Overheating Only Under Load With a Full Coolant Level
A highway tractor with an electronically controlled viscous fan and a charge-air-cooled radiator overheats only when pulling a grade at 65 mph, reaching 225°F coolant with the fan commanded to 100%. At idle and under light load it holds 195°F. The coolant is full and clean, the cap holds 15 psi on the bench, and the thermostat was replaced last month. The tech notices the fan roars at startup for about 90 seconds and then goes quiet, and that the fan does not seem to roar again even when the scan tool shows 100% fan command and 220°F coolant. Ambient is 95°F.
1. The tech wants to replace the radiator because it is 'probably plugged.' Evaluate that and describe how to separate an air-side restriction from a coolant-side restriction from a fan-drive fault.
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Replacing the radiator first is a guess. The strongest clue is the fan behavior: it roars cold (normal viscous engagement) but does not roar at 100% command and 220°F. That points at the fan drive or its control circuit before the radiator.
Separate the three: (1) Air-side restriction - measure charge-air cooler and radiator pressure drop or use an infrared gun across the core; a large inlet-to-outlet air delta with low coolant delta suggests air-side fouling (bugs, bent fins, debris between CAC and radiator). (2) Coolant-side restriction - check top-to-bottom coolant temperature spread and flow; a plugged core shows a large spread and low flow. (3) Fan drive - command the fan with the scan tool and verify actual fan speed with a photo-tach or listen for roar; check the fan control solenoid, PWM signal, and that the viscous clutch is receiving the command. If fan speed does not follow command, diagnose the drive first.
2. Scan data shows the fan solenoid commanded to 100% but the fan does not change pitch. List the checks in order and state what each result tells you.
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Order: (1) Verify the command is actually reaching the solenoid - backprobe the solenoid connector for supply voltage and ground/PWM at 100% command. No signal means a wiring/ECM output fault, not a fan fault. (2) If signal is present, measure solenoid resistance and compare to spec; open or shorted coil condemns the solenoid. (3) Check the viscous clutch fluid: a clutch that roars cold then quits and won't re-engage hot is often low on silicone fluid or has a failed control valve. (4) Verify the fan is not freewheeling by hand when hot and commanded - excessive free play indicates internal clutch failure. (5) Confirm the fan is the correct rotation and that no one installed a fixed adapter. A signal-present, solenoid-good, clutch-won't-engage result condemns the fan drive; a no-signal result sends you back to the harness/ECM. Only after the drive is proven good do you move to the heat exchangers.