For 1990-2009 cars

#### Theft Deterrent Diagnostic Circuit

### Circuit Description

The Theft Deterrent Module (Immobilizer) uses terminal 7 to send the vehicle speed signal and the frequency code (imo signal) to terminal 79 of the Engine Control Module (ECM).

When the ignition switch is turned to the ON position, a signal (transponder code) from the key is transmitted to the Theft Deterrent Module (TDM). During this period the ECM transmits a signal to the TDM requesting the frequency code (imo signal). The TDM compares the signal from the key and decides if an authorized key is being used. After verification, the TDM internally switches from the speed signal circuit and sends the imo signal to the ECM.

The ECM compares the received imo signal with the programmed stored signal. If both codes are the same, the engine control functions (fuel pump drive and injector signals) are released. Non matching codes block the engine functions and the car cannot be started.

### Conditions for Setting the DTC

1. DTC 16 indicates that a short to voltage exists in the speed/imo signal circuit (PA14/PA79.)
2. The above conditions must be fulfilled for at least 0.51 seconds.

### Action Taken When the DTC Sets

• | The engine starts then stalls.

• | The Theft Deterrent Module stores a DTC 16 in memory.

### Conditions for Clearing the DTC

• | The conditions for the fault are no longer present.

• | A history DTC will clear after 20 consecutive ignition cycles if the condition for the fault is no longer present.

• | The use of the scan tool.

### Diagnostic Aids

• | Using a DMM, check the ground circuit at terminal 4. Typical voltage drop readings are 100mV or less. When measuring the resistance it should measure at close to 0 (zero) ohms.

• | Use Terminal Repair Kit J 38125-A or J 35616-A to gauge all terminal contacts. Check for damaged connectors and poor wire to terminal connections.

• | If the DTC is an intermittent or history DTC, try performing the tests shown while "wiggling" wiring and connectors, this can often cause the malfunction to appear.

### Test Description

The following numbers refer to the step numbers on the diagnostic table:

1. This step checks if the Theft Deterrent Module transmitted the immobilizer signal.

2. This step checks for a short to voltage. The check is made with the ignition switch in the OFF position and then with the switch in the ON position.

3. This step rechecks for a current DTC 16.

4. The module may be at fault if no trouble is found in the power and ground circuits, or if no trouble is found with the connection at the Theft Deterrent Module.

#### DTC 16 Vehicle Speed Signal (Voltage High)

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Was the Theft Deterrent System Check performed? | -- | Go to Step 2 | Go to Theft Deterrent System Check
2 |

1. Connect the scan tool to the Data Link Connector (DLC).
2. Turn the ignition switch to the ON position.
3. Select IMMOBILIZER SIGNAL.

Does the scan tool display TRANSMITTED?

--

Go to Step 4

Go to Step 3

3 |

1. Turn the ignition switch to the OFF position.
2. Disconnect the wiring harness connector from the Theft Deterrent Module.
3. Disconnect the connector from the Engine Control Module (ECM).
4. Use the DMM to measure the voltage between terminal 7 of the Theft Deterrent Module connector and ground.
5. Turn the ignition switch to the ON position.
6. Measure the voltage at terminal 7.

Are both voltage readings within the specified value?

Less than 0.1 V

Go to Step 4

Go to Step 5

4 | Check the circuits between the Theft Deterrent Module and the ECM for possible intermittent malfunctions. Refer to Diagnostic Aids. Was a problem found? | -- | Go to Step 7 | Go to Step 6
5 |

1. Locate a short to voltage in the circuit between the Theft Deterrent Module terminal 7 and ECM terminal 79.
2. Repair the short to voltage.

Is the repair complete?

--

Go to Theft Deterrent System Check

--

6 |

• | Reassemble all connectors/components removed in previous steps.

• | Install the scan tool and clear codes.

• | Recheck for a current DTC 16.

Is a current DTC 16 present?

--

Go to Step 8

Go to Theft Deterrent System Check

7 | Locate and repair the intermittent malfunction. Is the repair complete? | -- | Go to Theft Deterrent System Check | --
8 |

1. Check the power and ground circuits.
2. Check for poor connections at the Theft Deterrent Module. Refer to Diagnostic Aids.
3. If no problems were found with the circuits replace the Theft Deterrent Module. Refer to Steering Wheel and Column On Vehicle Service.
4. Reprogram the Theft Deterrent Module. Refer to Programming Theft Deterrent Module.

Is the replacement complete and is the reprogramming complete?

--

Go to Theft Deterrent System Check

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