Thomas School Bus-Checking erratic oil pressure guage

TROUBLESHOOTING ENGINE OIL PRESSURE SYSTEM

Originally the oil pressure guage was jumping up and down but now it's reading below zero with the engine running. This indicates and "open circuit" that can be easily found with an ohm-meter.
Unhook both ends of the wire from the "oil pressure guage" and the "sender" and ground one end, then with your ohm-meter, probe the other wire end and touch the second probe to a good ground. An infinity reading means no continuity at all which indicates a definite open circuit. Replacing the sender wire completely or using the ohm-meter to narrow down the open is the next step.

TIP #1: Always check wiring closest to the environment first when looking for faults.

TIP #2: If you ground the "sender" end of the wiring the dash guage will read full to the right with the ignition in the "run" position, this indicates the wiring is OK.


MANUALLY CHECKING ENGINE OIL PRESSURE

This second clip has poor audio but it just shows a pressure guage hooked up to the oil pressure manifold [where the oil pressure sender usually screws into] the oil pressure is running evenly at 50 p.s.i. which is a good reading. This test tells us our mechanical end is OK and the focus should be on the electrical side of the oil pressure system. I gave this [Cat 3116] Diesel some revs at the end just for effect..



UPDATE! As with anything mechanical sometimes the strangest things happen. We found a bad ground at the mounting bolt that fastens the oil pressure manifold to the bus body, where the sender is threaded into. The path to ground from the sender switch had a high resistance which accounted for the faulty readings.

Quick Tip: Always check for the simple things first!