The diesel engine on a school bus pusher is a strange design with a transversely mounted radiator. Operators love these units because are very quiet (engine in the rear) with a stubby driveshaft. Low occurrences of driveline vibration and hearing yourself think is a bonus.
One thing that does cause problems is the hydraulic fan drive system that runs the engine cooling fan. The steering pump driven off the compressor runs the fan hydraulic motor as well. For those who find this confusing, the engine fan assy. is attached to the hydraulic motor shaft. Since the motor is driven by the steering pump it is labelled as a motor.
If you find you have stiff steering and the engine is overheating as well, you might want to look at the hydraulic steering pump. Check the hydraulic filter and magnet that resides inside the oil reservoir. If there is anything metallic then you're on the right track.
Older fan drives have a control manifold that has a electric solenoid that is triggered by an engine coolant temperature sensor. once the 200 degree (approx.) engine temperature is reached then the flow control opens the gates to full flow directed to the hydraulic fan drive motor. There is no need to worry about a fan belt on these rigs.
Newer pusher school buses have a 2 stage steering pump with sensors and manifold that controls the engine fan with a module bolted to the side of the frame. One stage of the hydraulic pump runs the steering and the other stage runs the engine fan. You also need the software to hookup to the fan control module to check out the entire system for proper flow and other parameters.
School buses have come a long way from the old belt drive and incredibly noisy gear box system that were popular in the early 90s. I think I've just dated myself, OK now you know I'm a child of the 60s... Hooray for Bob Dylan and the Chevy 283 cu. in. engine.
You can have plenty of services of good mechanics online. And they are correct person that can recommend solution for your problem.
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