Diagnostic Tools & Circuit Testing
Circuit Testing Tools
You should know when to use and when NOT to use a 12-volt test light during diagnosis of electronic controls (Do NOT use this tester unless specifically instructed to do so by a test procedure). Instead of using a 12-volt test light, you should use a DVOM or Lab Scope with a breakout box whenever a diagnostic procedure calls for a measurement at a PCM connector or component wiring harness.Electricity & Electrical Circuits
You should understand basic electricity and know the meaning of voltage (volts), current (amps), and resistance (ohms). You should understand what happens in an electrical circuit when it is open or shorted, and you should be able to identify an open circuit or shorted circuit using a DVOM. You should also be able to read and understand automotive electrical wiring diagrams and schematics.Electronic Controls
You should have a basic knowledge of electronic controls when performing test procedures to keep from making an incorrect diagnosis or damaging components. Do NOT attempt to diagnose an electronic control problem without this basic knowledge!Hand Tools & Meter Operation
To effectively use this or any diagnostic information, you should have a solid understanding of how to operate required tools and test equipment.
Emission regulations require that a Malfunction Indicator Lamp (MIL) be illuminated when an emissions related fault is detected and that a Diagnostic Trouble Code be stored in the vehicle controller (PCM) memory.
When the MIL is illuminated, it is an indication of a problem within one of the electronic components or circuits. When the scan tool is attached to the Data Link Connector (DLC) in the vehicle, it can access the DTCs. In some situations, without the use of a scan tool, the MIL can be activated to flash a series of long and short flashes, which correspond to the numbering of the DTC.
OBD II guidelines define when an emissions-related fault will cause the MIL to activate and set a Diagnostic Trouble Code (DTC). There are some DTCs that will not cause the MIL to illuminate. OBD II guidelines determine how quickly the onboard diagnostics must be able to identify a fault, set the trouble code in memory and activate the MIL (lamp).Scan Tools
Domestic vehicle manufacturers designed their computers to have an accessible data line where a diagnostic tester could retrieve data on sensors and the status of operation for components.
These testers became known in the automotive repair industry as -Scan Tools- because they scanned the data on the computers and provided information for the technician.
The Scan Tool is your basic tool link into the on-board electronic control system of the vehicle. Scan Tools are equipped with, or have separate software cards, for each OEM needed to be diagnosed. In this case, always secure a scan tool that has the latest OEM-specific diagnostic software included.
Ford specifies the use of an -NGS- scan tool with its diagnostic processes. However, there are aftermarket scan tools, when equipped with the right software, that can provide proper diagnosis as well.
If you are reasonably certain that the problem is related to a particular electronic control system, the first step is to check for any stored trouble codes in that controller.
On vehicles with more than one vehicle controller (i.e., PCM, BCM, MIC, TCM, etc.), if you are unsure whether the problem is Powertrain related, start by checking for codes in the other controllers to determine if the problem is related to another vehicle system.
If there are no codes set, and you are certain which Powertrain subsystem has a problem, you can start by checking one of the subsystems. The subsystems include the Charging, Cooling, Fuel, Ignition and Speed Control systems.
If a wiring problem is found during testing, you will need to refer to wiring diagrams in the appropriate information resource. Using a wiring schematic can help you determine:
Once you decide how to repair the vehicle, in addition to performing the repair, it is a good idea to clear any trouble codes that were set and to verify they do not reset.
To verify a repair, you should confirm that the Check Engine Light is operational and goes out after the 4-second key-on bulb check. Then, you need to duplicate the conditions present when the customer complaint occurred or when a trouble code set; these are the actual code conditions that caused a code to set. The individual code conditions and possible causes are included under Section 3. You can use this information to find out how to drive a vehicle for problem verification.
How To Use This Section
The General Motors (GM) OBD II Section contains information grouped into the three main categories described below:
To use this information, first look up the year, make, model, and in some cases, the specific engine application. Then read the code conditions containing the enable criteria that explain why a code set.
