For the Voltage Reference Circuits (VREF) function diagram, see Figure
“Voltage Reference Circuits Function Diagram”
.
| 1. Engine Oil Pressure Sensor | 5. Accelerator Position/Idle Validation |
| 2. Camshaft Position Sensor | 6. Barometric Pressure Sensor |
| 3. Injection Control Pressure Sensor | 7. Manifold Absolute Pressure Sensor |
| 4. ECM | |
Figure 1. Voltage Reference Circuits Function Diagram
The VREF circuit is a 5 ± .5 volt power supply from the ECM that provides power to the three wire engine and vehicle sensors and provides a benchmark or reference voltage for the ECM.
There are three separate VREF circuits in this system:
- VREF B — for chassis sensors
- VREF C — for body builder sensors
- VREF D — for engine sensors
Section 46.1.1
Fault Detection/Management
There is no fault detection specifically for the VREF signal directly, but if there is a VREF circuit fault, the sensor(s) in the section of the circuit affected may set an out of range high or low code. Multiple high or low codes are usually an indication of a VREF or, in some instances, a Signal Ground fault condition.
A VREF signal shorted to ground will cause the ECM to reset, causing either a stumble or no start condition.
For the Voltage Reference Circuits, see Figure
“Voltage Reference Circuits”
.
Figure 2. Voltage Reference Circuits
Connector VREF voltage checks are listed in Table
“Connector VREF Voltage Checks”
. If multiple faults are set, remove and measure the VREF at suspected sensor circuits.
| Sensor | Test Points | Spec | Comments |
| ICP | B to Grd | 5 ± .5v | Check VREF at suspected sensors one at a time. Identifying which sensors do not have VREF and which ones share common VREF feed will more quickly help isolate the area of a short or open circuit. If disconnecting a sensor causes VREF to be present at a circuit that had previously lost VREF, it is likely that the disconnected sensor had shorted VREF to ground.
|
| CMP | B to Grd | 5 ± .5v | |
| MAP | B to Grd | 5 ± .5v | |
| APS/IVS | C to Grd | 5 ± .5v | |
| BARO | B to Grd | 5 ± .5v |
Connector checks to chassis ground are listed in Table
“Connector Checks to Chassis Ground”
. Check with the sensor connectors disconnected and the ignition key OFF; all accessories OFF.
| Sensor | Test Points | Spec | Comments |
| ICP | B to Grd | > 1000 Ω | Resistance less than 1000 Ω indicates a short to ground. If a short to ground condition is identified, remove all sensor connectors that are connected to VREF and ECM to determine if the short is in a sensor, ECM or wire harness. If the short is identified in the harness, remove the intermediate Deutsch connectors and measuring to ground will identify in which part of the harness the short is located.
|
| CMP | B to Grd | > 1000 Ω | |
| MAP | B to Grd | > 1000 Ω | |
| APS/IVS | C to Grd | > 1000 Ω | |
| BARO | B to Grd | > 1000 Ω |
Connector checks to chassis ground are listed in Table
“Connector Checks to Chassis Ground”
. Check with the sensor connectors disconnected and the ignition key OFF; all accessories OFF.
| Sensor | Test Points | Spec | Comments |
| ICP | B to No. 19 | < 5 Ω | The measurement is taken from the sensor connector to the ECM 60-pin connector. Resistance greater than 5 Ω indicates high resistance or an open in the VREF supply circuit.
|
| CMP | B to No. 19 | < 5 Ω | |
| MAP | B to No. 19 | < 5 Ω | |
| APS/IVS | C to Grd | < 5 Ω | |
| BARO | B to Grd | < 5 Ω |
Section 46.1.2
Function
The ECM contains a regulated 5 volt DC voltage reference source to power engine and vehicle control sensors. The sensor signals are compared to the VREF to determine actual sensor output signal values. These values are processed by the ECM for engine operation.
Section 46.1.3
Operation
The ECM is supplied with Battery Voltage when the ignition key is ON at terminals 40 for the engine (gray connector) and terminals No. 3, and No. 5 for the chassis and body builder connections (black connector).
Section 46.1.4
ECM Voltage Reference Diagnostics
If multiple sensor codes are set, it is possible that the VREF signal circuit is open or the return signal circuit is open. Follow troubleshooting procedures to determine if VREF circuits are at fault. The engine will not run without a valid VREF signal.
| Series 40E Troubleshooting Manual – 6SE241 |
| Generated on 10-13-2008 |