Electronic Control System
This section uses diagnostic tables, wiring diagrams, and descriptive text in order to direct you through specific tests in order to locate and repair a problem. The diagnostic table is a step by step procedure to use in order to isolate the circuit or component that is the source of the problem. The wiring diagrams, the text describing the system, the test conditions, the failure conditions, and the actions taken when a DTC sets are located on the page facing the diagnostic chart.
Clearing DTCs
Important: Using the scan tool to clear DTCs causes also clears both the TCM and the ECM DTCs, regardless of which system is being diagnosed.
DO NOT clear DTCs unless instructed to do so by a diagnostic procedure.
Clearing the DTCs also clears the ECM's Freeze Frame data. Several TCM DTCs cause a MIL request signal to be sent to the ECM. The ECM will record the Freeze Frame data when a MIL Request signal is present. The scan tool has the capability to save any data that is stored in the Freeze Frame and then display that data at a later time (refer to Capture Info). This data may prove useful during transmission diagnosis. Once you correct and verify a problem, it is a good idea to clear the DTCs so that any future service work is not needlessly confused.
Many DTCs have complex test and failure conditions. Therefore, simply clearing the DTCs, exiting the diagnostics, and watching to see if the DTC sets again may not indicate whether a problem has been corrected. To verify a repair after the repair is complete, you must look up the test conditions (located on the facing page to the trouble chart) and duplicate those conditions. Then, if the DTC does not set, chances are good that the problem is fixed.
Capture Info
Selecting this option on the scan tool allows the technician to record the Freeze Frame data. The data is stored in the scan tool. When the starting diagnosis, capture the Freeze Frame data. Later, this information can be reviewed, even if you disconnect the ECM or the battery, or if the DTCs clears during a diagnostic procedure.
ECM Snapshot
The ECM snapshot feature is designed to assist in diagnosis of intermittent problems. When the scan tool takes an ECM snapshot, a subset of the ECM's data parameters at that moment in time stores for retrieval and for use in diagnosing a problem. The scan tool can be set up in order to take a data snapshot when a specific DTC sets, any DTC sets or when manually triggered with the scan tool.
Diagnosing Intermittents Using ECM Snapshots
The ECM snapshots may be very useful in diagnosing intermittent engine problems. The stored data may be retrieved and used in order to determine if a particular input was incorrect when the problem occurs. Many of the ECM DTC trouble charts will use the Freeze Frame information in order to pinpoint a problem if the problem is not current. For example, a vehicle comes in with an intermittent rough engine condition and a scan tool Snapshot was taken while the engine was running rough. The scan tool retrieves several data parameters, for example, the MAP sensor, the ECT sensor, the IAT sensor, and the Knock Sensor parameters. Comparison of the normal values of those parameters to any of the values that are out of the normal range may warrant an investigation. Also, fuel injector pulse width or spark advance may be retrieved. This may indicate that a fueling or ignition irregularity should be investigated. While this method may result in an increased diagnosis time, this also causes less actual repair time and fewer service comebacks.
Using Diagnostic Tables
When diagnosing the Catera transmission control system, you will almost certainly need to use the diagnostic procedures in this section. These diagnostic procedures are mostly in the form of tables. In the supporting text for each diagnostic table is a circuit diagram (in most cases), a circuit description, the conditions under which the TCM will run the diagnostic, the conditions that will cause the DTC to set, a description of action taken (STL or MIL operation, default actions), any diagnostic aids, and test descriptions which elaborate on certain steps in the table. Reading the supporting text will help you to understand the DTC or the condition and what the trouble chart is trying to accomplish. The following are definitions of the facing page information:
| • | Circuit Diagram -- The circuit diagram will show the circuits and components involved in setting the DTC. This diagram may be used as a reference when circuit checks are required in the table. If more detailed circuit information is required, refer to Engine Controls Schematics , or Engine Electrical, Electrical Diagnosis . |
| • | Circuit Description -- The circuit description explains the operation of the system and/or the affected circuits. |
| • | Conditions For Running The DTC -- The Conditions For Running the DTC are the conditions must meet the criteria before the ECM will test a particular system or circuit. |
| • | Conditions For Setting The DTC -- The Conditions For Setting the DTC are the conditions that cause the diagnostic to fail. The ECM checks these conditions only after the test conditions (described above) meet the criteria. |
| • | Action Taken When The DTC Sets -- A DTC may or may not illuminate the MIL, depending on DTC type (refer to DTC Type). The ECM may also use a default strategy to compensate for the problem. These actions serve the following purpose: |
| - | Preserve the driveability of the vehicle |
| - | Prevent the failure from causing any damage to the vehicle |
| - | Prevents unsafe vehicle operation |
| • | Diagnostic Aids -- Most trouble charts have one result which reads Fault Not Present. This means that the conditions that were present when the DTC set are no longer there. Sometimes, with the help of the Freeze Frame data or information from the driver, the problem may still be identified or at least narrowed down to a short list of possible intermittent failures. When this is true, the Diagnostic Aids will explain what to look for and which direction to head in looking for an intermittent failure. |