Example of Trouble Code Information
|DTC||Trouble Code Title, Conditions & Possible Causes|
|P0131 1T O2S, MIL Yes 2000, 2001, 2002, 2003, 2004, 2005, 2004, 2005 C/K Series Truck, G Series Van, S/T Series Pickup and Blazer, Envoy Escalade & TrailBlazer 4.8L VIN V, 5.3L VIN P, 5.3L VIN T, 5.3L VIN Z, 6.0L VIN N, 6.0L VIN U, 8.1L VIN G Transmissions All||HO2S-11 (Bank 1 Sensor 1) Circuit Low Input Conditions DTCP0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0120, P0121, P0122, P0123, P0169, P0178, P0179, P0200, P0220, P0300, P0442, P0446, P0452, P0453, P0455, P0496, P1125, P1258, P1514, P1515, P1516, P1518, P2108 and P2135 not set, engine started, engine running in closed loop, system voltage from 10-18v, Fuel Alcohol content less than 90%, fuel level over 10%, TP angle from 3-70% more than the idle value, then during the Lean Test period, the PCM detected the HO2S signal was less than 200 mv for 165 seconds or with engine runtime over 30 seconds, and during the Power Enrichment test period, the PCM detected the HO2S signal was less than 400 mv for 10 seconds. Possible Causes Engine misfire condition present (look for P0300 series codes) Fuel system too lean (possible low fuel pressure, water in fuel) HO2S signal circuit is shorted to the sensor or chassis ground HO2S is damaged (i.e., cracked) or air reference hole clogged PCM has failed|
|P0131 2T O2S, MIL Yes 2000, 2001, 2002, 2003, 2004, 2005, 2004, 2005 Deville, Eldorado & Seville 4.6L VIN 9, 4.6L VIN Y Transmissions A/T||HO2S-11 (Bank 1 Sensor 1) Circuit Low Input Conditions DTCP0101-P0103, P0106-P0108, P0112-P0113, P0116-P0118, P0121-P0123, P0125, P0128, P0200, P0300, P0410-P0446, P0452-P0453, P1258, P1415-P1416 and P1441 not set, AIR, Catalyst and EGR Flow tests off, system voltage 8-18v, no injectors disabled, TP angle from 3-25%, A/F ratio at 14.5-14.81, engine running in closed loop for 3 seconds, and the PCM detected the HO2S signal was less than 75 mv for 5 seconds while in closed loop, or it was less than 575 mv for 8 seconds while operating in Power Enrichment mode. Possible Causes Air leaks in the exhaust system, intake manifold, vacuum lines Engine misfire condition present (look for P0300 series codes) Fuel system too lean (possible low fuel pressure, water in fuel) HO2S signal circuit is shorted to the sensor or chassis ground HO2S is damaged (i.e., cracked) or air reference hole clogged PCM has failed|
Notes & Cautions
Before servicing any vehicle, please be sure to read all of the following precautions, which deal with personal safety, prevention of component damage, and important points to take into consideration when servicing a motor vehicle:
A key to the diagnosis of the PCM and its subsystems is to determine which subsystems are on a vehicle. Examples of typical subsystems appear below:
Changes In Diagnostic Routines
In some cases, a new Engine Control system may be similar to an earlier system, but it can have more indepth control of vehicle emissions, input and output devices and it may include a diagnostic "monitor" embedded in the engine controller designed to run a thorough set of emission control system tests.OBD II System Diagnostics
The diagnostic approach used in OBD II systems is more complex than that of the one for OBD I systems. This complexity will effect how you approach diagnosing the vehicle. On an OBD II system, the onboard diagnostics will identify sensor faults (i.e., open, shorted or grounded circuits) as well as those that lose calibration. Another new test that arrived with OBD II is the rationality test (a test that checks whether the value for one input makes rational sense when compared against other sensor input values). The changes plus the use of OBD II Monitors have dramatically changed OBD II diagnostics.
The use of a repeatable test routine can help you quickly get to the root cause of a customer complaint, save diagnostic time and result in a higher percentage of properly repaired vehicles. You can use this Diagnostic Flow Chart to keep on track as you diagnose an Engine Control problem or a base engine fault on vehicles with OBD II.
Here are some of the steps included in the Diagnostic Routine:
OBD II introduces common terms, connectors, diagnostic language and new emissions-related monitoring procedures. The most important benefit of OBD II is that all vehicles will have a common data output system with a common connector. This allows equipment Scan Tool manufacturers to read data from every vehicle and pull codes with common names and similar descriptions of fault conditions. In the future, emissions testing will require the use of an OBD II certifiable Scan Tool.History Of OBD Systems
Starting in 1978, several vehicle manufacturers introduced a new type of control for several vehicle systems and computer control of engine management systems. These computer-controlled systems included programs to test for problems in the engine mechanical area, electrical fault identification and tests to help diagnose the computer control system. Early attempts at diagnosis involved expensive and specialized diagnostic testers that hooked up externally to the computer in series with the wiring connector and monitored the input/output operations of the computer.
