Theory Of Operation

TIPM and WCM Operation 

The TIPM and WCM require two signals to verify the ignition is in the run or start position. The WCM receives a signal from the Ignition Switch on the G20 Ignition Switch Sense circuit which is a muxed voltage. At the same time the Ignition Switch supplies a 12 volt signal on the F20 circuit to the WCM and TIPM. This is a second signal to the WCM and one of the signals to the TIPM. The Ignition Switch status can be monitored in both the WCM and TIPM data display screens as the Ignition Run/Start under the "Sensor" value. The run or start position voltage on the F20 circuit should be at or close to battery voltage.

Once the WCM confirms the signals received on the F20 and G20 circuits, the WCM busses the Ignition Run/Start status to the TIPM. The TIPM monitors the CAN C bussed input information as the second signal/verification on the Ignition Run/Start status. The bussed messages can be monitored in both the WCM and TIPM data display screens.

The F20 and the bussed message information received by the TIPM regarding the Ignition Run/Start status must agree to enable starting. Once the vehicle is in operation, the loss of only one ignition signal will not cause the vehicle to turn off as long as the other feed still indicates a "run status".

The G20 circuit approximate key position voltages and status can be viewed in the data display screen of the WCM and compared to the values listed below:

• Open Circuit = 5v
• Key Out = 4.6v
• Key In /Lock = 4.04v
• Accessory = 3.2v
• Unlock = 2.2v
• Run = 1.12v
• Start = 0.41v
• Short/Grd = 0.00v

Diagnostic Aids 

The No Crank Diagnostic Test assumes all of the following to be true: 

• The condition is currently present.
• The TCM, PCM, WCM, and TIPM are at the latest calibration (part number) levels.
• There are no active DTCs.
• There are no applicable Recalls.
• There are no applicable TSBs
• There are no PCM inhibits (i.e. RPM, Trans Range, Clutch)
• The Keys/FOBs complete function and operation is correct.
• All battery cables and connections are clean and tight.
• There are no open fuses.
• The battery is fully charged and can pass a load test from the GR8 tester.
• The engine rotates freely.
• Any aftermarket equipment/wiring that has been added was installed and is operating correctly

Starter Relay Operation 

In a no crank condition the Starter Relay can be missing power at pin 85, feed at pin 30, controlled ground at pin 86 or output at pin 87. Further explanation of how a relay operates is listed below.

• Pin 85 receives power from the TIPM once the parameters for the Ignition Switch position and status, and all inhibit monitors (i.e. clutch switch, RPM, etc.) are satisfied.
• Pin 30 is a constant B+ feed.
• Pin 86 is a controlled ground by a module (i.e. PCM or BCM). Once power is sent from the TIPM to the pin 85 of the relay it passes through a coil and leaves the relay through pin 86 to control module.
• Pin 87 is an output circuit to the Starter solenoid. Once the circuit on pin 86 is grounded by the control module, the coil winding in the relay magnetically pulls the internal switch closed. When the internal switch is closed the power at pin 30 is sent to pin 87 of the relay.