Electrical Power

ATA Chapter 24 — N720AK Systems Reference

Overview

N720AK’s electrical system uses a dual-bus architecture managed by the flyEFII System32 Bus Manager. Power distribution and electronic circuit breaker protection are provided by the Vertical Power VPX Sport. Two EarthX ETX900 lithium batteries provide redundant power with fully isolated charging systems: a 60-amp primary alternator charges Battery 1, and a Monkworkz MZ-30 generator charges Battery 2.

Components

How It Works

Bus Architecture

• Essential (Endurance) Bus: Powers critical engine systems — ignition, fuel injection, fuel pumps. Managed by System32 Bus Manager. Protected by conventional physical circuit breakers, one per item — these are not VPX channels.
• Main Bus: Powers avionics and other aircraft systems via VPX Sport electronic breakers.

The only physical breakers in this airplane are on the essential bus bar. Most feed the things that keep the engine running — that is the inverse of a conventional airplane, where the pullable breakers are avionics and the engine needs no electrical power at all. Anyone reaching for a breaker panel out of habit may be reaching for ignition, fuel injection, and the fuel pumps. Brief this before engine start. See sections/sys-73-efii.md for why the engine stops without them.

Essential Bus Bar Breakers

Per the SteinAir power & lighting schematic (verified 2026-08):

COM 1 / NAV 1 split: pulling COM 1 kills only the GTN’s com transceiver (stuck-mic response — screen, GPS, and VLOC keep running). Pulling NAV 1 kills the GTN main and VLOC boards; the com board keeps operating on its last frequency with no display or tuning.

Known issue (fix planned at annual — task plan: plans/electrical-mods-2026-annual.md): the GTN’s COM/NAV power relays are held closed by the AV MSTR switch from a main-bus feed — so any main-bus loss opens them and the GTN goes dark despite its essential-bus breakers being hot. Planned fix: re-source the relay coil feed from the essential bus. The transponder/ADS-B (VPX J10-7) and GMA 245/headset power (VPX J10-4) also die with the main bus; planned fixes are an essential dual-feed and a dedicated essential breaker respectively.

Display Backup Power — Deliberately Not Installed

Dynon’s SV-BAT-320 backup batteries are not part of this system, by choice. With the System32 Bus Manager arbitrating two independent EarthX ETX900 batteries on fully isolated charging systems, bus current stays stable through a single charging-source or battery failure, so the per-display backup batteries solve a problem this architecture does not have. This matches what RV builders running the Bus Manager with dual batteries have generally concluded.

Consequence worth knowing: there is no display-level power reserve. Display availability depends entirely on the bus, which is what the dual-battery and dual-charging architecture exists to protect.

How the Bus Manager Actually Works

Verified against Bus Manager Drawing 5/5A (2026-08). Three facts define the architecture:

1. The key drives the Main Bus Relay (the only control of the main bus) and, cascaded from it, the Essential Bus Relay. The IGN 1 / IGN 2 panel toggles — not the key — are the ECU enables, sitting between the ECU breakers and the ECUs.
2. The essential bus is continuously diode-OR fed from both batteries. There is no switching logic — the essential bus passively draws from whichever battery node is higher, and when the nodes are close (alternator ~14.3 V, MZ-30 ~14.2 V) both sources share load through their diodes. This is normal and by design. Diode paths drop ~0.5–0.7 V below battery voltage under load. Charge isolation is separate and intact: the alternator cannot charge Battery 2 and the generator cannot charge Battery 1.
3. The EMERGENCY POWER switch is a hard jumper from the both-battery diode node directly to the essential bus output, bypassing the key, both relays, and all Bus Manager logic. It exists for Bus-Manager-internal failure; flipping it in normal flight changes nothing observable (it arms a parallel path). The Bus Manager does not automatically shed loads — the “automatic” protection is purely the passive diode-OR.

In-flight main-bus shed maneuver: E-PWR ON → verify essential bus voltage → KEY OFF. The engine keeps running (ECUs fed from the essential bus through the IGN toggles), the main bus and everything on the VPX goes dark, and both charging sources keep working. Order is critical — key-off with E-PWR off (or failed) stops the engine. Note the interaction with the engine-fire procedure: “Key OFF” only stops the engine while E-PWR is off. Main-bus loss also kills flaps, electric trim (AP panel), and pitot heat; the AP servos are essential-fed and the autopilot remains flyable from PFD1 minus auto-trim.