By early 1980, vehicle manufacturers had designed systems in which the onboard computer incorporated programs to monitor selected components, and to store a trouble code in its memory that could be retrieved at a later time. These trouble codes identified failure conditions that could be used to refer a technician to diagnostic repair charts or test procedures to help pinpoint the problem area.
The OBD II system was developed as a step toward compliance with California and Federal regulations that set standards for vehicle emission control monitoring for all automotive manufacturers. The primary goal of this system is to detect when the degradation or failure of a component or system will cause emissions to rise by 50%. Every manufacturer must meet OBD II standards by the 1996 model year. Some manufacturers began programs that were OBD II mandated as early as 1992, but most manufacturers began an OBD II phase-in period starting in 1994.
The changes to On-Board Diagnostics influenced by this new program include:
OBD System Diagnostics
One of the most important things to understand about the automotive repair industry is the fact that you have to continually learn new systems and new diagnostic routines (the test procedures designed to isolate a problem on a vehicle system). For OBD I and II systems, a diagnostic routine can be defined as a procedure (a series of steps) that you follow to find the cause of a problem, make a repair and then verify the problem is fixed.
System Control Modules
Before attempting diagnosis of the Electronic Engine Control system, familiarize yourself with the basics of how the system is designed to operate. It consists of a central processing unit: Powertrain Control Module (PCM), Engine Control Module (ECM), Transmission Control Module (TCM) and/or the Body Control Module (BCM). These units are the -heart- of the electronic control systems on the vehicle. In some cases, these units are integral with one another, and on some applications, they are separate. As you get deeper into actual diagnostic testing, you will find out which units are used on the vehicle you are testing.
The PCM is a digital computer that contains a microprocessor. The PCM receives input signals from various sensors and switches that are referred to as PCM inputs. Based on these inputs, the PCM adjusts various engine and vehicle operations through devices that are referred to as PCM outputs. Examples of the input and output devices are shown in the graphic below.
Where To Begin
Diagnosis of engine performance or drivability problems on a vehicle with an onboard computer requires that you have a logical plan on how to approach the problem. The -Six Step Test Procedure- is designed to provide a uniform approach to repair any problems that occur in one or more of the vehicle subsystems.
The diagnostic flow built into this test procedure has been field-tested for several years at dealerships - it is the starting point when a repair is required!
It should be noted that a commonly overlooked part of the -Problem Resolution- step is to check for any related Technical Service Bulletins.
Six-Step Test Procedure
The steps outlined below were defined to help you determine how to perform a proper diagnosis. Refer to the flow chart that outlines the Six Step Test Procedure on the previous page as needed. The recommended steps include:Accessing Components & Circuits
Every vehicle and every diagnostic situation is different. It is a good idea to first determine the best diagnostic path to follow using flow charts, wiring diagrams, TSBs, etc. Part of choosing steps is to determine how time-consuming and effective each step will be. It may be easy to access a component or circuit in one vehicle, but difficult in another. Many circuits are integrated into a large harness and are difficult to test. Many components are inaccessible without disassembly of unrelated systems.
In the graphic, you will note that the protective covers have been removed from the PCM connectors, and any circuit can be easily identified and back probed. In other cases, PCM access is difficult, and it may be easier to access circuits at the component side of the harness.
Another important point to remember is that any circuit or component controlled by a relay or fused circuit can be monitored from the appropriate fuse box.
There is generally more than one of each type of relay or fuse. Therefore, swapping a suspect relay from another system may be more efficient than testing the relay itself. Relays and fuses may also be removed and replaced with fused jumper wires for testing circuits. Jumper wires can also provide a loop for inductive amperage tests.
Choosing the easiest way has its limitations, however. Remember that an appropriate signal on a PCM controlled circuit at an actuator means that the signal at the PCM is also good. However, a sensor signal at the sensor does not necessarily mean that the PCM is receiving the same signal. Think about the direction flow through a circuit, and not just what signal is appropriate, to save time without making costly assumptions.Base Engine Tests
To determine that an engine is mechanically sound, certain tests need to be performed to verify that the correct A/F mixture enters the engine, is compressed, ignited, burnt, and then discharged out of the exhaust system. These tests can be used to help determine the mechanical condition of the engine.
