Fan Troubleshooting Guide: Part 2 – Cummins Engines

A reliable cooling fan is essential for keeping your Cummins engine running strong under heavy loads. If the fan is always on (hurting fuel economy) or fails to engage when needed (causing overheating), a step-by-step diagnostic approach will get you to the root cause quickly.

This is the second installment in our three-part Fan Troubleshooting Guide series. Today, we’re covering Cummins engines. Part 1 covered Caterpillar, and Part 3 will address Detroit. Follow these steps in sequence for the best results.

1. Verify the Fan Solenoid Type

Always start here: Confirm the correct fan solenoid configuration. The key is understanding whether your fan clutch requires air pressure to engage/lock up the fan hub or air pressure to release/disengage it.

• Air to engage/lock-up: Requires a normally OPEN fan solenoid.
• Air to release: Requires a normally CLOSED fan solenoid.

Note: This is the only factor determining the proper solenoid—regardless of current fan behavior.

If the solenoid is incorrect:

• Some models allow re-plumbing by swapping the air supply line and exhaust port to reverse the state.
• Others cannot be re-plumbed and require full replacement with the correct solenoid. At Kustom Truck Parts, we stock both normally open and normally closed fan solenoids to get you back on the road quickly. Shop our selection of fan solenoids here.

Once confirmed correct, proceed to Step 2.

2. Check Fan Solenoid Electrical Connections

Next, inspect the electrical side. Cummins controls the fan with a +12-volt supply signal from the ECM:

• ECM sends +12V to the solenoid → Fan OFF (disengaged)
• ECM removes +12V → Fan ON (engaged)

Use a multimeter on the two leads at the solenoid:

• One lead should have constant solid ground.
• The other lead will show power (or not) based on ECM command.

Important: This logic holds only if Step 1 solenoid type is correct.

• No ground on one lead? Trace it and connect to a solid chassis ground.
• Ground good, but no power? Check continuity on the power lead to the ECM output pin:
  • Cummins Celect Plus: Pin #C (21-pin round sensor harness interface connector)
  • Cummins CM570: Pin #12
  • Cummins CM870: Pin #38
  • Cummins CM871: Pin #38
  • Cummins CM2250: Pin #15
  • Cummins CM2350/2450/X15: Pin #78
• No continuity? Trace and repair the wiring.
• Good continuity and ground? Proceed to Step 3.

3. Verify ECM Input Status

Now confirm the four inputs that dictate fan operation. You'll need diagnostic software (Cummins INSITE, ProLink, etc.) to monitor live status.

Connect and navigate to the status/data monitor section. Monitor:

• Cooling fan output
• Four control inputs: Coolant temperature, Intake/manifold temperature, A/C high pressure switch, Fan override switch

Check the three common inputs first:

• Coolant Temperature → Triggers fan ON above 200°F. Verify reading is accurate and below threshold. Fix sensor or wiring if inaccurate.
• Intake/Manifold Temperature → Triggers fan ON above 110°F. Verify accuracy.
  Tip: After shutdown, heat soak can cause high readings. Briefly run the engine to purge hot air for a true value.
• A/C High Pressure Switch → Normally CLOSED circuit (constant ground = no fan needed). Opens at high pressure → removes ground → signals fan ON.
  Test: Unplug/replug the switch while watching ECM status—it should toggle. Confirm one connector lead has solid ground. Simulate closed circuit by jumping the two pins.

Fan Override Switch (Manual Fan Switch):

• Cummins Celect Plus: Analog input only at Pin #U (21-pin connector).
  Toggle switch while monitoring status—it should change. Celect Plus requires normally CLOSED input (ground when OFF, open when ON). Verify feature is enabled and circuit is good.
• Cummins CM570, CM870, CM871, CM2250, CM2350/2450/X15: Direct input (analog or J1939 datalink, depending on chassis).
  Analog pins:
  • CM570: Pin #15
  • CM870: Pin #17
  • CM871: Pin #17
  • CM2250: Pin #25
  • CM2350/2450/X15: Pin #11
  Toggle switch while monitoring—it should change state.
  For analog inputs (CM570, CM870, CM871): Requires normally CLOSED (ground when OFF, open when ON).
  Verify ECM programming: For J1939 glider kits, disable “accessory fan switch” under standard fan control; for analog, enable it.
  Trace circuit if status does not change.

All inputs correct? Proceed to Step 4.

4. Confirm ECM Output Status

Final check: With all inputs below thresholds and correct (Coolant <200°F, Intake <110°F, A/C switch OFF, Override OFF), the ECM should command fan OFF.

In Cummins INSITE:

• Look for the “Fan Clutch” parameter in the data monitor/logger.
  When “Fan Clutch” shows OFF → ECM output active (+12V) → fan should be OFF.
  When “Fan Clutch” shows ON → ECM output open (no +12V) → fan should be ON.

• Matches expected? System working correctly.
• Doesn't match? Re-trace from Step 1. Persistent issues may require a test ECM.

Following this guide methodically resolves most Cummins fan issues. Stay tuned for Part 3 on Detroit engines!

Happy wrenching!
—The Kustom Truck Parts Team

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