What causes the voltmeter to indicate high voltage?

Either high battery voltage (service the charging system as required) or an inoperative voltmeter (replace the voltmeter).

How do I test whether the Oil Pressure Switch is causing the warning light to stay on?

Re-connect the 39 (R/W) wire to the Coolant Temperature Sender, then jumper the 31 (W/R) wire from the Oil Pressure Switch to ground. If the indicator goes off, replace the Oil Pressure Switch.

What wire colors are used for the Engine Warning Indicator circuits?

Circuit 39 is R/W (red/white) to the Coolant Temperature Sender and circuit 31 is W/R (white/red) to the Oil Pressure Switch.

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1990 Ford Bronco Instrument Cluster Troubleshooting 60-9 - Voltmeter & Warning

Ford instrument cluster troubleshooting hints page 60-9, voltmeter and engine warning indicator

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Shop manual troubleshooting chart (60-9) covering voltmeter low/high voltage and always-on engine warning indicator; check shorts on 39 R/W or open 31 W/R wire.

AI caption June 2012 · Photo 1418 of 1799

This document is page 60-9 of a Ford service manual section covering the Instrument Cluster, presenting a continuation of a troubleshooting hints table. The table is organized into three columns: Condition, Possible Cause, and Action. Three conditions are covered: (1) voltmeter indicates low voltage/no voltage, with causes including low battery voltage, wiring/connector faults, a damaged printed circuit, a blown fuse, or an inoperative voltmeter, and matching corrective actions such as servicing the battery/charging system, checking wiring and connectors, inspecting the printed circuit, servicing shorts and replacing fuses, or replacing the voltmeter; (2) voltmeter indicates high voltage, caused by high battery voltage or an inoperative voltmeter, corrected by servicing the charging system or replacing the voltmeter; and (3) Engine Warning Indicator always on, caused by actual high coolant temperature or low oil pressure, a short to ground on the 39 (R/W) wire or an open on the 31 (W/R) wire, an inoperative Coolant Temperature Sender or Oil Pressure Switch, or an inoperative Engine Warning Indicator Module. Detailed diagnostic steps are given, including disconnecting and jumpering the 39 (R/W) and 31 (W/R) circuits to isolate the faulty component before replacing the module.

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Frequently asked questions

Why does my voltmeter read low or no voltage?: Per the table, possible causes are low battery voltage, wiring and/or connector faults, a damaged printed circuit, a blown fuse, or an inoperative voltmeter. Actions include servicing the battery/charging system, checking wiring for breaks and connectors for tightness/corrosion, checking the printed circuit for cracks, servicing the circuit short and replacing the fuse, or replacing the voltmeter.
What causes the voltmeter to indicate high voltage?: Either high battery voltage (service the charging system as required) or an inoperative voltmeter (replace the voltmeter).
Why is the Engine Warning Indicator always on?: Possible causes: an actual high coolant temperature or low oil pressure condition, a short to ground on the 39 (R/W) wire or an open on the 31 (W/R) wire, an inoperative Coolant Temperature Sender or Oil Pressure Switch, or an inoperative Engine Warning Indicator Module.
How do I test whether the Oil Pressure Switch is causing the warning light to stay on?: Re-connect the 39 (R/W) wire to the Coolant Temperature Sender, then jumper the 31 (W/R) wire from the Oil Pressure Switch to ground. If the indicator goes off, replace the Oil Pressure Switch.
What wire colors are used for the Engine Warning Indicator circuits?: Circuit 39 is R/W (red/white) to the Coolant Temperature Sender and circuit 31 is W/R (white/red) to the Oil Pressure Switch.