33-C-3928865 | Genuine Cummins® Electric Fan Clutch

Cummins ISB 5.9
Cummins ISB 6.7
Cummins QSB

• Freightliner medium- and heavy-duty trucks with ISB 6.7L engines
• Kenworth trucks with ISB or compatible QSB engines
• Peterbilt trucks with ISB or compatible QSB engines
• Blue Bird school buses with Cummins ISB 6.7L
• IC Bus and Thomas Built Buses with Cummins ISB
• Various vocational trucks and industrial equipment powered by Cummins Engines

1. Prevent engine overheating

Its primary job is to keep the engine from getting too hot.

When the engine is under heavy load—such as towing, climbing hills, or operating in hot weather—the fan clutch engages the radiator fan to increase airflow through the radiator and cool the engine.

2. Improve fuel economy

A large engine-driven fan can require 10 to 50 horsepower (or more on some heavy-duty trucks) when operating at full speed.

By disengaging the fan when extra cooling isn't needed, the engine doesn't waste power spinning it all the time, which can reduce fuel consumption.

3. Increase available engine power

When the fan is disengaged:

• More engine power is available for acceleration.
• More power reaches the drivetrain instead of driving the cooling fan.

This is especially helpful for trucks hauling heavy loads.

4. Reduce fan noise

Cooling fans are loud at high speed.

An electric fan clutch keeps the fan slowed or disengaged whenever possible, making the vehicle quieter, particularly during cruising or idle.

5. Speed up engine warm-up

A cold engine runs more efficiently once it reaches its normal operating temperature.

By keeping the fan off during warm-up, the engine reaches its ideal temperature more quickly, which can improve efficiency and reduce emissions.

6. Improve emissions performance

Modern engines operate most efficiently within a specific temperature range.

1. Engine overheating

This is the most common symptom.

If the fan clutch doesn't engage when commanded, there isn't enough airflow through the radiator, especially:

• At idle
• In stop-and-go traffic
• While climbing hills
• When towing or hauling heavy loads
• In hot weather

Possible causes:

• Failed clutch
• Faulty actuator or solenoid
• Electrical problem
• Failed temperature sensor
• ECM control issue

2. Fan runs all the time

A fan that's constantly engaged may cause:

• Loud "roaring" fan noise
• Reduced fuel economy
• Slower engine warm-up
• Reduced engine performance

This can happen if the clutch is mechanically stuck or receives an incorrect command.

3. Excessive fan noise

A healthy fan is usually quiet except when maximum cooling is needed.

A failing clutch may produce:

• Constant roaring
• Grinding
• Rattling
• Squealing

Grinding or rattling can indicate worn bearings inside the clutch.

4. Poor fuel economy

If the fan remains engaged continuously, the engine must use extra power to spin it, increasing fuel consumption.

5. Reduced engine performance

A locked fan can consume significant engine power.

Symptoms include:

• Sluggish acceleration
• Reduced hill-climbing ability
• Lower overall performance

6. Check Engine Light

Modern heavy-duty trucks often monitor fan clutch operation.

You may see:

• Check Engine Light
• Cooling system warning
• Fan control fault messages

Common diagnostic trouble codes relate to:

• Fan speed mismatch
• Fan clutch circuit faults
• Fan actuator faults
• Communication errors

7. Fan doesn't engage under heavy load

If the engine temperature rises but the fan never speeds up, the clutch may not be responding.

This can result from:

• Failed clutch
• Damaged wiring
• Blown fuse
• Faulty relay
• Failed ECM output

8. Intermittent operation

The fan may:

• Engage and disengage unexpectedly
• Operate only after hitting bumps
• Work only when cold or only when hot

These symptoms often point to:

• Loose electrical connectors
• Damaged wiring
• Internal electronic failure

9. Visible damage

Inspect the fan clutch for:

• Oil or grease leakage (if applicable)
• Burned electrical connector
• Corroded terminals
• Broken fan blades
• Excessive bearing play
• Signs of overheating