To diagnose an engine-related complaint, compare the results of the Compression, Cylinder Balance, Engine Cylinder Leakage (not included) and Engine Vacuum Tests.Check For Trouble Codes Or Symptoms
Determine if the problem is a Code or a No Code Fault. Then refer to the appropriate published service diagnostic information to make the repair.Engine Compression Test
The Engine Compression Test is used to determine if each cylinder is contributing its equal share of power. The compression readings of all the cylinders are recorded and then compared to each other and to the manufacturer's specification (if available).
Cylinders that have low compression readings have lost their ability to seal. It this type of problem exists, the location of the compression leak must be identified. The leak can be in any of these areas: piston, head gasket, spark plugs, and exhaust or intake valves.
The results of this test can be used to determine the overall condition of the engine and to identify any problem cylinders as well as the most likely cause of the problem.
Compression Test Procedure
- Allow the engine to run until it is fully warmed up.
- Remove the spark plugs and disable the Ignition system and the Fuel system for safety. Disconnecting the CKP sensor harness connector will disable both fuel and ignition (except on NGC vehicles).
- Carefully block the throttle to the wide-open position.
- Insert the compression gauge into the cylinder and tighten it firmly by hand.
- Use a remote starter switch or ignition key and crank the engine for 3-5 complete engine cycles. If the test is interrupted for any reason, release the gauge pressure and retest. Repeat this test procedure on all cylinders and record the readings.
The lowest cylinder compression reading should not be less than 70% of the highest cylinder compression reading and no cylinder should read less than 100 psi.Evaluating The Test Results
To determine why an individual cylinder has a low compression reading, insert a small amount of engine oil (3 squirts) into the suspect cylinder. Reinstall the compression gauge and retest the cylinder and record the reading. Review the explanations below.
Reading is higher - If the reading is higher at this point, oil inserted into the cylinder helped to seal the piston rings against the cylinder walls. Look for worn piston rings.
Reading did not change - If the reading didn't change, the most likely cause of the low cylinder compression reading is the head gasket or valves.
Low readings on companion cylinders - If low compression readings were recorded from cylinders located next to each other, the most likely cause is a blown head gasket.
Readings are higher than normal - If the compression readings are higher than normal, excessive carbon may have collected on the pistons and in the exhaust areas. One way to remove the carbon is with an approved brand of -Top Engine Cleaner.-
Engine Vacuum Tests
An engine vacuum test can be used to determine if each cylinder is contributing an equal share of power. Engine vacuum, defined as any pressure lower than atmospheric pressure, is produced in each cylinder during the intake stroke. If each cylinder produces an equal amount of vacuum, the measured vacuum in the intake manifold will be even during engine cranking, at idle speed, and at off-idle speeds.
Engine vacuum is measured with a vacuum gauge calibrated to show the difference between engine vacuum (the lack of pressure in the intake manifold) and atmospheric pressure. Vacuum gauge measurements are usually shown in inches of Mercury (in. Hg).
Engine Cranking Vacuum Test Procedure
The Engine Cranking Vacuum Test can be used to verify that low engine vacuum is not the cause of a No Start, Hard Start, Starts and Dies or Rough Idle condition (symptom).
The vacuum gauge needle fluctuations that occur during engine cranking are indications of individual cylinder problems. If a cylinder produces less than normal engine vacuum, the needle will respond by fluctuating between a steady high reading (from normal cylinders) and a lower reading (from the faulty cylinder). If more than one cylinder has a low vacuum reading, the needle will fluctuate very rapidly.
- Prior to starting this test, set the parking brake, place the gearshift in P/N and block the drive wheels for safety. Then block the PCV valve and disable the idle air control device.
- Disable the fuel and/or ignition system to prevent the vehicle from starting during the test (while it is cranking).
- Close the throttle plate and connect a vacuum gauge to an intake manifold vacuum source. Crank the engine for three seconds (do this step at least twice).
The test results will vary due to engine design characteristics, the type of PCV valve and the position of the AIS or IAC motor and throttle plate. However, the engine vacuum should be steady between 1.0-4.0 in. Hg during normal cranking.Engine Running Vacuum Test Procedure
- Allow the engine to run until fully warmed up. Connect a vacuum gauge to a clean intake manifold source. Connect a tachometer or Scan Tool to read engine speed.
