Follow these procedures to diagnose problems with the sensor or actuator circuits.
Section 15.1.1
Basic Procedure
The basic diagnostic procedure recommended for most sensor and actuator circuits is to disconnect the harness at the connector and inspect for corrosion, bent pins, spread pins, or any condition that could cause a loose or intermittent connection. See Figure
“Connector Removal”
.
Figure 1. Connector Removal
Section 15.1.2
Connector Voltage Checks
Turn the ignition key to the ON position and measure if the expected voltages are present at the connector. On circuits with expected voltages, this test will verify the integrity of that circuit. On circuits without an expected voltage, this test will determine if that circuit is shorted or miswired to a voltage source.
- Signal return (marked “A” on all engine sensor harness connectors) should measure less than 0.25 volts.
- VREF should measure 5.00 volts ± 0.50 volts. If this is higher or lower than expected, disconnect sensors one at a time to determine if a sensor is biasing the circuit and refer to VREF procedures. See Figure
“Measuring VREF Voltage”. - Sensor signal lines will measure 0 to 0.25 volts if the circuit is designed to pull down when disconnected, or a higher voltage (normally 4.6 to 5, or 12 volts) if it is designed as a pull up circuit. A pull up signal circuit that measures the expected value normally indicates a good circuit.
- Actuator circuits may be either on/off type circuits (normally 12 volts) or pulse width modulated circuits (12 volts controlled by a percent duty cycle.
Figure 2. Measuring VREF Voltage
Section 15.1.3
Connector Checks to Ground (B-)
Next, measure the resistance of all wiring harness connectors to ground (preferably the negative battery cable) to determine if a short to ground condition is present. It is important that during this test all accessories including the dome light be turned off. Current flow in the system will affect resistance readings. If the reading is fluctuating greatly, disconnect the positive battery cable and measure to the negative battery cable.
- Signal ground (marked “A” on all engine sensor harness connectors) should measure less than 5 Ω. Signal ground on vehicle sensor connectors vary. See Figure
“Measuring Resistance to Ground”. - The VREF and signal lines, with the processor connected, will normally measure greater than 1,000 Ω.Note: The symbols < (less than) and > (greater than) are used on each diagnostic circuit page. e.g. < 5 Ω is less than 5 Ω; > 5 Ω is more than 5 Ω.
- Power ground on an actuator circuit should measure less than 5 Ω. The control side of an actuator circuit will normally measure greater than 1,000 Ω.
Figure 3. Measuring Resistance to Ground
Section 15.1.4
Harness Resistance Tests
The harness resistance tests are performed when a circuit is suspected of having high resistance or being open. These tests are performed with the breakout box connected and by measuring resistance from the sensor connector end to the processor connector. If an open circuit or high resistance is encountered, the problem is easily isolated by separating the circuit at the interim connectors (normally the Deutsch connector on the valve cover or the Packard 22 pin on the cowl) and measuring resistance through both halves of the circuit.
Section 15.1.5
Operational Signal Checks
These checks are made with the breakout box installed and are normally measuring a signal voltage or frequency. They are useful for determining an in-range type fault or an intermittent connection.
If an intermittent fault is detected, monitoring a suspected circuit and recreating the environmental or physical conditions that caused the complaint will help verify if a problem is in a particular circuit.
It is critical when measuring the signal level of a circuit to understand its function and whether it is an analog voltage, digital frequency, sine wave or digital communication signal. A standard Digital Volt Ohm Meter (DVOM) has certain limitations in measuring any circuit that has a frequency.
| Series 40E Troubleshooting Manual – 6SE241 |
| Generated on 10-13-2008 |