Endurance bus radio behavior: The GTN 650 is COM1. If power goes out on the GMA 245, it hard-connects COM1 from the GTN 650 directly to the headphones. This ensures radio communication is maintained even if the audio panel loses power on the endurance bus.

Essential Bus Walkthrough

The picture above is the whole story of the two busses in one place; the numbered walkthrough is the version to be able to say out loud.

1. Normal. The alternator charges Battery 1 and the MZ-30 charges Battery 2 — two isolated charging systems. The two busses do not share a source. Per Bus Manager Drawing 5A, the Main Bus Relay’s contact is fed from the alternator / Battery 1 rail alone; only the Essential Bus Relay is fed from the diode-OR node, which sees both batteries. The key drives the Main Bus Relay coil and, cascaded from it, the Essential Bus Relay coil (essential bus bar → ECUs, ignition coils, pumps, PFD 1, GTN, servos).
2. Battery 1 dies — the main bus dies with it. Once the alternator stops carrying, nothing is left on that rail, so the main bus goes away while the essential bus keeps running on Battery 2 through its diode. This is automatic and it has been observed in this airplane, but there is no sensor and no logic behind it: the busses simply have different sources.
3. The alternator fails but Battery 1 is good. The main bus stays up on Battery 1 until it runs down. The failure shows as alternator amps falling to zero and bus voltage sagging — not as a battery discharge, because the shunt is on the alternator lead (see Ammeter Shunt Placement).
4. Main-bus shed (smoke, runaway load, VPX fault): EMERGENCY POWER ON → verify essential-bus voltage → KEY OFF. The switch is a hard jumper from the diode node to the essential bus output, so with it on the key can open both relays without touching the engine. Order is life: key off with the switch off (or failed) opens the Essential Bus Relay and the engine stops. What survives: engine, PFD 1, both fuel pumps, autopilot servos (no auto-trim), both charging sources. What dies today: the second and third displays, transponder, audio panel, flaps, electric trim, pitot heat — and the GTN, because its COM/NAV power-relay coils are fed by AV MSTR from the main bus (the annual’s fix list).
5. EMERGENCY POWER does not work. You are on whatever the Essential Bus Relay gives you: leave the key on and land now. There is no mechanical ignition or fuel fallback on this engine, and no essential breaker is an isolation tool — every one of them feeds something the engine needs.
6. Fire / secure. Key OFF and EMERGENCY POWER OFF. Both paths must be open, because the jumper alone re-powers the ECUs, the pumps, and the starter.

Verified on the ground 2026-08-29 (EMERGENCY POWER ON, key OFF): engine, PFD 1 and pumps stayed up; GTN, transponder and audio dropped.

Ammeter Shunt Placement — Reading an Alternator Failure

The system’s current shunt sits between the alternator and the system, not between the battery and the bus. The consequence for failure recognition: on an alternator failure the SkyView does not show a battery discharge — battery drain is downstream of the shunt and invisible to it. What you see is alternator amps dropping toward zero, followed by bus voltage sagging as Battery 1 begins to carry the main-bus load unassisted.

Alerting implication: because the failure won’t announce itself as a “discharge,” the EMS alerts should be tuned to catch it directly — a low-alternator-output (amps-near-zero) and/or low-bus-voltage alarm — so the first indication isn’t a slow, silent battery drain. Confirm these alarm thresholds are set on the ground. (Battery 2 / MZ-30 charging is independent and unaffected — see the MZ-30 section.)

VPX Sport

The VPX Sport provides:

• Electronic circuit breaker protection for main bus loads — these channels have no physical breakers to pull or reset; they are reset from the EFIS. (The endurance bus is separate and does use physical breakers — see Bus Architecture above.)
• Load monitoring and display on EFIS
• Per-channel overcurrent trip and thermal protection — automatic, but per channel, not bus-level
• Programmable power channels

What Is and Is Not Automatic

Nothing in this airplane senses a low bus and sheds the main bus for you. One thing does drop the main bus automatically, but by topology rather than by detection — a dead Battery 1 takes the main bus with it, because that bus has no other source (walkthrough step 2). Do not describe that as the Bus Manager shedding load; it is the absence of a source, and the Bus Manager is not aware of it.

