sending units
Note that there are two different sending units - those for the trucks with a gauge, and those are variable-resistance units, and those for trucks with a light - which are nothing more than pressure switches. However, also note that D4AZ 9278-A shows in both the gauge and light columns. That's because it was used in TSB #88-5-14, as described here.
Basically, Ford substituted a variable-resistance sending unit with a 6 psi pressure switch and a 20 ohm resistor. So any time the oil pressure was at or above 6 psi the switch would close and the gauge would read mid-scale - regardless of how much pressure there was.
Basically, Ford substituted a variable-resistance sending unit with a 6 psi pressure switch and a 20 ohm resistor. So any time the oil pressure was at or above 6 psi the switch would close and the gauge would read mid-scale - regardless of how much pressure there was.
See the page on Fuel System Part #'s.