition switch is turned to “Start”.
        • passes high-current power from the battery to the starter solenoid, engaging the starter motor
        • isolates the high current required by the starter from the ignition switch itself for safety and longevity
        • if the starter relay fails, the vehicle may not crank at all, even though other electrical systems (lights, radio, etc.) function normally
      • this may be prevented from starting if there are issues with the engine immobiliser security system
      • this may fail if the solenoid for the start motor is faulty
      • this requires an initial very high current of ~400A to overcome the inertia of the fly wheel
    • fuel pump to send petrol into the engine
      • turning the key to “ON” signals the Engine Control Unit (ECU) to energize the fuel pump relay momentarily, allowing current from the battery to power the fuel pump
        • there is a separate ECU fuse to the starter motor relay fuse in most vehicles although this fuse may also be used for other systems
        • the ECU determines when to activate the relay based on ignition and engine status
        • the fuel pump relay serves as the main switch, completing the circuit to power the pump only when needed
        • the pump will also briefly run when the key is first turned to “ON” to prime the system, even without cranking
      • when the engine starts cranking, the ECU continues to ground the relay, keeping the pump running
      • the pump runs continuously as long as the engine is running and the ECU detects RPM; otherwise, it shuts off for safety
    • ignition to send power to the spark plugs to ignite the compressed fuel in the cylinder which has been compressed by the piston driven up by the starter motor
      • this is done in sequence to each spark plug via the ignition coils and distributor
      • the spark plug ignition is activated and precisely controlled by an electronic ignition system that centers around the ECU, ignition coils, and electronic sensors
      • when the ignition key is turned and the engine is cranking or running, the ECU uses signals from the crankshaft and camshaft position sensors to determine the exact timing for ignition
      • the ECU sends a low-voltage signal to the ignition coil for the appropriate cylinder.
      • the coil rapidly builds a magnetic field, then collapses it, converting the battery's 12V into a high-voltage pulse (typically 20–40 kV) across the spark plug
      • this high voltage is sent via a spark plug wire (or directly if coil-on-plug) to the spark plug, causing a spark at precisely the right moment to ignite the air-fuel mixture in the cylinder
      • most modern cars have a fully electronic ignition system — there is no mechanical distributor as with older cars
        • each ignition coil is activated for its corresponding cylinder by input signals from the ECU
        • faults in the ignition coil or ECU signals may cause misfires or no-start conditions
  • once the engine has started, the engine drives the alternator which provides ongoing electrical supply to the vehicle's needs as well as recharging the battery
  • modern vehicles have a multitude of sensors connected to BUS networks which connect to the vehicle's computer system which monitors and adjusts systems as needed

OBD2 scanners

  • the OBD2 is a 16 pin wired plug port to allow external OBD2 scanners to analyze the system's sensor status and reset errors
  • the Vehicle Connection Interface (VCI) which plugs into the port can be wired or Bluetooth enabled
  • there are many on the market - not all of them work well
  • the better ones are bidirectional so you can clear error codes and even activate certain functions
  • the newer ones also have a built-in AI assistant
  • BEWARE: some charge annual subscription fees for updates and you NEED UPDATES for newer vehicles
  • they will generally NOT have every function for every vehicle due to restrictions by vehicle manufacturers
    • Not all vehicles support all reset functions.
    • Some brands require AutoAuth or FCA (a subscription service) to allow clearing codes, or perhaps even access to diagnostics (eg. Chrysler series, Maserati series, Fiat series, Renault series, Nissan series, Mercedes-Benz series, Volkswagen series.) in addition you may need a special adapter to unlock the security gateway:
      • Chrysler 12+8 adapter will be needed for most Chrysler, Ram, Alfo Romeo, Fiat models.
      • Nissan 16+32 adapter cable for B18 Nissan Sylphy, Sentra
      • Mercedes-Benz W206 W223 chassis vehiclesC series/S series models after 2020
  • Error codes:
  • vehicles older than 2006 may not have compatible ports or systems
    • eg. Toyotas older than 2006 may have a 22-pin plug, and will likely run the older OEM Toyota protocol (MOBD)
    • petrol vehicles from 2006 onward are required to have OBD2 compliance
    • electric vehicles (EVs) generally have an OBD2 port, but their OBD2 implementations can differ from traditional internal combustion engine vehicles and ODB2 devices and apps may not be compatible