| Important: Not all of the steps are supported. |
| • | Test Description -- The Test Descriptions are further explanations of why certain checks are done and what the check is supposed to uncover. The notes correspond to the steps on the trouble chart. If you are performing a check and wonder why the check is being made or what the check is supposed to find, refer to the note under Test Description that has the same number for an explanation. |
| • | Diagnostic Table -- Diagnostic tables are an organized and systematic approach to diagnosing a DTC. The diagnostic table consists of five separate columns: step number, action, value, yes, and no. The step number indicates which step is being performed. The action column contains all of the necessary information about how to perform a certain test. The last sentence in each action block will always be a question. The question can only be answered yes or no. The answer to the question will dictate which column you will go to next and you will need to answer yes or no. The yes or no answer to each test will lead you to the next step to be performed or a particular action to take. Most boxes will have a Go to Step X statement. If a yes or no block does not have a Go to Step X statement, the text inside will be the last step that you take on the trouble chart because the statement will either direct you to another diagnostic procedure or section, a Fault Not Present condition, or System OK. |
| Always begin with Step 1 at the top of the diagnostic table unless there is a note or caution above this step. (Some tables will have notes at the top in order to warn you of possible damaging conditions or existing conditions that may hamper the diagnosis). Never skip steps or jump ahead in the diagnostic table. Always go where the Yes or No box tells you to go. Taking short cuts often leads to misdiagnosis. When a problem is found, make the necessary repairs and then verify the repair. |
| • | Verify Repair -- The last few steps of the diagnostic table intend to verify any repairs that have been made. This is one of the places you will be instructed to clear the DTCs. Then, the vehicle should be run within the Conditions For Running the DTC, and rechecked for DTCs. Additional DTCs that may have been set, should have been written down during the OBD System Check. At this point, if the DTC did not re-set, continue on to the next DTC, if multiple DTCs were stored. |
Diagnosing Intermittents
Most diagnostic tables have one result which reads Fault Not Present. This means that the conditions that were present when the DTC set are no longer there. This does not mean that the problem is fixed, but simply means that the problem is intermittent. The problem was present but is not present now. The problem should be diagnosed and repaired if at all possible in order to prevent the problem from returning. The only way to diagnose a problem that is not present is to gather information from the time when the DTC sets. This can be done in a few ways: through the Snapshot data, the Freeze Frame data, and the driver observations.
Use the Snapshot data in order to check the data parameters to see if they fall within the normal operating range. For example, a snapshot TP sensor parameter value of 100% while the engine was idling is not normal and would indicate that the TP sensor voltage went high either due to an open in the sensor return circuit or a short to voltage on the sensor signal circuit. Driver observations may give additional information about the system or area of the car that should be checked for an intermittent. For example, while diagnosing an intermittent DTC P0300, you will discover that the driver noticed that the MIL only comes on in the rain. This would lead you to check a secondary ignition wiring, because it is likely that an ignition component is arcing to ground when wet.
This kind of information, while not airtight, may be very helpful in diagnosing intermittent conditions. Besides, this information may be the only information you can get about a failure and the only chance to find the cause of an intermittent.
Checking Terminal Contact
Many diagnostic tables will have you checking terminal contact before replacing a component. This is done because the checks performed in diagnostic tables can only check the continuity of a circuit across a wire or an in-line connection, not the continuity across the connection at a component. Checking the terminal contact will prevent the replacement of good components, prevent comebacks due to any intermittent connection problems, and make some repair jobs easier (e.g. replacing a terminal instead of a component). For this reason it is very important to check terminal contact when instructed to do so.
Checking the terminal contact is easy as long as you have a supply of new terminals handy. The Terminal Repair Kit, J 38125, is a good source of terminals for checking terminal contact. This kit contains a supply of all of the most current terminals. To check terminal contact, start by inspecting the male terminals. The terminals should be straight and aligned with the other terminals in the row. The terminals should not be twisted, bent or otherwise damaged. The female terminal should be, likewise, inspected for alignment and damage. Finally, take a new male terminal of the same series (e.g. Metripack 150, Weatherpack, etc.) and connect the terminal to the female terminal(s) to check. The terminal should not fall out or be easily jarred out of connection. The terminal should require some force in order for disconnection. The force required to disconnect the terminal will depend on the size of the terminal being checked. Larger terminals, Metripack 630 series for example, should be very difficult to remove by hand. Smaller terminals, Micropack series for example, should be easier to disconnect by hand but still should not fall out. Replace, do not repair, any damaged terminals. Refer to Engine Electrical and the Terminal Repair Kit, J 38125, for identification of terminal series and repair procedures.
