In this diagram, Fuse 18 is shown to Instrument Cluster
Fuse Block Diagram in a 88 by Jem270 at SuperMotors.net
4 is for instrument illumination & exterior lamps so if those lamps are ok, the fuse is good..
10 is for instrument illumination & ?
18 (Red/Yellow) is power to Instr Cluster
Long Version of how that Oil Pressure Gauge is Designed
Circuit Operation 87-88 F Series & Bronco;
"...In recent years, Ford Motor Company has been doing something I view as a bit sneaky. They have taken the oil pressure gauge and turned it into a glorified warning light. It says it's an oil pressure gauge, it looks like an oil pressure gauge, but in reality it is not. starting in 1986 a change was made on some models that altered the function of the oil pressure gauge as we knew it. The variable resistance sending unit was replaced with an open/closed switch and an in-line 20 ohm resistor was installed between the gauge and the switch. Anytime you have more than 4.5 psi of oil pressure, the switch closes, completing the circuit from the gauge (with the 20 ohm resistor in-line) to ground. This then results in a gauge that reads just above the middle. As you can see, there is no variation to the gauge needle in this circuit. It will either read no oil pressure or half-scale (normal oil pressure). On this style of gauge circuit, if the oil pressure gauge reads in the middle, it is telling you only one thing - you have more than 4.5 psi of oil pressure. How much more is anyone's guess at that point. When oil pressure drops below 4.5 psi, the gauge will return to 0 and alert the driver of a problem...Ford released a TSB (#88-5-14) for 1987-1988 F Series and Broncos that called for converting the variable resistive style oil pressure gauge circuit to the switch style circuit - if the customer complained of low or erratic oil pressure readings. The kit contains an oil pressure switch and an in-line 20 ohm resistor that is to be connected between the end of the existing sending unit signal wire and the new switch. The variable resistance oil pressure unit removed from the vehicle is then discarded. I want to point out some diagnostic problems that can arise from this circuit. First, if you are working on this switch style oil pressure circuit (with the in-line 20 ohm resistor) and install a variable resistive sending unit in place of the oil pressure switch, the result will be a gauge that reads lower than normal due to having two resistors in series, the fixed 20 ohm in-line resistor and the variable resistive sending unit. On the other hand, installing the open/closed switch on a vehicle that incorporates the conventional oil pressure gauge circuit will result in a pegged gauge needle after starting the engine. Sometimes you can tell that you need a switch instead of the variable resistive sending unit by the presence of the in-line resistor a few inches from the switch connector. This is not a good rule of thumb though, since many vehicles have the fixed resistor on the back of the cluster, and some of the digital dashes have that circuit built into the cluster. To determine what style circuit you have, simply ground the sending unit wire with the key on. If the gauge reads exactly half-scale, you can be fairly sure you need a switch-style sender. If you ground the sender wire and the gauge pegs, a variable resistor-style sender is needed..."
Source: by James M at asashop.org
This is Ford's Version:
Fake Oil Pressure Gauge TSB 88-05-14 for 87-88 Bronco & F Series
LIGHT TRUCK: 1987-88 F-SERIES, BRONCO
ISSUE: An oil pressure gauge that indicates an erratic or low reading when the engine oil pressure is within specification may be caused by the oil pressure sender. The oil pressure sender may not work properly with the magnetic oil pressure gauge.
ACTION: To correct this, install a new design oil pressure switch and new design resistor wire assembly using the following service procedure. Refer to the oil pressure switch and resistor wire application chart for the correct service parts.
OIL PRESSURE SWITCH AND RESISTOR WIRE APPLICATION CHART
Engine Application Description Service Part No.
5.0L, 5.8L, 7.3L and 7.5L Oil Pressure Switch E6SZ-9278-A
4.9L Oil Pressure Switch E8TZ-9278-A
All Resistor Wire Assembly E6SZ-9F291-A
1. Remove the existing oil pressure sender.
2. Install the new design oil pressure switch. Torque oil pressure switch to 10-18 lbs-ft (13-34 N-m).
3. Connect the female terminal end of the 20 ohm resister wire assembly to the oil pressure switch.
4. Connect the male terminal end of the 20 ohm resistor wire assembly to the vehicle wire harness.
PART NUMBER PART NAME
E6SZ-9278-A Oil Pressure Switch
E8TZ-9278-A Oil Pressure Switch
E6SZ-9F291-A Resistor Wire Assembly
Oil Pressure Mod to Actual PSI "...short R in PCB..." in 87-88 F series trucks and Broncos
btw, Jeff at the Bronco Graveyardhas offered a 2% discount to members of The BroncoZone for on line orders. To get your discount, enter the discount code BZMEMBER. Also you must include your BroncoZone User Name with the order.
Thanks for this input. I now have an idea where to start looking and probing with my trusty test light. I have the rest of the gauges in the cluster that still work, it's just the oil pressure one that don't work and it's causing the engine light to come on. I'm not overly concerned by it because it's been like that since I bought it a year ago and simple logic tells me that if it actually was a pressure issue or an oil pump problem, it wouldn't be with us in this world any longer. When I was running tow trucks in the past we had the same problem and there was some sort of a breaker that had to be reset on that Power Stroke for it to work again. What I'm wondering now that I've read your input here is, if I go out and install an aftermarket oil pressure gauge (one that actually has numbers on the face), will that engine warning light still stay on or will it go out and work properly?
Adding a wet pressure gauge will not change anything as far as the engine light is concerned. However, you will have a better indication of what it actually is and it can confirm or disprove that you are having actual pressure problems. A good rule of thumb is 10 PSI per 1000 RPM. When cold you will normally have around a 50 PSI reading which translates to a little above mid scale on the resistive type sender system.
If you have a older engine, you can keep it going strong for years by increasing the oil you use to 20W 50. I have been running my engine with 5 quarts of 20W 50, 1 quart of Lukas Oil Stabilizer and a high quality filter for several years now all 4 seasons. That makes it a little stiff until it warms up when temperatures are in the teens, but that is easily solved by using a block heater to keep the oil warm.
One thing I try to mention to people who want to trouble shoot their electrical problems is to get a Ford EVTM (Electrical and Vacuum Troubleshooting Manual) for the year truck that they have. This is one of the best investments a wrencher can make short of a complete set of Service manuals. I would only get a Chilton's or Haynes as a last resort, the reason being that they are just a readers digest form of the real thing with the editors eliminating vital information that they think is not necessary, WRONG. Also their electrical diagrams are guaranteed to give you a headache with lines running all over the place. The EVTM diagrams only show you what you need to know for the circuit you are troubleshooting.