• The Bus Manager does not shed loads. Its automatic behaviors are the passive diode-OR at the both-battery node and the fuel-pressure-triggered switch to fuel pump 2. Neither watches bus voltage. Its install instructions describe no voltage-sensing bus logic of any kind.
• The VPX Sport does not shed loads either. The word does not appear anywhere in the Rev G4 manual. Its automatic behaviors are per-channel overcurrent and thermal protection. Low voltage is detected and reported: “A low voltage condition is detected and reported by the EFIS.” Set the SkyView low-voltage alarm (the manual’s own example is 13 V) and decide what to turn off yourself.
• The main-bus shed is a manual maneuver. EMERGENCY POWER ON → verify essential-bus voltage → KEY OFF. Order is life.

The one genuinely voltage-sensing automatic response is the MZ-30’s standby regulator, which sits idle while the node is held high and engages on its own as the node sags below about 13.7 V — see the MZ-30 section.

What an alternator failure actually looks like: ALT amps fall to zero and bus voltage sags. It does not show as a battery discharge, because the shunt is on the alternator lead.

Channel Assignments

Channel names and pin numbers are read from the SteinAir power & lighting schematic; channel names are the VPX’s own (5A-8 = the eighth 5-amp output). The J-connector each pin belongs to is inferred from Vertical Power’s worksheet pinout — confirm against the VPX Sport pinout before wiring anything. Two rows corroborate it independently: the schematic’s audio and transponder pins land on J10-4 and J10-7, which is exactly what the Bus Architecture section above already recorded.

Switch Inputs (VP-X J2, DB25)

The pilot switch panel feeds the VPX’s external switch inputs rather than switching power directly. Which channels each input gates is stored in the VPX’s own configuration, not in the wiring.

AV MSTR is a DPDT switch doing two separate jobs. One pole drives VPX Switch Input #1, which gates a configured group of the channels above — that is the avionics-master function proper. The other pole (Wht/Blk) holds the GTN COM/NAV power relay coils closed, which is the known issue described under Bus Architecture. Section 4 notes a third consequence: there is no dedicated trim breaker, so AV MSTR is the only kill switch for the trim servos.

A/P MSTR is not a switch input — it feeds autopilot servo power directly (20 AWG red).

Fuses (Non-VPX)

Discrete fuses outside the VPX electronic breaker system, located above the essential bus bar behind the pilot PFD:

Monkworkz MZ-30 Generator

Installed 2026-03-09. The MZ-30 is a permanent-magnet generator driven off the engine’s vacuum pump pad, providing independent charging for Battery 2. Manual version 4, dated 2024-10-22.

Specifications

Voltage selection: Ships configured for 14.6 VDC. For lithium batteries (EarthX), the lower 14.2 VDC setting is recommended per Monkworkz Feb 2026 guidance. To select 14.2V, disconnect pin 1 (VSEL) on the input side Pico-Lock connector. Voltage is set at startup and cannot be changed while running.

N720AK is confirmed on the 14.2 V setting — flight data (2026-06 and 2026-08 logs) shows the Battery 2 node regulating at 14.20 V against the alternator’s 14.30 V. This is exactly Monkworkz’s recommended standby-generator geometry (primary slightly above the MZ).

Standby Behavior — Read This Before Toggling the Enable Switch In Flight

Per the manual (v4, Operation, p. 21), the regulator produces output only when the attached bus voltage is below 13.7 V (in addition to RPM/temperature limits). Consequences observed in flight (2026-08-31):

• Toggling enable OFF then ON in flight does not bring the generator back while the alternator is healthy — Battery 2’s surface charge holds the node above 13.7 V, so the regulator sits in standby, possibly for the rest of the flight. It re-engages on its own after landing as the node sags. This is designed standby-generator behavior, not a fault.
• The functional test of the MZ-30 is turning the alternator field OFF (briefly): the bus sags, the MZ engages within seconds, ALT back on. Toggling the MZ’s own switch proves nothing.
• Load sharing is normal: with the nodes 0.1 V apart, cruise logs show the MZ contributing ~6 A (peaks ~18 A) alongside the alternator’s ~23 A through the essential-bus diode-OR. This is supply-sharing into the essential bus, not a charge-isolation failure.

