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 test 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 DTC sets.
Conditions for Running the DTC
| • | The ignition switch 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. |
| • | The above condition is met for more 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. |
| • | A message center or an indicator displays Reduced Engine Power. |
| • | Under certain conditions the control module commands the engine OFF. |
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
Important: Reprogramming the PCM may cause a communication error between the PCM and the TAC. If the PCM detects a communication error, DTC P1518 sets. Clear any DTCs from the memory that may have been set by Reprogramming.
| • | DTC P1518 sets if the battery voltage is low. If the customer's concern is slow cranking or no crank 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 when 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--i.e. cruise, brake. 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 no communication is detected by the TAC module from the PCM. Inspect related circuits for being shorted to a 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. If water intrusion occurs, multiple DTCs may set without any circuit or component conditions found during diagnostic testing. |
| • | When the TAC module detects a problem 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 an individual condition may correct more than 1 DTC. Remember this if you review the stored information in Capture Info. |
| • | For an intermittent condition, refer to Intermittent Conditions . |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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This step determines if the ignition relay is supplying a voltage to the ETC fuse.
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Increasing the engine speed to 3,000 RPM aids in locating a shorted throttle actuator motor control circuit. Depending on the polarity of the throttle actuator motor transistors, this DTC may not set with a fault in the control circuits. The throttle actuator motor is a bi-directional DC motor. Raising the engine speed changes the polarity of the transistors in the throttle actuator motor. This occurs because 1 set of the transistors are low, 0 volts, and the other set are high, B+. Therefore, if 1 set of transistors are 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 related harnesses and connectors.
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Locating and repairing an individual condition may correct more than 1 DTC.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Powertrain Control Module Connector End Views or Engine Controls Connector End Views | ||||
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |
Does the test lamp illuminate on at least 1 side of the fuse? | -- | Go to Step 3 | Go to Ignition Relay Diagnosis | |
3 |
Does the test lamp illuminate? | -- | Go to Step 22 | Go to Step 4 |
4 | Connect a scan tool. Is DTC P0604 also set? | -- | Go to Step 5 | |
|
Important: If the Driver Information Center is displaying Reduced Engine Power, go to step 6.
Does the scan tool indicate this DTC failed this ignition? | -- | Go to Step 6 | Go to Diagnostic Aids | |
6 |
Does the DMM indicate voltage on both circuits above the specified value? | 1 V | Go to Step 12 | Go to Step 7 |
7 |
Does the DMM indicate continuity to ground? | -- | Go to Step 10 | Go to Step 8 |
8 |
Does the DMM indicate continuity to ground? | -- | Go to Step 9 | Go to Step 11 |
9 |
Did you find and correct the condition? | -- | Go to Step 28 | Go to Step 24 |
10 |
Did you find and correct the condition? | -- | Go to Step 28 | Go to Step 24 |
11 |
Did you find and correct the condition? | -- | Go to Step 28 | Go to Step 24 |
12 |
Did you find and correct the condition? | -- | Go to Step 28 | Go to Step 13 |
13 |
Did you find and correct the condition? | -- | Go to Step 28 | Go to Step 14 |
14 | Test for an open or high resistance in the TAC module ground circuit with a DMM. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 28 | Go to Step 15 |
15 | Test for voltage on the serial data circuits at the TAC module 16-way harness connector with a DMM. Does the DMM indicate voltage within the specified values for both circuits? | 0-4.5 V | Go to Step 16 | Go to Step 18 |
16 |
Does the DMM indicate OL for both circuits? | -- | Go to Step 20 | Go to Step 17 |
17 |
Did you find and correct the condition? | -- | Go to Step 28 | Go to Step 18 |
18 | Test for a short between both serial data circuits and all other circuits at the PCM and TAC module harness connectors with a DMM. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 28 | Go to Step 19 |
19 | Test for a short to voltage on both serial data circuits at the TAC module 16-way connector with a DMM. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 28 | Go to Step 25 |
20 |
Did you find and correct the condition? | -- | Go to Step 28 | Go to Step 21 |
21 |
Does the DMM indicate voltage at the specified value? | 0 V | Go to Step 25 | Go to Step 24 |
22 |
Did you find and correct the condition? | -- | Go to Step 28 | Go to Step 23 |
23 |
Did you find and correct the condition? | -- | Go to Step 28 | Go to Step 24 |
24 | Test for poor connections at the TAC module harness connector. Refer to Testing for Intermittent Conditions and Poor Connections and Repairing Connector Terminals in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 28 | Go to Step 26 |
25 | Test for poor connections at the PCM harness connector. Refer to Testing for Intermittent Conditions and Poor Connections and Repairing Connector Terminals in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 28 | Go to Step 27 |
26 | Replace the TAC module. Refer to Electronic Throttle Actuator Control Module Replacement . Did you complete the replacement? | -- | Go to Step 28 | -- |
27 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 28 | -- |
28 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 29 |
Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK | ||
