Powertrain System Diagnosis

POWERTRAIN SYSTEM DIAGNOSIS
After the visual inspection is complete, the Powertrain System Check, Chart [1] should be performed. The Powertrain System Check will provide direction for diagnosis of:
- PCM inoperative or damaged (No PCM data or "SERVICE ENGINE SOON" MIL inoperative).
- PCM trouble codes current or history.
- Fuel System Rich or Lean.
- Customer Complaint Driveability Symptoms.
The Powertrain System Check is designed as a "Master" chart for driveability and emissions system diagnosis. The System Check should always be used as the starting point for all Powertrain diagnosis.
The Powertrain System Check is an organized approach for identifying a problem. Driver Comments normally fall into one of the following areas:
- Steady "SERVICE ENGINE SOON" MIL.
- Driveability problem; or
- Engine will not start or stalls after start.
UNDERSTANDING THE CHART AND USING IT CORRECTLY WILL REDUCE DIAGNOSIS TIME AND PREVENT THE UNNECESSARY REPLACEMENT OF PARTS.

POWERTRAIN DIAGNOSIS PROCEDURE
VISUAL UNDERHOOD INSPECTION
One of the most important checks is a visual underhood inspection. This can often fix a problem. Inspect all vacuum hoses for pinches, cuts, or disconnects. Inspect all the wires in the engine compartment for good connections, burned or chaffed spots, pinched wires, or contact with sharp edges or hot exhaust manifolds. These quick tests takes only a few minutes, can save you valuable time, and help you repair the problem.

All Powertrain diagnosis should begin with a thorough visual inspection. Visual inspection can often lead to repair of a simple problem without use of the charts. An example might be an unstable idle complaint with history Code P030. Visual inspection may reveal a vacuum hose disconnected or a spark plug wire disconnected or cut, ISC not connected or ISC connector not latched, or other obvious causes for an unstable idle condition.
- Inspect all vacuum hoses for being pinched, cut or disconnected. Be sure to inspect hoses that are difficult to see such as beneath the upper intake, generator, etc.
- Check for proper ground connections, ground eyelets connected to ground points, star washers installed, if applicable. Check both battery positive junction blocks for loose retainer nuts.
- Inspect other wiring in the engine compartment for good connections, burned or chaffed spots, pinched wires or harness contact with sharp edges or hot exhaust manifolds.
- Check for blown or missing fuses and for relays missing or installed in the wrong locations.
- Inspect plug wires for proper routing, connection, cuts, visible signs of arcing to ground.

USING TROUBLE TREES
When diagnosing the Northstar powertrain, you will almost certainly need to use the diagnostic procedures in this or other powertrain sections. These diagnostic procedures are mostly in the form of trouble trees, because trouble trees are generally the easiest type of diagnostic chart to follow. On the facing page of each trouble tree will be a circuit diagram, descriptions and notes about the condition or code diagnosed in the trouble tree. Reading these descriptions and notes will help you understand the code or condition and what the trouble tree is trying to accomplish. Below are explanations of the facing page information and trouble tree for diagnostic codes.

Circuit Diagram
The circuit diagram on the trouble tree facing page will show the circuits and components involved in setting the code. This diagram may be used as a reference when circuit checks are required in the trouble tree. If more detailed circuit information is required, Section 8A may be referenced.

Circuit Description:
The circuit description explains the sensor and/or circuits involved in setting the code. It also gives a brief description of when the code is set.

Test Condition(s):
The test conditions are the conditions that must be met before the PCM will test for a failure. These conditions are generally set up so that an input or system may be reliably checked and not give a false failure indication.

Failure Condition(s):
The failure conditions are the conditions that must be met for the code to set. Failure conditions are checked only after the test conditions (described above) have been met. If they are true, then an input or system failure is present and the code is set.

Action Taken:
The actions taken are the steps the PCM takes after the code is set. These actions serve one of three purposes; 1) to inform the driver of the problem, 2) to preserve the driveability of the vehicle, or 3) to prevent the failure from causing any damage to the vehicle.
Many trouble trees will have you checking terminal contact before replacing a component. This is done because the checks performed in trouble trees can only check the continuity of a circuit across a wire or in-line connection, not the continuity across the connection at a component. Checking terminal contact will prevent the replacement of good components, prevent comebacks due to 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.

Notes On Fault Tree:
The notes on fault tree are explanations of the reason certain checks are done and what the check is supposed to uncover. The notes are numbered and correspond to the circled numbers on the trouble tree. If you are performing a check and wonder why the check is being made or what the check is supposed to find, note the circled number next to the check you are performing. Refer to the note under "Notes on Fault Tree" that has the same number for an explanation.

Notes On Intermittents:
Most trouble trees have one result which reads 'Fault not present.' This means that the conditions that were present when the code set are no longer there. Sometimes, with the help of snapshot on code set 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 notes on intermittents will explain what to look for and which direction to head in looking for an intermittent failure.

Trouble Tree
Trouble trees are charts with boxes and lines connecting the boxes. They are similar to flowcharts without the arrows and various symbols. Always start with the top left box of the chart (except when there is a note above it). (A few trees will have notes above the top-most box of the tree. These notes must be read and followed first.) Each box will have instructions to perform a test, a result of a test, or inspections/repairs to perform.

The top left box will normally be a test to perform with two or more possible results in the row of boxes below it. After performing the test as instructed, choose the result box which best matches the result you got. Next, perform the repair or test indicated in the box below the result box chosen. Continue performing tests or inspections until you find a problem. Once you have found a problem, repair it and follow any instructions at the bottom of the tree. When a problem is found, it is not necessary to complete the inspections or repairs listed in the box. However, it is very important to read and follow any instructions or notes at the bottom of the trouble tree page after repairs are complete.

Most test boxes will have a circled number to the left of them. This number refers to a note on the facing page under 'Notes on Fault Tree'. Find the note numbered the same as the test box for an explanation of the test (see 'Notes on Fault Tree' above for more information).

CHECKING TERMINAL CONTACT
Checking 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. It contains a supply of all currently used terminal series. To check terminal contact, start by inspecting the male terminals. They should be straight and aligned with other terminals in the row. They 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 it to the female terminal(s) to be checked. It should not fall out or be easily jarred out of connection. It should require some force to disconnect it. The force required to disconnect it 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 Section 8A and the Terminal Repair Kit, J 38125, for identification of terminal series and repair procedures.

DIAGNOSING INTERMITTENTS
Most trouble trees have one result which reads 'FAULT NOT PRESENT'. This means that the conditions that were present when the code set are no longer there. This does not mean that the problem is fixed, it simply means that the problem is intermittent. The problem was present but is not present now. And it is likely to return in the future so it should be diagnosed and repaired if at all possible. The only way to diagnose a problem that is not present is to gather information from the time when the code was set. This can be done in two ways; through snapshot data and driver observations.

Snapshot data, either snapshot on code set or customer snapshot, can be used to check data parameters to see if they fall within normal operating range. For example, a snapshot MAP parameter value of 100 while the engine was idling is not normal and would indicate that the MAP sensor voltage went high either due to an open in the 5 volt return 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 Code P040, you find out the driver noticed that the "SERVICE VEHICLE SOON" message only comes on after an extended wide open throttle maneuver. This would lead you to check the wiring harness that carries the power steering pressure switch circuit underhood, because it is likely that powertrain movement (during WOT) is stretching the harness, causing an open circuit.

This kind of information, while not airtight, may be very helpful in diagnosing intermittent conditions. Besides, it may be the only information you can get about a failure and the only chance to find the cause of an intermittent.