Starter Motor Technical Pages


Starter motor function - What it does:

Cranking : When electrical power is applied to terminal 50 of the starter motor solenoid, the solenoid relay main contacts close applying battery current to the starter motor.
The Vehicle engine starter motor engages with the engine flywheel ring gear causing the engine crankshaft to rotate from stationary until fuel ignition occurs, ie, engine started.

Automotive starter motors are mostly either permanent magnet or series wound direct current (dc) electric motors, though large industrial engines may use a heavy duty dc starter motor, or hydraulic, or pneumatic (air operated) starter motor , or even a seperate engine (donkey engine) to provide cranking.

Call 01743 884888 for starter motor supply enquiries

Starter Motor Fitting :

How to wire a typical Negative earth type Pre-engaged starter motor:
The pre engaged vehicle starter motor used on most modern cars and light commercial requires three connections;

  • B+  battery positive supply from battery to the B+ (terminal 30) of the starter motor solenoid. (this is normally a heavy gauge red cable on many vehicles)
  • Ignition switched trigger wire : to terminal 50 of the starter motor solenoid. This wire provides the power supply to energise the starter solenoid coil when the ignition switch is in the engine cranking position.
  • B- Battery negative : supply from the battery negative terminal to starter motor casing or engine block. (often a heavy guage braided cable)
  • Diagnosis  : List of possible starter faults




    Key On No Crank : Starter does not turn engine over when ignition switch in crank position

    Discharged battery due to defective battery ,alternator, wiring fault, component left switched on overnight.
    Loose or corroded battery terminals.
    Defective ignition switch.
    Defective wiring.
    Immobiliser function active due to incorrect key/transponder, faulty immobiliser.
    Defective Starter Solenoid.
    Starter motor brushes worn out or stuck, brush springs weak or broken.
    commutator dirty or worn
    Field coil windings defective.
    Broken Magnets on Permanent magnet starter motor

    Recharge and test battery . Get alternator dynamo regulator and wiring checked, tested , renewed or repaired as applicable.

    Clean battery terminals & check security.

    Check cable connections and wiring.

    Test ignition switch / relay / wiring & renew as required.

    Renew immobiliser key fob batteries if <2 years old. Have immobiliser operation checked (may require diagnostic equipment) and rectify key coding if required, or immobiliser removed and renewed (depending on system).

    Repair, renew, or overhaul starter motor as required.

    Key On No or Slow Crank - Starter solenoid clicks, starter engages, but will not turn engine,or turns slowly.

    Battery discharged.
    Terminals,contacts or wiring in poor or corroded condition causing resistance to current flow.
    Brushes worn out or sticking.
    Motor or bearings partially seized.
    Dirty or worn commutator.
    Field coil windings defective.
    Engine ignition timing incorrect.
    Engine seized or damaged

    Recharge or renew battery.Test alternator.
    Check wiring terminals,cabling,contacts.
    Repair,overhaul, or renew starter motor.
    Rectify engine fault as required.

    Noisy operation of starter pinion

    Starter drive gear defective.
    Starter mounting bolts loose.
    Engine flywheel ring gear teeth damaged.

    Overhaul,repair or renew starter motor as necessary.
    Check security of mounting bolts.
    Check flywheel ring gear and renew as necessary.

    Starter spins but does not engage

    Starter drive defective,starter solenoid defective, driveshaft dirty/corroded. Broken linkage in starter. Starter
    reduction gears stripped or damaged.
    Incorrect unit fitted for application.

    Overhaul, repair or replace motor as necessary.

    Starter continues to turn after ignition key is released, or remains engaged whilst engine is running.

    Defective ignition switch,relay,wiring, or starter solenoid contacts.

    Switch off engine immediately!
    Renew ignition switch, relay or repair starter motor as necessary. A starter motor stuck engaged whilst the engine is running may overheat and cause fire, therefore requires immediate attention.

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