Engine Light But Won't Scan - Pontiac G6 Forum
 
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post #1 of 6 (permalink) Old 08-14-2011, 11:45 AM Thread Starter
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Engine Light But Won't Scan

Here is a stumper. Last week, while driving around 40 MPH, the check engine light on my 2007 G6 2.4L with 60,500 miles came on and the DIC said engine power reduced. I pulled over and shut the car down and restarted it, but it still had little power. I went to Autozone and their scan tool would not link up with it, however upon leaving there, it ran at full power, though the light was still on. Later that day, it went out. Today, I went to start it and there was a delay in the starter engaging, and then the light came back on and the engine stuttered badly and struggled to hit 20 MPH. When I got home, I restarted it and it ran fine, but the light was still on. I took it to my nephew who is an ASE certified mechanic and a former GM mechanic who couldnt get it to link with a Mac diagnostic computer. He said that usually means a sensor is shorting out and causing the computer port to get no power. Anyone heard of this? I am aware there is a catalytic converter notification about them failing (I got one last November). I am hoping this is the problem though I dont get any weird smells. I really can't afford the $100 the dealer wants just to scan the car and internet searches aren't really finding anything. My last Pontiac was a POS, I am really hoping this one isn't going to be the same.
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post #2 of 6 (permalink) Old 08-14-2011, 02:33 PM
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Engine Power Reduced typically means a problem in the throttle actuator control system. But you need to resolve that diagnostic link connector issue first. If the scan tool is not powering up when connected & key ON, there is a power or ground issue at the connector. I'll try to find the schematic to help.

If the scan tool does not power up when connected, check this fuse in the cabin fuse box:

HVAC CTRL (BATT)




Last edited by greenman; 08-14-2011 at 03:00 PM.
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post #3 of 6 (permalink) Old 08-14-2011, 04:38 PM Thread Starter
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The fuse is OK. The second time when the engine really lost power, the throttle was almost unresponsive. The Mac scan tool couldnt read any of the modules. Im going to call the dealer to see what they have to say and if its something they have seen before. The car has never had the link played with before, so it wasn't messed up prior.
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post #4 of 6 (permalink) Old 08-14-2011, 05:41 PM
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Of course you'll need to get the DTC/s before you can proceed with diagnosing the TAC.
_____________________________________

Service Information
2007 Pontiac G6 | G6 (VIN Z) Service Manual | Document ID: 1617626
--------------------------------------------------------------------------------

Throttle Actuator Control (TAC) System Description
Purpose
The throttle actuator control (TAC) system delivers improved throttle response and greater reliability and eliminates the need for mechanical cable. The TAC system performs the following functions:

Accelerator pedal position (APP) sensing

Throttle positioning to meet driver and engine demands

Throttle position (TP) sensing

Internal diagnostics

Cruise control functions

Manage TAC electrical power consumption

The TAC system includes the following components:

The APP sensors

The throttle body assembly

The engine control module (ECM)

Accelerator Pedal Position (APP) Sensor
The accelerator pedal contains 2 individual accelerator pedal position (APP) sensors within the assembly. The APP sensors 1 and 2 are potentiometer type sensors each with 3 circuits:

A 5-volt reference circuit

A low reference circuit

A signal circuit

The APP sensors are used to determine the pedal angle. The engine control module (ECM) provides each APP sensor a 5-volt reference circuit and a low reference circuit. The APP sensors provide the ECM with signal voltage proportional to the pedal movement. The APP sensor 1 signal voltage at rest position is near the low reference and increases as the pedal is actuated. The APP sensor 2 signal voltage at rest position is also near the low reference and increases as the pedal is actuated.

Throttle Body Assembly
The throttle assembly contains the following components:

The throttle blade

The throttle actuator motor

The throttle position (TP) sensor 1 and 2

The throttle body functions similar to a conventional throttle body with the following exceptions:

An electric motor opens and closes the throttle valve.

The throttle blade is spring loaded in both directions and the default position is slightly open.

There are 2 individual TP sensors within the throttle body assembly.

The TP sensors are used to determine the throttle plate angle. The TP sensors provide the engine control module (ECM) with a signal voltage proportional to throttle plate movement. The TP sensor 1 signal voltage at closed throttle is near the 5-volt reference and decreases as the throttle plate is opened. The TP sensor 2 signal voltage at closed throttle is near the low reference and increases as the throttle plate is opened.

Engine Control Module
The engine control module (ECM) is the control center for the throttle actuator control (TAC) system. The ECM determines the drivers intent and then calculates the appropriate throttle response. The ECM achieves throttle positioning by providing a pulse width modulated voltage to the TAC motor.

Modes of Operation
Normal Mode
During the operation of the throttle actuator control (TAC) system, several modes or functions are considered normal. The following modes may be entered during normal operation:

Minimum pedal value--At key-up the engine control module (ECM) updates the learned minimum pedal value.

Minimum throttle position (TP) values--At key-up the ECM updates the learned minimum TP value. In order to learn the minimum TP value, the throttle blade is moved to the closed position.

Ice break mode--If the throttle is not able to reach a predetermined minimum TP, the ice break mode is entered. During the ice break mode, the ECM commands the maximum pulse width several times to the throttle actuator motor in the closing direction.

Battery saver mode--After a predetermined time without engine RPM, the ECM commands the battery saver mode. During the battery saver mode, the TAC module removes the voltage from the motor control circuits, which removes the current draw used to maintain the idle position and allows the throttle to return to the spring loaded default position.

Reduced Engine Power Mode
When the ECM detects a condition with the TAC system, the ECM may enter a reduced engine power mode. Reduced engine power may cause one or more of the following conditions:

Acceleration limiting--The ECM will continue to use the accelerator pedal for throttle control; however, the vehicle acceleration is limited.

Limited throttle mode--The ECM will continue to use the accelerator pedal for throttle control; however, the maximum throttle opening is limited.

Throttle default mode--The ECM will turn off the throttle actuator motor and the throttle will return to the spring loaded default position.

Forced idle mode--The ECM will perform the following actions:

- Limit engine speed to idle by positioning the throttle position, or by controlling the fuel and spark if the throttle is turned OFF.

- Ignore the accelerator pedal input.

Engine shutdown mode--The ECM will disable fuel and de-energize the throttle actuator.
2011 General Motors Corporation. All rights reserved.



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post #5 of 6 (permalink) Old 08-26-2011, 08:12 PM Thread Starter
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Angry

Turned out it was a bad throttle body. I had to have the whole unit replaced for about $276 parts and labor. Normally that is something I would have done myself, but since I couldn't get the car to scan and a trusted longtime family mechanic diagnosed it, I figured I would give him the business and have him do it. So far so good.
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post #6 of 6 (permalink) Old 08-26-2011, 08:48 PM
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Quote:
Originally Posted by bswasey View Post
Turned out it was a bad throttle body. I had to have the whole unit replaced for about $276 parts and labor. Normally that is something I would have done myself, but since I couldn't get the car to scan and a trusted longtime family mechanic diagnosed it, I figured I would give him the business and have him do it. So far so good.
Thanks for coming back with the results.

Have you made any progress on the failing to scan issue? I'm very interested in finding out what that's about.



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