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A pulse width modulated stepper motor valve (PSV) is used to bias gas flow to the venturi mixer as a means of air/fuel ratio control. The PSV is connected to the low pressure regulator through a special connector tube with two O-rings that seal against the inside diameter of the gas inlet port in the PSV. It is mounted to the mixer housing with 4 attaching bolts and uses an O-ring to seal the mounting flange. Gas flows through the PSV housing. The gas outlet passage in the PSV is intersected by a bore in which a piston operates. Gas flow is controlled by precise positioning of the piston and the amount that the piston protrudes into the gas outlet passage. Greater protrusion into the gas passage increases restriction and results in reduced gas flow and a leaner air/fuel ratio. Similarly, less protrusion reduces restriction resulting in higher gas flow and a richer air/fuel mixture. ‪

The piston is driven by a stepper motor which positions the piston based on a (PWM) position command signal from DDEC. The PSV is electrically connected to the ECM and 12 volt battery power through an 7 pin connector that mates to the engine sensor harness. The PSV supplies a "gas valve position" analog signal to DDEC which can be monitored using the diagnostic data reader (DDR). The signal indicates valve opening position and is scaled from 0 to 100 percent. A diagnostic signal is supplied to DDEC for loss of command signal, piston obstruction or valve electronics failure. See Figure 33389 . ‪

1. PSV ‪

5. Mixer ‪

2. Bolt ‪

6. Connector O-rings ‪

3. Washer ‪

7. Fuel Transfer Tube ‪

4. PSV O-ring ‪

Series 50G Automotive PSV 1998 and Later Engines

Replacement of Pulse Width Modulated Stepper Motor Valve for the Series 50G Engine

The PSV is non-serviceable and should be replaced as a unit. No adjustment is required. ‪

Removal of Pulse Width Modulated Stepper Motor Valve for the Series 50G Engine

Remove the PSV as follows: ‪


To avoid injury from the explosion of natural gas, the engine must be kept in a well ventilated area away from open flames, sparks, and electrical resistance (heating) coils.

  1. Vent fuel system, refer to Section .
  2. Disconnect the seven wire connector from the engine sensor wiring harness.
  3. Remove the four bolts connecting the PSV to the fuel mixer.
  4. Slide the PSV forward to separate from the connecting tube.

Installation of Pulse Width Modulated Stepper Motor Valve for the Series 50G Engine

Install the PSV as follows: ‪

  1. Lubricate connector O-rings and PSV O-ring.
  2. Loosen bolts attaching fuel mixer to throttle.

    Note: The bolt holes in the throttle are slotted to allow adjustment to align the PSV with the connector tube in the low pressure regulator.

  3. Position PSV against mixer with O-ring in place and align gas inlet port with connector. Slide rearward to engage connector until bolts holes are aligned.
  4. Secure PSV to the fuel mixer with four bolts and torque to 20 Nm (15 lbft).
  5. Tighten fuel mixer to throttle attaching bolts.


    To avoid injury from an explosion of natural gas, the following precautions must be taken:

    • Use a combustible-gas detector. Liquefied natural gas (LNG) is odorless and cannot be detected by smell. Compressed Natural gas (CNG) may be odorless and may not be detected by smell.
    • Equipment fuel systems are the responsibility of the Original Equipment Manufacturer (OEM). Equipment fuel system guidelines must be closely adhered to when installing or servicing equipment. Refer to OEM guidelines specifying which maintenance procedures require venting of fuel lines and fuel tanks.
    • LNG systems are pressurized and contain extremely cold (-260 F [-162 C]) fluids. Contact the fuel supplier or OEM for LNG safety requirements. Contact with LNG may cause personal injury (freezing).
    • Vent systems on the equipment should be ducted to a safe area whenever equipment is in an enclosed area.
  6. Clear AFR learn table using the Diagnostic Data Reader (DDR).
  7. Check for diagnostic codes using DDR.
  8. Check for leaks, refer to Section .