Final answer:
The first thing to check when troubleshooting a hard steering problem on a highway tractor while towing a loaded trailer is the fifth wheel adjustment.
Step-by-step explanation:
When troubleshooting a hard steering problem on a highway tractor that occurs only when towing a loaded trailer, the first thing that should be checked out is the fifth wheel adjustment. The fifth wheel is the mechanism that allows the trailer to be connected to the tractor. If it is not properly adjusted, it can cause the trailer to exert too much force on the tractor's steering, leading to a hard steering problem.
The other options listed - the power steering pump, tire pressure, and kingpin alignment - can also contribute to steering issues, but the fifth wheel adjustment is the most likely culprit in this scenario.
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