Ford Truck Oil Pressure Gauge - Is it an idiot light?
The Ford truck line from 1987 to 1991, which includes F150, F250, F350 and Broncos, used a full set of gauges that included oil pressure and voltage gauges. Unfortunately, halfway through the production run Ford hobbled the oil pressure gauge, turning it into a glorified indicator light.
Sensor vs Switch
The 1987 to 1988 trucks included a fully functional oil pressure gauge. They used an engine mounted oil pressure sensor that sends a variable resistance signal to drive the gauge. The sensor is grounded to the engine block. When oil pressure is low the signal wire will show roughly 100 plus ohms to ground, which causes the gauge needle to read low. The resistance drops as the oil pressure increases. Under normal operating conditions the sender will show roughly 20 ohms, causing the gauge to read a little over halfway. It's not a precise system, but it works.
Starting in 1989 Ford replaced the oil pressure sensor with an oil pressure switch. When oil pressure is low the switch will be off or open, producing a very high (infinite) resistance signal to the gauge. With normal oil pressure the switch will close, producing a very low (near zero) resistance signal to the gauge. In order to make the gauge appear to read normal, Ford started installing a 20 ohm resistor on the back of the gauge cluster.
Circuit Board Differences
1987 and 1988 trucks used cluster circuit boards without a resistor. Note that 1988 used a slightly different circuit board that included a 510 ohm resistor for other purposes. Like the 87 circuit boards pictured, they did not have the oil pressure resistor.
1989 to 1991 trucks all used the same circuit board that included the 20 ohm resistor for the oil pressure circuit. The 20 ohm resistor has stripes colored red, black, black, and gold.
Modification
Since the 89-91 trucks still used the same physical gauge, it's common to modify them in order to have a fully functional yet stock looking oil pressure gauge. The first step is to replace the oil pressure switch with the sensor used on early trucks. For the cluster side you can replace the circuit board (or the entire cluster) with one from an earlier year truck. Or you can remove the resistor and solder a small wire in its place, although this can be risky since the base plastic of the circuit board melts easily. We usually recommend replacement with earlier year parts.
Tachometer vs Non Tachometer Clusters
Through the 87-91 range trucks were sold with and without a tachometer. Functionally all of the gauges are identical, including the oil pressure differences listed above. In addition, all the gas powered trucks included all the wiring for tachometer gauge clusters. This makes upgrading to a tachometer cluster a simple drop in operation. Even the differences between 6 cylinder and V8 are worked out in the truck wiring harness.