Antitheft and Alarm Systems

DTC 18 MIL Circuit:

DTC 18 MIL Circuit (Part 1 Of 2):

DTC 18 MIL Circuit (Part 2 Of 2):

CIRCUIT DESCRIPTION
When the ignition switch is turned ON, the Engine Control Module (ECM) sends a signal over the lamp circuit to terminal 2 of the Theft Deterrent Module (Immobilizer) requesting the frequency code (imo signal).

After the Theft Deterrent Module (TDM) receives and recognizes the transponder code signal from the key, the module internally switches from the speed signal circuit and sends the imo signal to the ECM from terminal 7.The ECM receives the imo signal at terminal 79.

CONDITIONS FOR SETTING THE DTC
The TDM did not receive the request signal from the ECM after the ignition was switched On.
^ A short to ground or voltage in the imo request circuit.
^ An open in the imo request circuit.
^ All the above conditions must be fulfilled for at least 0.51 seconds.

ACTION TAKEN WHEN THE DTC SETS
The engine will start then stall.

CONDITIONS FOR CLEARING THE DTC
^ 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.
^ Using a 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 3561 6-A to gauge all terminal contacts.
^ Check for damaged connectors and poor wire to terminal connections.
^ If the DTC is an intermittent DTC, try performing the tests shown while "wiggling" wiring and connectors, this can often cause the malfunction to appear.

TEST DESCRIPTION
The number(s) below refer to the step number(s) on the diagnostic table.
2. The scan tool displays and indicates whether the Theft Deterrent Module (TDM) has received a request signal from the Engine Control Module (ECM).
8. If the DTC continues to reset and the circuits are OK, the possibility exists that either the TDM or the ECM may be at fault. The only way to to determine which one is by trail and error. Replace the TDM first.
10. After determining that the TDM is OK, replace the ECM.