Page 1 of 6 DTC P1518 Throttle Actuator Control (TAC) Module Serial Data Circuit Circuit Description The throttle acuator control (TAC) module and the powertrain control module (PCM) communicate via a dedicated serial data circuit. This serial data circuit is separate from any other serial data circuit on the vehicle. Accurate transmitting and receiving of serial data requires not only good circuit integrity, but also adequate system voltage. This diagnostic monitors the accuracy of the serial data transmitted between the TAC module and the PCM. If a loss of data or invalid data is detected, this DTC will set. Conditions For Running the DTC The ignition is in the crank or run position. The ignition voltage is more than 5.23 volts. Conditions for Setting the DTC Invalid or missing serial data messages are detected for a predetermined amount of time. Above condition met for less than 1 second. Action Taken When the DTC Sets The PCM will illuminate the malfunction indicator lamp (MIL) and the Reduced Engine Power light will be on when this diagnostic fails. The PCM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records. If no other TAC System DTCs are set, the TAC System will operate in Reduced Engine Power mode, and this DTC will be stored. If certain TAC System DTCs are set at the same time, the TAC System will either default to a more tightly restricted mode of operation if it is determined that limited safe operation is possible, or command engine shut down.
Page 2 of 6 Conditions for Clearing the MIL/DTC The PCM will turn the MIL OFF after 3 consecutive drive trips that the diagnostic runs and does not fail. 40 consecutive warm-up cycles has occurred A History DTC will clear after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic. A last test failed (current DTC) will clear when the diagnostic runs and does not fail. PCM battery voltage is interrupted. Important: The clear DTC info function may have to be performed twice in order to clear a throttle actuator Control System DTC. Using a scan tool Diagnostic Aids Inspect for the cause of intermittents. An intermittent may be caused by the following conditions: - Poor connections - Corrosion - Misrouted harness - Rubbed through wire insulation - Broken wire inside the insulation - Water intrusion Inspect for intermittent connections at the TAC module power and ground circuits and the TAC Module/PCM serial data circuits. Inspect the TAC module connectors for signs of water intrusion. When this occurs, multiple DTCs should be set and no circuit or component problems can be located. Inspect for adequate terminal tension. Refer to Wiring Repairs in Wiring Systems. Inspect for further causes of intermittents. Refer Symptoms. If a repair is completed and the DTC was cleared using a scan tool on the same ignition cycle, the default action will not clear until an ignition cycle has occurred. Test Description The numbers below refer to the step numbers on the diagnostic table. 2. 16. If this DTC does not Fail This Ignition, continue to monitor this DTC status while moving related harnesses and connectors. Refer to Diagnostic Aids if the harness movement causes this DTC to set. At this point you have ruled out the harness as the cause of the problem. Since there is no method to verify the transmission of serial data with commonly available tools, it is most cost effective to replace the TAC module first. Refer to Electronic Throttle Actuator Control Module Replacement If the fault is still present after replacing the TAC module, reinstall the original TAC module and replace the PCM. Refer to Powertrain Control Module Replacement.
Page 3 of 6 19. When a problem is detected within the TAC System, more than one TAC System related DTC may set. This is due to the many redundant tests run continuously on this system. It is possible that locating and repairing one individual problem may correct more than one DTC. Keep this in mind when reviewing captured DTC info. DTC P1518 - PCM to TAC Module Serial Data Circuit Step Action Values Yes No 1 Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? 2 Go to Powertrain On Board Diagnostic (OBD) System Check 2 Turn OFF the ignition for 15 seconds. Start the engine. Monitor the Fail This Ignition option under the Diagnostic Trouble Code (DTC) option using the scan tool. Did this DTC fail this ignition? 3 Go to Diagnostic Aids 3 Turn OFF the ignition. Disconnect the TAC module harness connector that contains the serial data circuits. Turn ON the ignition, with the engine OFF. Measure the voltage from the TAC Module ignition feed circuit in the TAC module harness connector to battery ground using a DMM. B+ Does the DMM indicate the specified voltage? 4 9 4 Turn OFF the ignition Inspect the continuity of the TAC module ground circuit from the TAC module harness connector to chassis ground Is there continuity? 5 10 5 Turn ON the ignition, with the engine OFF. 1.0-4.0 V 14 6
Page 4 of 6 Measure the voltage from both of the serial data circuits in the TAC Module harness connector to battery ground Is the voltage within the specified range on both circuits? 6 Turn OFF the ignition. Leave the TAC module disconnected. Disconnect the PCM connector. Refer to Powertrain Control Module Replacement. Turn ON the ignition, with the engine OFF. Inspect the harness for a voltage on both of the serial data circuits Is any voltage present? 11 7 7 Check both of the serial data circuits for continuity from the TAC module harness connector to the PCM harness connector Is continuity indicated on both circuits? 8 12 8 Inspect for continuity between each of the serial data circuits and a ground Is continuity indicated on either circuit? 13 16 9 Repair the open in the TAC module ignition feed circuit. 20 10 Repair the open in the TAC module ground circuit. Refer to Wiring Repairs in Wiring Systems. 20 11 Repair the short to voltage on the affected serial data circuit. Refer to Wiring Repairs in Wiring Systems. 20
Page 5 of 6 12 Repair the open/high resistance on the affected serial data circuit. Refer to Wiring Repairs in Wiring Systems. 20 13 Repair the short to ground on the affected serial data circuit. Refer to Wiring Repairs in Wiring Systems. 20 14 Inspect for poor connections/terminal tension at the TAC Module harness connector. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? 20 15 15 Replace the TAC Module. Refer to Electronic Throttle Actuator Control Module Replacement. 20 16 Replace the TAC Module with a known good unit. Select the Clear DTC Information option under the Diagnostic Trouble Code (DTC) option using a scan tool. Idle the engine at normal operating temperature. Select the Specific DTC option under the Diagnostic Trouble Code (DTC) option using a scan tool. Enter DTC P1518. Operate vehicle within the conditions for setting this DTC as specified in the supporting text. Does the scan tool indicate that this diagnostic ran and passed? 21 17 17 Re-install the original TAC module. 18
Page 6 of 6 18 Inspect for poor connections/terminal tension at the PCM harness connector. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? 20 19 Important: Program the replacement PCM. 19 Replace the PCM. Refer to Powertrain Control Module Replacement. 20 20 Select the Diagnostic Trouble Codes (DTC) option and the Clear DTC Information option using the scan tool. Idle the engine at the normal operating temperature. Select the Diagnostic Trouble Codes (DTC) option and the Specific DTC option, then enter the DTC number using the scan tool. Operate the vehicle within the Conditions for Setting the DTC as specified in the supporting text, if applicable. Does the scan tool indicate that this diagnostic ran and passed? 21 2 21 Select the Capture Info option and the Review Info option using the scan tool. Does the scan tool display any DTCs that you have not diagnosed? Go to the applicable DTC table System OK