P1518

CIRCUIT DESCRIPTION
The throttle actuator 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 the PCM detects a loss of data or invalid data, this diagnostic trouble code (DTC) sets.

CONDITIONS FOR RUNNING THE DTC
^ DTC P1517 is not set.
^ The ignition switch is in the crank or run position.
^ The ignition voltage is greater than 5.23 volts.

CONDITIONS FOR SETTING THE DTC
^ Invalid or missing serial data messages are detected for a predetermined amount of time.
^ All of the above conditions are met for less than 1 second.

ACTION TAKEN WHEN THE DTC SETS
^ The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
^ The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame and/or the Failure Records.
^ The control module commands the TAC system to operate in the Reduced Engine Power mode.

OR

^ Under certain conditions the control module commands the engine OFF.
^ The message center displays Reduced Engine Power.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
^ A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
^ A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
^ Clear the MIL and the DTC with a scan tool.

DIAGNOSTIC AIDS
^ DTC P1518 sets if the battery voltage is low. If the customer's concern is slow cranking or a lack of cranking because battery voltage is low, ignore DTC P1518. Clear any DTCs from memory that may have set from the low battery voltage condition.
^ DTC P1518 sets if there is a short to B+ on the TAC module ground circuit. Inspect the fuses for the circuits that are in the TAC module harness, such as the cruise, and brake circuits. An inspection of the fuses may lead you to the circuit that is shorted to the TAC module ground circuit.
^ DTC P1518 sets if the TAC module ignition feed circuit is shorted to a B+ supply circuit. The TAC module stays powered-up when the ignition switch is turned OFF. When the ignition switch is turned ON, the TAC module is powered-up before the PCM. DTC P1518 sets because communication is not detected by the TAC module from the PCM. Inspect related circuits for possible shorts to the B+ supply circuit.
^ Inspect the TAC module power and ground circuits and the TAC module/PCM serial data circuits for intermittent connections.
^ Inspect the TAC module connectors for signs of water intrusion. When this occurs, multiple DTCs could be set without circuit or component conditions found during diagnostic testing.
^ When the TAC module detects a condition within the TAC system, more than 1 TAC system related DTC may set. This is due to the many redundant tests run continuously on this system. Locating and repairing 1 individual condition may correct more than 1 DTC. Disconnecting components during testing may set additional DTCs. Be aware of this if you review the stored information in Capture Info.
^ For an intermittent condition, refer to Intermittent Conditions. Intermittent Conditions

TEST DESCRIPTION

Steps 1-6:

Steps 7-14:

Steps 15-23:

Steps 24-29:

The numbers below refer to the step numbers on the diagnostic table.
5. Increasing the engine speed to 3,000 RPM aids in locating a shorted TAC control circuit. Depending on the polarity of the TAC transistors, this DTC may not set with a fault in the control circuits. The TAC motor is a bi-directional DC motor. Raising the engine speed changes the polarity of the transistors in the TAC motor. This occurs because one set of the transistors are low, around 0 volts, and the other set are high, B+. Therefore, if one set of transistors is at a low voltage, and the corresponding circuit is shorted low, DTC P1518 will not set. When the polarity of the transistors change, this DTC sets. If this DTC does not Fail This Ignition, continue to monitor this DTC status while moving the related harnesses and connectors.
29. When the TAC module detects a condition within the TAC system, more than 1 TAC system related DTC may set. This is due to the many redundant tests run continuously on this system. Locating and repairing 1 individual condition may correct more than 1 DTC. Disconnecting components during testing may set additional DTCs. Be aware of this if you review the stored information in Capture Info.