yo beast runner,
Wiring Diagram in 87-89 Bronco & F series (Mitchell)
Source: by equivalent (Beetlejuice) at SuperMotors.net http://www.supermotors.net/registry/media/280828_1
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
Source: by Mike & Debbie at http://www.rowand.net/Shop/Tech/FordOilPressureGaugeFix.htm
btw, Jeff at the Bronco Graveyard
has 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.