Description and Operation — Diagnostic Methods

This document provides important diagnostic knowledge required for a successful repair outcome. It defines the technical competencies expected of users of this manual.

Ford Diagnostic Assumptions

Ford diagnostics assume that the vehicle concern described in the test title is currently present. Exceptions to this rule are noted in each test. Do not replace modules or other components according to diagnostic instructions if the concern is not present at the time of testing.

Using Flex Probes and Checking Wiring Pin (or Terminal) Fit

• To avoid damaging wiring terminal contacts, use Rotunda Flex probes when connecting test equipment or jumper wires to the contacts.
• Proper fit of the male pin (terminal) to the female terminal is critical for a stable and secure electrical connection.
  • Pin fit can be checked by using a mating pin to perform a normal pull-out force test. A damaged pin will have a very low pull-out force from the mating pin.
  • Correct pull-out force checking of small contacts may require removing the contact from the rigid connector housing (contact retainer or locking tab) if this increases resistance when inserting or removing the pin.
• Replace damaged connectors, contacts or terminals.

Measuring Automotive Circuits

Checking Power Supply Circuits

Measuring the voltage of a power circuit under expected load, as opposed to with the circuit disconnected, will only identify open circuits (for example, a blown fuse or open circuit). This measurement practice will not identify excessive circuit resistance.

• Circuits with a current of approximately 200-1000 mA can be tested with a 250-350 mA test lamp. Measure the circuit voltage with a digital multimeter while the test lamp is connected. A drop in existing voltage during the test lamp load indicates excessive circuit resistance.
  • 24 gauge (0.5 mm) or smaller wire is typically used to carry approximately 1000 mA (1 ampere) or less. Use a 250-350 mA test lamp to load these circuits.
• Circuits carrying more than one ampere must be loaded with a device requiring a similar current (for example, a brake lamp). A voltage drop during loading indicates excessive resistance.
  • Wires of gauge 20 (0.8 mm) or larger are typically used to carry 1 ampere (1000 mA) or more.
  • Using voltage drop measurement is best practice and is more accurate for high-current circuits.

Checking Ground Circuits

The best way to check ground circuits is to measure circuit voltage drop while the component operates (or while attempting to start it).

• An ohmmeter can be used accurately only if the battery has been disconnected.
  • Expect less than 2 ohms for most small-gauge wires (18 gauge and smaller).
  • Disconnecting the battery is critical, because digital multimeter ohmmeter readings are usually distorted by normal voltage (battery connected) between body ground and chassis ground circuits.

Checking Resistance or Circuit Continuity

• Expect resistance of less than 2 ohms for most wiring harness circuits.
• A standard digital multimeter's low ohmmeter resolution (approximately 0.1 ohm) limits its accurate use to circuits carrying less than approximately 5 amperes. This is because very small resistances that are not detected by a standard digital multimeter can cause significant voltage and power losses in high-current circuits.
  • Voltage drop measurement is required for high-current circuits.
• A standard digital multimeter applies a small voltage to the circuit or component to calculate resistance. As a result, these ohmmeters are very sensitive to any voltage level. Voltage present in the circuit will distort digital multimeter readings.
  • Recommended practice: Swap the test leads and check for changes in the resistance reading. Reversing the digital multimeter leads should never change the resistance reading (unless the circuit contains a semiconductor). Differences in readings when the leads are swapped at the test points indicate incorrect test results.

Checking for Unintended Short to Other Circuits

A digital multimeter can be used to detect unwanted connections to:

• Ground
  • Recommended practice: disconnect both ends of the circuit. Measure the resistance between the suspect circuit and ground. Expect resistance greater than 10,000 ohms.
• Other De-energized Circuits
  • Recommended practice: disconnect both ends of the two circuits. Using an ohmmeter, measure between the two circuits. Expect resistance greater than 10,000 ohms.
• Energized Circuits
  • Recommended practice: disconnect both ends of the circuit. Turn the ignition on/operate (key on). Using a voltmeter, measure the voltage between the suspect circuit and ground. Expect no voltage.

Checking Circuits by Back Probing the Connector

Back probing should be used only when the diagnostic step requires the circuit to be tested under actual operating conditions. Back probing is a risky test method due to uncertainty of probe connection and the possibility of damaging terminals.

