Case IH 158-17 - VCM – VCM / ECM: Keyswitch Battery Potential - Voltage Low

Indicates an issue with the switched power circuit supplying the controller. It typically manifests as either low switched voltage reaching the controller from the ignition/keyswitch, or an ECU power down error where the controller does not power down properly because the switched voltage does not drop completely to 0V.

• MAGNUM AFS Connect
• Steiger
• Magnum
• Puma
• Maxxum
• 7120
• 8120
• 9120
• 140/240 series

1. 1

   Verify that the batteries are fully charged and pass a load test, as low system voltage will trigger this code.

2. 2

   Inspect main chassis grounds, cab grounds, and the specific ground wire on the firewall/driver's side leading to the VCM for high resistance.

3. 3

   Back-probe the keyswitch to ensure it is outputting full battery voltage when in the 'ON' position and dropping completely to 0.0V when in the 'OFF' position.

4. 4

   Inspect the VCM/ECM power relays and fuses in the cab distribution box for corroded pins, melted plastic, or loose terminal tension.

5. 5

   Measure the voltage drop on the switched power wire between the keyswitch/relay and the VCM connector (Input A2 or equivalent switched power pin).

1. 1

   Disconnect Power

   Turn off the master disconnect switch or remove the negative battery cables to prevent short circuits.

2. 2

   Clean Grounds

   Remove the main ground connections at the chassis and firewall. Clean the terminals and mounting surfaces with a wire brush until bare metal is exposed, then reattach and tighten to spec. Apply dielectric grease to prevent future corrosion.

3. 3

   Replace Faulty Relays

   If the VCM power relay or ignition relay shows signs of pitting, high resistance, or sticking (which prevents the voltage from dropping to 0V on shutdown), replace the relay.

4. 4

   Repair Wiring

   If a voltage drop was found during diagnostics, trace the switched power wire from the ignition switch to the VCM. Repair any chafed sections, broken wires, or corroded connectors using OEM-approved weather-pack connectors and heat shrink.

5. 5

   Replace Keyswitch (If Applicable)

   If the ignition switch failed the diagnostic test (bleeding voltage when off or dropping voltage when on), remove the dash panel and replace the switch assembly.

6. 6

   Re-test

   Reconnect the batteries. Turn the key to the ON position and verify via the diagnostic display or ServiceMaxx/EST tool that the switched voltage matches the battery voltage. Turn the key OFF and verify the VCM powers down correctly without logging the 158-17 code.