- Start the engine and let the idle speed stabilize. Raise the engine speed rapidly to just over 2000 rpm. Repeat the test (3) times. Compare the idle and cruise readings.
If the engine wear is even, the gauge should read over 16 in. Hg and be steady. Test results can vary due to engine design and the altitude above or below sea level.
This next section provides an overview of ignition tests with examples of Engine Analyzer patterns for a Distributor Ignition System.Preliminary Inspection
- Perform these checks prior to connecting the Engine Analyzer:
- Check the battery condition (verify that it can sustain a cranking voltage of 9.6v).
- Inspect the ignition coil for signs of damage or carbon tracking at the coil tower.
- Remove the coil wire and check for signs of corrosion on the wire or tower.
- Test the coil wire resistance with a DVOM (it should be less than 7 k/ohm per foot).
- Connect a low output spark tester to the coil wire and engine ground. Verify that the ignition coil can sustain adequate spark output while cranking for 3-6 seconds.
- Connect the Engine Analyzer to the Ignition System, and choose Parade display. Run the engine at 2000 RPM, and note the display patterns, looking for any abnormalities.
Perform the following checks prior to connecting the Engine Analyzer:
- Check the battery condition (verify that it can sustain a cranking voltage of 9.6v).
- Inspect the ignition coils for signs of damage or carbon tracking at the coil towers.
- Remove the secondary ignition wires and check for signs of corrosion.
- Test the plug wire resistance with a DVOM (specification varies from 15-30 k/ohm).
- Connect a low output spark tester to a plug wire and to engine ground. Verify that the ignition coil can sustain adequate spark output for 3-6 seconds.
- Connect the Engine Analyzer to the ignition system.
- Turn the scope selector to view the -Parade Display- of the ignition secondary.
- Start the engine in Park or Neutral and slowly increase the engine speed from idle to 2000 rpm.
Compare actual display to the examples below.
It is a good idea, prior to tracing any faults, to clear the DTCs, attempt to replicate the condition and see if the same DTC resets. Also, once any repairs are made, it will be necessary to clear the DTC(s) - PCM Reset - to ensure the repair has totally resolved the problem. For procedures on PCM Reset, see DIAGNOSTIC TROUBLE CODES section.Problem Resolution & Repair
Once the problem component or circuit has been properly identified and verified using published diagnostic procedures, make any needed repairs or replacement to restore the vehicle to proper working order. If the condition has set a DTC, follow the designated repair chart to make an effective repair. If there is not a DTC set, but you can determine specific symptoms that are evident during the failure, select the symptom from the symptom tables and follow the diagnostic paths or suggestions to complete the repair or refer to the applicable component or system in service information.
If the vehicle does not set a DTC and has only intermittent operating failures or concerns, to resolve an intermittent fault, perform the following steps:
Once a repair is completed, the next step is to verify the vehicle operates properly and that the original symptom was corrected. Verification Tests, related to specific DTC diagnostic steps, can be used to verify a repair.Symptom Diagnosis
To verify the customer complaint, the technician should understand the normal operation of the system. Conduct a thorough visual and operational inspection, review the service history, detect unusual sounds or odors, and gather diagnostic trouble code (DTC) information resources to achieve an effective repair.
This check should include videos, newsletters, and any other information in the form of TSBs or Dealer Service Bulletins. Analyze the complaint and then use the recommended Six Step Test Procedure. Utilize the wiring diagrams and theory of operation articles. Combine your own knowledge with efficient use of the available service information.
Verify the cause of any related symptoms that may or may not be supported by one or more trouble codes. There are various checks that can be performed to Engine Controls that will help verify the cause of a related symptom. This step helps to lead you in an organized diagnostic approach.
To determine whether vehicle problems are identified by a set Diagnostic Trouble Code, you will first have to connect a proper scan tool to the Data Link Connector and retrieve any set codes. See DIAGNOSTIC TROUBLE CODES section for information on retrieving and reading codes.
If no codes are set, the problem must be diagnosed using only vehicle operating symptoms. A complete set of -No Code- symptoms is found in the SYMPTOM DIAGNOSIS (NO CODES) section.
Do NOT attempt to diagnose driveability symptoms without having a logical plan to use to determine which engine control system is the cause of the symptom - this plan should include a way to determine which systems do NOT have a problem! Remember, there are 2 kinds of NO CODE conditions: