Yale 94-1 – Engine Control Module (ECM) / Vehicle System Manager (VSM): Primary Fuel Pressure Lower Than Expected

The primary fuel pressure reading is falling below the acceptable threshold required for proper engine operation.

• Yale and Hyster Forklifts
• Yale VX line (GLP/GLC models)
• Hyster forklifts
• PSI 2.0L engine
• PSI 2.4L engine
• PSI 4.3L engine
• Doosan G420F LPG/Gasoline engine

1. 1

   Measure Primary Fuel Pressure: Connect a pressure gauge to the LD 2-Stage convertor to manually verify the actual primary fuel pressure.

2. 2

   Inspect Sensor Wiring: Check the fuel pressure sensor wiring harness for faults. Look for an open circuit, a short circuit to ground, or a short to the battery.

3. 3

   Check the Electric Fuel Lock-Off Valve: Verify that the electric lock-off valve is receiving proper voltage and ground. The voltage to this valve is controlled directly by the ECM.

4. 4

   Inspect Fuel Supply: Ensure the LPG tank valve is fully open and check the fuel level sensor. Inspect the fuel filter and fuel lines for physical restrictions, debris, or blockages.

5. 5

   Check the Controller: If all wiring and mechanical pressures are correct, the fault may lie in a functional failure of the ECM/controller itself.

1. 1

   Safely Isolate the Fuel System

   Close the fuel supply valve at the LPG cylinder and run the engine until it stalls to depressurize the fuel lines.

2. 2

   Inspect and Replace Filters

   Remove the fuel filter housing and check for contamination or debris. Tap the filter on a clean cloth; if heavily soiled or restricted, replace the fuel filter.

3. 3

   Test the Lock-Off Valve

   Using a multimeter, test the electric fuel lock-off valve for proper ECM voltage and ground while cranking. Replace the valve if it fails to open.

4. 4

   Test/Replace the Fuel Pressure Sensor

   If manual pressure at the LD 2-Stage convertor is within spec but the ECM still reports low pressure, the sensor is likely faulty. Disconnect the sensor, clean the connector pins, and test for proper reference voltage. If the wiring is intact, replace the fuel pressure sensor.

5. 5

   Reassemble and Clear Codes

   Reconnect all fittings, slowly open the LPG tank valve, and check for leaks using a soapy water solution. Use a diagnostic service tool to clear DTC 94-1 and verify that the actual fuel pressure matches the commanded pressure in the ECM data stream.