PostgreSQL Trigger and rows updated

am trying to update a table according to this trigger :

    CREATE TRIGGER alert
    AFTER UPDATE ON cars
    FOR EACH ROW
    EXECUTE PROCEDURE update_cars();

Trigger Function :

    CREATE FUNCTION update_cars()
    RETURNS 'TRIGGER'
    AS $BODY$
    BEGIN
    IF (TG_OP = 'UPDATE') THEN
    UPDATE hello_cars SET status = new.status
    WHERE OLD.ID = NEW.ID;
    END IF;
    RETURN NULL;
    END;
    $$ LANGUAGE plpgsql;

the trigger works fine. when cars table updated, the hello_cars table
updated but status column in each row is updated and contains same new
status ! it must be updated according to car ID.
i think my problem is in condition : WHERE OLD.ID = NEW.ID;
but can't tell what's wrong exactly

Thanks in advanced

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