Architecture

The two battery charging systems are fully isolated:

The bus manager’s internal screw that previously allowed the primary alternator to cross-charge Battery 2 has been removed. Each battery has a dedicated charging source.

Generator → Battery 2 Wiring

The generator output connects to the Battery 2 stud inside the bus manager (Drawing 5A, bus manager installation guide page 13). A BOSCH 0332019155 normally-open relay (30A, 12V, internal diode) sits between the generator output and Battery 2 to prevent parasitic draw when the engine is off.

Relay coil power: the essential bus output, and only that — the generator enable switch is not in the coil circuit (Sam, 2026-09-23). An earlier revision of this page said “essential bus + generator enable switch”, which wrongly implied the switch was in series with the coil. This design means:

• Turning off the ignition key de-energizes the essential bus, opening the relay and disconnecting the generator from Battery 2 — regardless of enable switch position
• Turning on the master, or the emergency bus, closes the relay and connects the MZ-30 to Battery 2
• The enable switch is separate, driving the regulator’s Output pin 1 Enable. So the regulator can be connected-but-disabled, which is the state behind the floating amps reading documented below
• The generator cannot turn itself on. The Monkworkz reference diagram drives the contactor coil from the Enable node through diode D2; N720AK deliberately drives it from the essential bus instead

Note: The manual recommends connecting to the switched side of the master contactor to avoid parasitic draw from the regulator’s ~30mA diagnostic blink code circuit. The BOSCH relay achieves the same purpose.

MZ Regulator Connections

The MZ regulator is mounted on the engine mount with the following connections:

Generator side (3-phase, #6 screw terminals, 12 AWG):

• Terminal 7 (~1), Terminal 8 (~2), Terminal 9 (~3) → MZ Generator (phases can be connected in any order)

Input Molex Pico-Lock (pre-wired, 22-24 AWG):

• Pin 1: Voltage_Select (connected = 14.6V, disconnected = 14.2V)
• Pin 5: GEN_Thermistor → Generator thermistor (required — system will not produce output without it)
• Pin 6: GND

Output Molex Pico-Lock (22-24 AWG):

• Pin 1: Enable → Pilot enable switch (pin 1 to switch center, pin 6 to switch N.O.)
• Pin 2: Output_Active — pulls to ground when regulator is active and maintaining ≥14V (works with Dynon/Garmin EFIS contact inputs). Orange/brown wire, currently coiled up and unused near the Monkworkz enable switch.
• Pin 3: +i_Shunt (internal fuse used as shunt, 1.1 mΩ nominal)
• Pin 4: −i_Shunt
• Pin 5: Proportional_Current → Dynon EMS pin 31 (0–~2.7V = 0–30A, brown/blue wire)
• Pin 6: GND

Power output (#6 screw terminals, 10 AWG Tefzel):

• Terminal 17: Output_Power_Positive → BOSCH relay → Battery 2 stud (bus manager)
• Terminal 18: Output_Power_Ground (to clean metal airframe ground, no paint/corrosion)

Known issue: the amps reading floats to ~20 A with the generator disabled

Symptom (Sam, 2026-09-23): on the ground, essential bus powered, turning the generator enable switch off leaves the EMS MZ30 amps reading drifting up to about 20 A over some seconds rather than falling to zero.

What it is not. An earlier draft of this note blamed loss of regulator power — the relay opening and taking the regulator’s supply with it. That is wrong. The relay coil is driven from the essential bus output, separately from the enable switch, so with the master (or emergency bus) on the relay is closed, the MZ-30 stays connected to Battery 2, and the regulator is powered throughout. It is powered but disabled.

Working hypothesis. The regulator stops driving Output pin 5, Proportional_Current, when Enable is de-asserted, leaving that wire high-impedance rather than held at 0 V. EMS pin 31 then floats, and a floating high-Z analog input drifts slowly — which matches the “after a bit of float” behaviour.

The arithmetic corroborates the magnitude. Pin 5 is specified 0–~2.7 V = 0–30 A, and the SkyView sensor definition MONKWORKZ CURRENT carries c_x1 = 11.111 A per volt — 30 ÷ 2.7 = 11.111, so the two agree. A displayed 20 A implies 1.80 V on the pin, a plausible resting point for a floating CMOS input.

