Failure mode - Cummins 6.7L ISB Diesel (Ram HD)

Fan clutch failure (overheat while towing)

Reliability tier: verified - verify before ordering, see /disclosure/

Symptoms

temperature climbs only under load/towing/AC-high but stays normal unloaded
fan roars constantly (stuck engaged) or never engages
fan-clutch DTCs; belt noise from a wobbling clutch

Compare with the other causes of the same sign on this engine: overheating or temperature creeping up (3 causes) · ticking, knocking, rattle or whine (7 causes)

Root cause

The electronically-controlled fan clutch fails to engage (or stays locked). Undetected, the first symptom is a towing overheat - which on a 6.7 also stresses the head gasket and EGR cooler. Stuck-engaged failures waste fuel and scream instead.

Diagnosis

command the fan on with a scan tool and watch RPM response
watch coolant temp vs fan-speed PID on a loaded drive
inspect for wobble/free-play mechanically

Fixes

replace the fan clutch (electric-controlled unit); verify the cooling stack isn't debris-plugged at the same time ()

6.7 Cummins Fan Clutch (electronic-controlled)

Prevalence

common

Parts for this repair run about $230-$400: see them next to the other repairs on this engine in repair costs.