• Back probing can be used where the circuit must be analyzed by voltage drop method. All voltage drop tests measure a small amount of voltage (expect less than 5 percent of circuit operating voltage).
  • A zero voltage result indicates incorrect test conditions (no current flow or a poor back-probe connection).
• Sometimes the module's failure mode will change circuit operation when it is opened for testing. Back probing allows testing without changing normal module operation.

Back probing can be used for circuit analysis if the following precautions are strictly observed:

• Do not force test leads or other probes into connectors. Use care not to damage the terminal while making a good electrical contact with the circuit or terminal. Failure to follow these instructions may damage wiring, terminals, or connectors and cause subsequent electrical faults.
  • Use back-probe leads specifically designed for the purpose to help make a good test connection and prevent damage to the connector or terminal during back probing.
• Back probing is an incorrect test for a single-point voltage test. When zero volts is possible, you cannot distinguish a poor probe contact from a zero-voltage result. Disconnect the circuit and test normally.
• Back probing is an incorrect test for circuit continuity or an open circuit (using an ohmmeter) between two points. You cannot distinguish between poor probe contact and an open circuit. Disconnect and isolate the circuit and perform the test normally.

Circuit Analysis Using Jumper Wires (Creating Substitute Circuits)

Jumper wires can be used for circuit analysis if the following precautions are strictly observed:

• Always use jumper wires with fuses. The recommended fuse for a universal jumper wire is 5 amperes or less; higher-rated fuses should be used only when required by the load.
• Use flexible probes or similar to prevent damage to connector terminals.
  • Flexible probes are not designed for high current (more than 5 amperes). Do not use them to supply power to cooling fans, fan motors, or other high-current devices.
• Follow the diagnostic test instructions closely when using jumper wires to avoid damage to components or the wiring harness caused by incorrect jumper connections.
• Never repair a circuit by splicing a new wire in parallel with the old one (bypassing the circuit) without first fully understanding what caused the circuit failure. Always locate, investigate and correct the fault to eliminate the root cause and repair any adjacent wiring that was damaged.

Performing Voltage Drop Tests

A voltage drop test measures the loss of power or voltage in a circuit. Losses can be measured on the ground side or the power side (negative or positive) of any device circuit.

• The following is required to perform a voltage drop test:
  • A voltmeter connected at the start and end of the suspect circuit.
  • A working or attempting-to-work circuit. Power must be on and available for current flow.
• The voltmeter lead polarity must match the normal direction of current flow.
• A zero voltage reading indicates a poor voltmeter connection or that the component was not switched on.
• A small voltage reading indicates normal circuit loss. On 12-volt circuits this is typically less than 0.5 volt (not more than 5 percent of the circuit operating voltage).
  • A voltage reading greater than 0.5 volt indicates abnormal voltage loss. An abnormally high voltage drop indicates poor wires or connectors causing high circuit resistance.

Using the PID Module

Networked electronic modules usually offer diagnostic scanner access to internal data or operating values. This data is known as Parameter Identification (PID) data.

• Using a diagnostic scanner, PID input values, output states and diagnostic states can be read.
• Monitoring PID information in the diagnostic scanner data logger function allows detailed and accurate testing without disassembling the vehicle or using bulky equipment.
• Data logger displays show the "behind the scenes" information for almost every function of a modern vehicle.
• Most module outputs can be controlled using the PID controller. For example, a PID output state command is usually available for the cooling fan, engine speed, and many other module outputs.

Module Testing

Unnecessary module replacement results from incorrect or inadequate testing.

• Understand the correct function of the module. Read the system description and operation.
• Make sure programmable parameters are set correctly for the function in question (refer to Module Configuration 418-01 for additional information).
• First resolve any DTCs as directed in the workshop diagnostic manual.
• Check all inputs, both wired and network.
• Test outputs (refer to "Module Switched Circuit Testing" below).
• Check for module software updates (flash reprogramming).

Module Switched Circuit Testing

• Using the diagnostic scanner Output State Control function to activate components is a quick way to confirm the output can be switched on and off by the module. A test that shows normal module output function confirms the need to analyze module inputs.
• Do not connect ground or power directly to module-switched components with jumper wires unless directed to do so by the workshop manual, as the component may be damaged by direct connection to ground or power.