Pin 31 cannot be pulled down in configuration. The sensor-definition file states only pin 6 supports a pull-down; pins 8, 22, 23 and 31 support “no pull-up” but no pull-down. This cannot be fixed in SkyView setup.

Confirm before fixing. Essential bus on, enable switch off, measure DC volts at EMS pin 31 (or regulator Output pin 5) to airframe ground.

• ~1.8 V and drifting → hypothesis holds. Then temporarily bridge pin 31 to ground through 10 kΩ; the reading should collapse to 0 A.
• 0 V → hypothesis is wrong; something else is producing the indication.

Fix: a pull-down resistor, pin 31 to airframe ground, around 10 kΩ.

That the regulator is powered during the fault actually strengthens this. If pin 5 were driving a wrong value, a pull-down would fight it; the evidence says pin 5 is simply undriven, so a modest pull-down wins cleanly and draws only 0.27 mA at full scale. ⚠ Re-verify calibration against a clamp meter with the generator producing — if pin 5 is a resistive divider rather than a buffered output, the pull-down joins the divider and shifts the scale. The output impedance is not documented; ask Monkworkz on the same call as the pin 2 sink question.

Alternative, if the pull-down disturbs calibration: wire Output pin 2 and treat it as the authoritative producing indication. The orange/brown wire is already coiled and unused, and SkyView ships a matching sensor definition — MONKWORKZ STATUS, function CONTACT, name MZ30_S, available on all thirteen GP pins. Needs a freed pin. ⚠ Note the Monkworkz reference diagram labels pin 2 Active_High while this page records it as pulling to ground when active — settle that polarity by measurement before wiring it.

This retracts an earlier conclusion. A previous revision reasoned that because pin 31 already reads MZ-30 current, the Output Active discrete might be redundant. That is wrong: a reading that shows 20 A from a generator that is switched off cannot serve as an “is it producing” indication. Until the float is fixed, Output Active is the only trustworthy source of that fact.

Caution: The Pico-Lock connectors are delicate — add ~0.5“ maintenance loops in the connector wires, secured to nearby 12/10 AWG wires with a small pre-load toward the connector to prevent unseating.

Cooling

Both generator and regulator require cooling via 3/4“ holes in the engine baffling with corrugated nylon tubing routed from the baffling. The duct material is a force fit into the generator, regulator, and baffling holes. RTV can improve the seal.

Generator Enable Switch

Panel-mounted next to the primary alternator field switch. Controls the relay coil circuit (essential bus → enable switch → relay coil → ground).

The enable switch wiring connects pin 1 (Enable) to the switch center post and pin 6 (GND) to the switch’s normally-open terminal, grounded at the regulator end (not at the panel — grounding at the panel can cause ground loops and erratic behavior per manual section 2.1).

Startup Behavior

After engine start, when RPM exceeds ~1200, the MZ-30 performs an internal self-test (2–4 seconds). If all checks pass, it begins providing output voltage. The enable switch can be set ON as part of the pre-start checklist or activated at idle.

Protection Features

• Overvoltage Protection (OVP): Shuts down output in <1 second if overvoltage detected. If OVP triggers 3 times in one flight, system shuts down for the remainder of that flight. If this occurs across 3 separate flights (9 total OVP events), the regulator requires service/replacement from Monkworkz.
• Thermal protection: Two-stage — first reduces to half current limit, then full shutdown if temperature continues rising. Recovers automatically when temperature drops below recovery threshold.
• Current limiting: Electronic current limiting at 30A. Output fuse is internally replaceable by a repair shop only.
• Shear coupling: Located between input spline and motor shaft. Breaks if vacuum drive torque is exceeded — contact Monkworkz for replacement if generator rotates freely.

Regulator Blink Codes

The regulator has a diagnostic LED on the output side:

The Output Active pin (pin 2) also provides status: it “flashes” on an 8-second cycle (4s ground, 4s open) if the generator is overheating, providing an in-cockpit indication.

Dynon EMS Monitoring

The MZ-30’s proportional current output (pin 5, 0–~2.7V = 0–30A) is wired to Dynon EMS pin 31. This pin previously carried the CO Guardian PPM signal to the Dynon EMS — only the EMS display was removed when the wire was repurposed. The CO Guardian unit remains installed in the cabin and still provides an audible alarm above 50 PPM. The alarm is not currently wired into the GMA 245 audio panel, so it sounds in the cabin but is not heard through the headsets.