example Bluetooth AI OBD2 scanners

example smartphone BT OBD2 scanners

Electric bus networks

CAN bus

  • allows reduction in wires and connectors throughout the car
  • it consists of a pair of twisted wires - “CAN Hi” and “CAN Lo”
    • in general there are sequences of square wave pulses sent from each device connected
    • C-CAN BUS / “PowerTrain CAN”
      • the square wave pulse voltages range from a base of 2.5V to a peak of 3.5V for CAN-Hi where as on CAN-Lo the square wave voltages will drop from 2.5V base to 1.5V for each signal
      • this is a high speed BUS with the signals being sent at between 125kbps - 1000kbps
      • these two wires are connected to each other at each end via a 120 Ohm terminal resistor on each end giving a total resistance of 60 Ohm as they are in parallel - usually one is inside the ECU but the location of the other one varies between car models
      • there is no single control unit which maintains the CAN-BUS but rather each device connected to it has a control unit that can function independently of other units and send and receive data via the CAN-BUS - hence it is a Multi-Master Network
      • on the scan tool, C-CAN HI is usually OBD 6 and C-CAN-LO is usually OBD 14 which is opposite in position
        • can use a multimeter to check 1):
          • total resistance by putting pins in 6 and 14 and comparing them with ignition OFF - if it reads 120 Ohm instead of 60 Ohm - one of the resistors may be disconnected or faulty
          • voltage by putting pin in either 6 OR 14 with other probe on a good GND and ignition ON - they both should read around ~2.6V on Hi and ~2.3V on Lo
          • check short to GND - ignition OFF, put pin in either 6 OR 14 with other probe on a good GND and do a continuity check - there should NOT be any continuity with GND for either
        • can use an oscilloscope:
          • pos probe pin into 6 or 14 and neg probe to car GND, turn ignition ON, this will should the saw wave voltages pulse signals being sent
            • voltages close to 0V instead of a base of 2.5V suggests there is a short to GND somewhere on that BUS
      • example:
      • engines may have one crankshaft sensor which sends the all important revs per minute status of the engine to the rest of the vehicle, via the Engine control unit (ECU) and thence via the CAN-BUS to various control units, each of which just tap into the CAN-BUS in parallel for this information in a bi-directional manner without needing a direct dedicated wire to the sensor or to the ECU:
        • electronic power steering unit
        • transmission control unit
        • ABS brakes
        • ESP
        • EPS
        • 4WD / AWD unit if one is fitted
        • air conditioning unit
        • driver's instrument panel / cluster Gateway
        • smart key module
        • DLC / oBd2 connector for scan tool
    • LIN-BUS (Local Interconnect Network)
      • a specific low-cost, low-speed serial communication protocol used primarily for connecting various electronic components in vehicles.
      • typically employed in non-critical, localized control functions such as window control, door locks, mirror adjustment, seat controls, lighting, and climate control
      • uses a master-slave architecture over a single wire and is designed to be simpler and cheaper than high-speed buses like CAN-BUS
        • the ECU generally has a master node
        • the battery sensor is a slave node
      • voltage should be close to 11-12V with signals dropping by ~600mV in a square wave similar to a CAN-LO but different voltages and speed
    • Body Electrical CAN BUS
      • mainly for communicating between control units within the car itself but some of these will also connect to C-CAN usually via the cluster Gateway which converts the signals from one bus type to another
      • lower data transmission speed
      • can be a LIN-BUS, CAN-BUS design or combination
      • example connections:
        • driver's instrument panel /cluster Gateway
        • smart key module (SMK)
    • K-line
      • a diagnosis communication line to link the DLC / oBd2 connector for scan tool which is on C-CAN to the Body CAN
      • single wire for data - similar to LIN-BUS
      • on the scan tool, K-Line is usually OBD 12 and Chassis GRN is usually OBD 4 which is opposite in position (Signal GND is usually OBD 5)
    • M-CAN / Multimedia CAN
      • often used for audio, amplifier
      • connects to gateway (eg. A/C or navigation) to the C-CAN
        • body control module (BCM) - eg. auto door lock/unlock
      • on the scan tool, M-CAN HI is usually OBD 1 and M-CAN-LO is usually OBD 16 which is opposite in position
1)
https://www.youtube.com/watch?v=bvyHx-C2Xfs
australia/car_electrics.txt · Last modified: 2025/10/02 07:50 by gary1
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