Arduino GSM module does not work when USB to TTL module is unplugged !!!!!!!

Ive connected gsm module on my custom arduino board so that i can send command SMS commands to arduino and it execute some action or turn ON or OFF pin. It works perfectly when i connect CP2102(USB TO TTL )module(as serial interface) to this board SMS command also work. BUT when i remove USB TO TTL module and restart arduino it doesnt work ie …not accepts my commands

Here is my code::::

#include <SoftwareSerial.h>
SoftwareSerial SIM900(10, 11);
String textMessage;
#define LED1 2   
#define VALVE1 7
#define GSMRESET 8   
#define VALVE2 12
#define TANK_SENS4 A0  //full tank wigh buzzer
#define TANK_SENS3 A1
#define TANK_SENS2 A2
#define TANK_SENS1 A3  //low tank
int TKLVL = 0;
int LEVEL;    
int val ; 

void setup()
{ pinMode(LED1, OUTPUT);
  digitalWrite(LED1, LOW);     

  pinMode(GSMRESET, OUTPUT);
  pinMode(VALVE2, OUTPUT);
  pinMode(TANK_SENS4, INPUT); //FULL TANK WITH BEEP BUZZER
  pinMode(TANK_SENS3, INPUT);
  pinMode(TANK_SENS2, INPUT);
  pinMode(TANK_SENS1, INPUT); //LOW TANK WATER
  // digitalWrite(TANK_SENSPOWER, HIGH); //turns OFF tanksenspower OFFvia PNP transistor..
  Serial.begin(9600);
  SIM900.begin(9600);
  Serial.print("SIM900 ready...");
  SIM900.println("AT+CMGD=1,4\r");
  delay(1000);
  // AT command to set SIM900 to SMS mode
  SIM900.println("AT+CMGF=1\r");
  delay(400);
  // Set module to send SMS data to serial out upon receipt
  SIM900.println("AT+CNMI=2,2,0,0,0\r");
  delay(400);
  digitalWrite(VALVE1, LOW);

}

void loop()
{
  if (SIM900.available() > 0) {
    textMessage = SIM900.readString();
    Serial.print(textMessage);
    delay(10);
  }

  if (textMessage.indexOf("Von") >= 0)
  {
    G_valve1On();
    textMessage = "";
  }
  if (textMessage.indexOf("Voff") >= 0)
  {
    G_valve1Off();
      textMessage = "";
  }      

}

//-----------------------------------------------------------

void G_valve1On()
{
  digitalWrite(VALVE1, HIGH);
  Serial.println("...turning valve1 on...");
}
void G_valve1Off()
{
  digitalWrite(VALVE1, LOW);
  Serial.println("...turningvalve1 off...");
}[code]

[/code]

It works perfectly when i connect CP2102(USB TO TTL )module(as serial interface) to this board

What works? Be as specific as possible.

BUT when i remove USB TO TTL module and restart arduino it doesnt work

What doesn't work? Be as specific as possible.

ie ..not accepts my commands

If you've unplugged the module, how can you expect it to still communicate with the Arduino?