EMS general-purpose inputs — all 13 allocated

Read from the 2026-08-29 SkyView USER_CONFIG snapshot (GDrive Public/Configs/Dynon/), not from the panel. The SV-EMS-221’s general-purpose pins on the 37-pin connector are C37 pins 4, 6, 7, 8, 9, 10, 11, 12, 20, 21, 22, 23 and 31 — the set the sensor-definition file lists as supporting the 10 k / 991 Ω / 200.88 Ω pull-ups. Every one is configured.

Pin 31 monitors the MZ-30 by current — but that reading is not currently trustworthy. With the generator switched off it floats to about 20 A rather than falling to zero; see the known issue in the MZ-30 section below. Until that is fixed the current reading cannot stand in for an “is it producing” indication, so the Output Active discrete is not redundant.

Pin 9 PHEAT is the weakest claim of the thirteen. It is the only contact with its alarm OFF, so it displays pitot-heat controller status (the brown/blue status wire) without ever alerting on it. That makes it the candidate if a pin has to be freed for the CO detector. It is not useless — heater-working status is real information — so this is an owner judgement, not an obvious swap.

Non-GP pins for reference: 1 BATT1 and 2 BATT2 voltage, 14 MAIN fuel flow, 24/25 ALT ammeter shunt, 26 MAP, 32/34 and 33/35 RPM L/RPM R. Pins 19, 27/28 and 36/37 are unconfigured but are not general-purpose inputs.

References

• Monkworkz MZ-30 product page
• MZ-30 Installation and Operation Manual V4 (2024-10-22)
• VAF: Monkworkz Wiring for Amps Readout
• Bus Manager Installation Guide, Drawing 5A (page 13)

Wiring

KiCad migration: All connector pinouts and wiring data below should eventually be ported to proper KiCad schematics. The text tables here are transcribed from build notes and serve as the interim reference.

Wiring Stations (Station-by-Station Topology)

Complete wiring map of the aircraft, from tail to firewall.

Rear Tailcone

• ELT
• SunTail (rear position/strobe)
• Elevator trim servo

Tailcone / Baggage Area Junction (RIGHT)

• Elevator autopilot servo
• ADSB
• Net hub
• Battery 1 positive (large gauge)
• Battery 2 positive (large gauge)
• Main battery negative (large gauge)

Tailcone / Baggage Area Junction (LEFT)

• Alt static line from panel
• EFIS hub connection
• Starter solenoid 1 drive
• Starter solenoid 2 drive
• Starter annunciator wire
• Emergency air override (yellow and red lines)
• O₂ supply and pressure lines (blue and red)
• Oxygen wiring harness
• Pitot air hose (blue)
• AoA hose (green)

Passenger Area (LEFT)

• O₂ lines (blue and red)
• Left passenger O₂ harness
• Left door ajar sensor
• Power wire for left door ajar servos

Passenger Area (RIGHT)

• O₂ lines (blue and red)
• Right passenger O₂ harness
• Right door ajar sensor
• Power wire for right door ajar servos

Left → Center Tunnel

• Flap position sensor harness
• Flap motor wires

Wing Connection Area (LEFT)

• Pitot heat power and ground (14 AWG)
• Heated pitot status wire
• Fuel level sensor left
• AoA air hose (green)
• Pitot air hose (blue)
• 2× shielded 3-wire bundles for left lights
• Green sync wire for left lights
• 5-wire trim bundle for left roll trim

Wing Connection Area (RIGHT)

• AP splice for red, black, yellow — 3-way (rear, connector, front)
• AP bundle extension from connector
• 2× shielded 3-wire bundles for right lights
• Green sync wire for right lights
• Right wing COM antenna coax from panel (inline connector)
• Right wing NAV antenna coax from panel (inline connector)

Gear Support Mount (LEFT) — Under-Seat Terminal Blocks

• EMS ground for trim servo sensors
• Pitch trim motor power and ground
• Roll trim motor power and ground
• Roll trim position sensor
• Pitch trim position sensor
• Trim bundle from left wing
• 5V (red/white) EMS power for trim servo sensors

