Reverse Polarity & Isolation Relay Diagrams
Wiring reference for a positive-triggered reverse polarity relay pair, a negative-triggered reverse polarity relay pair, and a single isolation relay. Use these patterns for door lock actuators, camera arms, upfitter accessories, and any load that must never backfeed another circuit.
Standard SPDT Relay Pin Identification
| Pin | Name | Function |
|---|---|---|
| 30 | Common | Moving output contact — connects to 87a at rest, switches to 87 when the coil is energized. |
| 87 | Normally Open (NO) | Connects to pin 30 only while the coil is powered. Usually fed with fused +12V. |
| 87a | Normally Closed (NC) | Connects to pin 30 whenever the coil is off. Used to hold the motor lead at ground in the resting state. |
| 85 | Coil ground side | One side of the electromagnet coil. Goes to ground in a positive-triggered circuit — or to the switched negative trigger in a ground-triggered circuit. |
| 86 | Coil power side | Other side of the coil. Goes to the switched trigger (positive or negative depending on the design) or to a fused 12V source. |
1. Positive Reverse Polarity Relay Pair
How it works: A center-off (ON–OFF–ON) switch sends fused +12V to the coil of Relay A or Relay B — never both. Each relay's pin 30 feeds one motor lead, pin 87 is fed fused +12V, and pin 87a holds that same motor lead at ground.
- Switch off: both motor leads sit on 87a (ground). The motor is statically held — no drift, no chatter.
- Relay A energized: lead 1 gets +12V, lead 2 stays grounded → motor runs FORWARD.
- Relay B energized: lead 2 gets +12V, lead 1 stays grounded → polarity is reversed, motor runs in REVERSE.
Common uses: door lock actuators on positive-triggered systems, sliding camera arms, upfitter valve/slide actuators, light bars with a reversing motor drive.
2. Negative Reverse Polarity Relay Pair
How it works: The motor wiring is identical to the positive-triggered pair. The difference is only in the coils: pin 86 of each relay is fed fused +12V permanently, and the switch energizes the relay by supplying ground to pin 85.
- Use this when the trigger source (body control module, alarm output, keyless-entry driver) grounds its output when active instead of supplying +12V.
- Both relays stay off when no ground is present, so the actuator rests with both leads grounded on 87a.
- Grounding Relay A's coil runs the motor forward; grounding Relay B's coil reverses it.
Common uses: negative-triggered factory door lock systems, alarm/lock-actuator integration, and equipment that switches on the ground side of the circuit.
3. Isolation Relay
How it works: The relay's contact side (87 → 30) carries the device's power from a fused constant battery feed. The coil (86 → 85) is energized by a switched ignition source, so the device only receives power while the ignition is on.
- Ignition on: coil energizes, 30 connects to 87, and constant +12V flows to the load.
- Ignition off: the contact opens, so the constant battery feed cannot leak backward into the vehicle's ignition or accessory circuits — no module sleep disruption, no parasitic draw.
- Fuse the constant feed at the battery and keep the ignition coil feed small (the coil draws only ~150–200 mA).
Common uses: feeding a Fleet 3 hub, router, or radio from constant battery power while still controlling it from ignition — without backfeeding the ignition circuit or waking vehicle modules.
Bench check before vehicle power-up
Energize each coil and listen for the click; verify 30 switches from 87a to 87 with a continuity meter.
With everything off, confirm both motor leads read ground — the actuator must not move or drift.
Trigger each side once and confirm forward/reverse direction matches the switch position.
After installation, run a parasitic draw test to confirm the relays are not loading any circuit at rest.