2.9 OP Scam
Sometime in '89, Ford pulled its head out of its rear-end and realized they had a serious problem with the 2.9's oiling system. So, they changed the electric OP gauge circuit.
The older circuit used a sending unit, whose resistance varied with the oil pressure. This resulted in an OP gauge that actually read oil pressure, though not very accurately.
The new circuit used an OP switch that was normally open, and closed when it saw about 5 PSI or better oil pressure. This powered the OP gauge in the dash, through a resistor, to show what appeared to be good oil pressure. All it actually did was tell you you had more than about 5 PSI oil pressure, and when you didn't, the gauge needle dropped like a rock. Ask me how I know.
So Ford engineered an OP switch that worked the reverse of how OP switches usually work. They changed the printed circuit in the dash, and added a resistor to let the whole mess work. In doing so, they hoped to delay your discovering the low oil pressure problem until the vehicle was at least out of warranty, and probably old enough that people would say, "Hey, it's an old truck. What do you expect?"
Can you say cover-up?
Anyway, I digress. To solve your problem, get a new sending switch for the year of your truck, not the year of your motor.
Better yet, get a mechanical gauge. I would never trust anybody's factory gauge, especially Ford's.