Gear Support Mount (RIGHT) — Under-Seat Terminal Blocks

• Green sync wire for lights
• Wig-wag power
• Nav power
• Strobe power
• Taxi power
• Landing power
• Tail light bundle from rear
• 16 AWG ground wire for lights
• AP servo black, red, yellow spliced together
• 2× 3-wire light bundles + green sync crossing to left wing root
• 2× 3-wire light bundles + green sync from right wing root

Forward Center Tunnel

• 2× red power wires for fuel pumps
• Ground wire for fuel pumps (1 wire spliced to 2)

Control Sticks (LEFT and RIGHT)

• Main bundle from SteinAir panel harness
• COM flip sensor wire

Pilot Knee Side (LEFT)

• O₂ pilot wiring harness
• O₂ lines (blue and red)

Copilot Knee Side (RIGHT)

• O₂ copilot wiring harness
• O₂ lines (blue and red)

Left Firewall Penetration — Engine Sensors

• CHT wires × 6
• EGT wires × 6
• Fuel pressure ground and data
• Oil pressure ground and data
• Oil temp ground and data
• Spliced 5V wire (red/white) for fuel pressure, oil pressure, oil temp sensors

Right Firewall Penetration — EFII / Power

• 60A fuse → bus manager
• Ammeter shunt wires
• Spark plug wires for EFII
• Ignition coil wires (71 and 88)
• Pressure sensor wires
• All remaining EFII outputs

Terminal Blocks Under Seats

Two terminal blocks located under the front seats distribute wiring between the wing roots, center tunnel, and panel.

Left Terminal Block

Right Terminal Block

Wing Root Connectors (CPC)

CPC barrel connectors (series 1, 17–18 pin) at each wing root carry all wing wiring.

Right Wing Root Exit

Left Wing Root Exit

Autopilot Servo Wiring (Wing)

Wire color mapping between the wing harness and the Dynon roll/pitch servo connector.

Dynon EMS-220 Connector (37-Pin)

Complete pinout for the SV-EMS-220 engine monitoring module. Updated 2026-03-04.

Through Firewall

Across Panel

Down Left (Under Panel)

Shared Power

Unused Pins

Panel Mounting Fasteners

Screw/bolt specs for every panel-mounted component. Useful reference for panel removal and reinstallation.

Connector Inventory

Summary of all connector types used in N720AK wiring (all installed).

Inspection & Maintenance

EarthX Battery & Chargers

Chargers

Charger Behavior Notes

The OptiMate chargers enter sleep/maintenance mode once the battery is fully charged, checking voltage roughly once per hour. Important: If a continuous parasitic load is present (e.g. panel powered on for configuration), the charger may not detect the drain during its sleep intervals and the battery can discharge. This caused a drain-to-zero event during initial setup. The TM-275’s TUNE mode (continuous 13.6V power supply) avoids this problem. (Per EarthX Engineering, Dillon Hinners, 2026-01-13.)

Hartzell Alternator Maintenance

Per the AL 12-E160/V installation manual: battery terminal torque 50 in-lb, safety wire .032. Annual: check bearings. 5yr/1000hr: inspect brushes (replace if <0.250“ from holder case edge). 5A enable CB, 60A main breaker.

The drawing’s own title block reads “12 VOLT 60 AMP EXPERIMENTAL ALTERNATOR, W/INTERNAL VOLTAGE REGULATOR AND OVERVOLTAGE PROTECTION”. That matters beyond trivia: the alternator is a non-certificated product, so no Part 39 Airworthiness Directive can attach to it — the same reasoning that exempts the YIO-540. Hartzell’s SB #209 (alternator mounting brackets) reaches a different part-number family entirely. Both determinations are recorded in ad-sb-compliance.tsv.

Note that 99-9900V is a drawing number, not a part number — the manual is an installation drawing, sheet 1 of 2, Rev A.

References

• VPX Sport Installation & Operating Manual (Rev G4)
• VPX Sport Installation & Operating Manual (Rev G3) — previous revision
• VPX Getting Started Guide (Rev A)
• EFII Bus Manager Installation Instructions
• EarthX ETX900 Installation & Maintenance Manual
• Hartzell AL 12-E160/V Alternator Installation Manual (drawing 99-9900V Rev A)
• Power & Lighting Schematic
• VPX Pro/Sport Load Planning Worksheet