Wiring a classic vehicle like a 1972 Ford F-100 with a new harness can present some head-scratching moments, especially when dealing with the original equipment manufacturer (OEM) ignition switch. Many enthusiasts find themselves deciphering old wiring diagrams while trying to integrate modern aftermarket harnesses. One common point of confusion arises with identifying the function of each terminal on the OEM switch, particularly what might be termed the “Top Right Acc Position Counter.” Let’s break down how to approach this and ensure your vintage Ford truck is wired correctly and safely.
When tackling the wiring of your 1972 F-100’s ignition switch with an E-Z Wiring harness, understanding the terminals on your OEM switch is crucial. You mentioned struggling with the “top right” position, which you’ve labeled as the “top right acc position counter.” It’s important to clarify what this terminal actually controls. In most standard ignition switches of this era, the terminals are designed for specific functions related to starting and running your vehicle, as well as powering accessories.
Typically, an ignition switch will have connections for:
- Battery (B or BAT): This is the main power feed from the battery.
- Ignition (IGN): This terminal powers the ignition system and other engine-related components that need to run when the engine is on.
- Accessory (ACC): This powers accessories like the radio, heater blower, and wipers – items that can be used when the engine isn’t necessarily running.
- Start (STR or S): This terminal activates the starter solenoid to crank the engine when you turn the key to the start position.
Considering your description, the “top right acc position counter” you’re inquiring about is almost certainly the Accessory (ACC) terminal. The term “counter” might be a slight misunderstanding, as it’s not really counting anything. Instead, think of it as the terminal that provides power when the ignition switch is in the accessory position. This position is usually one click back from the “off” position or one click forward from “off” before the “ignition” position.
To verify this on your 1972 F-100 OEM switch, and to ensure you’re connecting it correctly to your E-Z Wiring harness, here’s a practical approach:
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Consult Wiring Diagrams: You’ve already referenced the 1972 master wiring diagram, which is a great starting point. Look closely at the ignition switch section to see if there’s a terminal labeled for accessories and its corresponding location on the switch diagram.
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Test with a Multimeter: The most reliable way to identify the function of the “top right” terminal is to use a multimeter.
- Set your multimeter to measure DC voltage.
- Connect the black lead of your multimeter to a good ground on your F-100.
- Probe the “top right” terminal of your ignition switch with the red lead of your multimeter.
- Turn your ignition key to the accessory position.
- If the multimeter reads approximately 12 volts (or your battery voltage), then this confirms that the “top right” terminal is indeed your Accessory (ACC) terminal.
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Connect to Your E-Z Wiring Harness: Your E-Z Wiring harness likely has labeled wires for “IGN Switch Accessory.” Once you’ve confirmed the “top right” terminal is the ACC terminal on your OEM switch, you should connect the “IGN Switch Accessory” wire from your new harness to this terminal.
Regarding your other connections and questions:
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Top Center (21Y to IGN Switch Power): Connecting the top center blade (21Y) to “IGN Switch Power” seems plausible if 21Y is indeed the main power feed (Battery). However, “IGN Switch Power” on your harness might be intended for the Ignition terminal, not the Battery terminal. Double-check your E-Z Wiring harness instructions to confirm where they intend the main battery feed to connect. It’s usually labeled “Battery” or similar.
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Top Left (98V/297 BK-G to IGN Switch Accessory): Connecting 98V/297 BK-G to “IGN Switch Accessory” might be incorrect if you’re also considering the “top right” as the Accessory terminal. 98V and 297 BK-G originally going to the instrument cluster suggests they might be related to gauges or indicators that are powered in the ignition or run position, not just accessory.
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Bottom Left (32R-B to IGN Switch Start): Connecting 32R-B to “IGN Switch Start” is likely correct as 32R-B going to the starter relay aligns with the Start function.
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Bottom Middle (16PK/932B/16APK/16R-G to IGN Switch Coil): Connecting 16PK etc., to “IGN Switch Coil” is questionable. “IGN Switch Coil” in an aftermarket harness typically implies it’s for the Ignition terminal, powering the ignition coil. However, the original wires going to the instrument cluster (st cluster/RPO gauge cluster) suggest these might be related to gauge power as well, potentially in the “Ignition” or “Run” position, not specifically the “Coil” which is usually directly engine-related.
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Bottom Left Original (977V – Gauges): If 977V goes to gauges, it’s likely another Ignition or Accessory feed for the instrument cluster.
Recommendation:
Before proceeding further, it’s highly recommended to:
- Accurately identify each terminal on your OEM ignition switch using a multimeter as described above (Battery, Ignition, Accessory, Start).
- Carefully review the E-Z Wiring harness instructions to understand what each labeled wire is intended for (IGN Switch Power, IGN Switch Accessory, IGN Switch Start, IGN Switch Ignition, Battery, etc.).
- Compare the original 1972 wiring diagrams with your findings and the E-Z Wiring harness instructions to create a clear connection plan.
Wiring issues can lead to electrical problems and even fire hazards. Taking the time to properly identify each terminal and understand its function, especially the “top right acc position counter” (Accessory terminal), is essential for a safe and functional wiring job on your classic 1972 F-100. If you’re unsure, consulting with a professional automotive electrician might be a worthwhile investment for peace of mind.
