Overheating School Bus on a Morning Route With Rising Coolant Temperature and Low Fan Engagement
A 2016 conventional school bus with a Cummins ISB 6.7 runs a 42-mile morning route with 31 stops. The driver reports the coolant temperature gauge climbing from 195°F to 228°F only on the route, never on the highway. The coolant level is correct, the radiator cap holds pressure, and no DTCs are stored. The shop finds the fan clutch is a viscous type with an air-actuated override. At idle in the bay the fan roars and the temperature holds at 198°F. During a loaded road test with the A/C on and repeated stops, the temperature climbs steadily above 215°F and the fan does not appear to engage at high speed. The charge-air cooler is clean, the thermostat was replaced last month, and the water pump belt is tight with no slippage. Ambient temperature is 88°F.
1. What is the most likely cause of the temperature rise specific to stop-and-go operation, and how would you confirm it before replacing parts?
Read the worked answer
The symptom pattern - normal at idle in the bay, normal at highway speed, rising only during loaded stop-and-go - points to the fan drive not engaging under the conditions that demand it, not to a coolant flow or heat-rejection problem. At highway speed ram air masks a weak fan; at idle the fan override may be commanded by the ECM, which explains the bay test passing.
Confirm by:
- Commanding the fan override with the scan tool during a loaded road test (or with a jumper at the fan solenoid if the tool cannot command it) and watching whether temperature drops and fan speed audibly increases.
- Checking the air supply to the fan clutch solenoid for pressure and leaks; a cracked line or failed solenoid will prevent engagement even though the clutch itself is good.
- Verifying the coolant temperature sensor reading against an infrared thermometer at the thermostat housing so a biased sensor is not mis commanding the fan.
- Only after the command path is proven good should the viscous clutch be condemned.
2. The fan override command produces no change and no air pressure is found at the clutch. Outline the diagnostic path and the repair decision.
Read the worked answer
With no command response and no air at the clutch, the fault is upstream of the clutch. Trace from the supply: check the chassis air tank pressure and the fan solenoid supply line for a closed valve or disconnected fitting; check the solenoid for voltage and ground when commanded; check for a blocked or crimped line between solenoid and clutch.
Decision logic:
- If voltage/ground are present at the solenoid but no air passes, replace the solenoid.
- If no voltage/ground, check the ECM driver circuit and the fuse; a shorted solenoid coil can pull the driver down.
- If air reaches the clutch but the fan still does not engage, the viscous clutch is failed and is replaced.
After repair, repeat the loaded road test with the A/C on and confirm temperature stabilizes below 210°F and the fan engages on command.