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MX-5 Miata NA6 1990-93 (B6-ZE, 1.6) → Speeduino Speeduino v0.4 (auto)

MX-5 Miata NA6 1990-93 (B6-ZE, 1.6) — Speeduino — Speeduino
⚠ NOT CHECKED. Don't build from this.
Drawn by Michael Gnankou · Rev 0 · draft · 2026-10-09

Before you start

Not confident with automotive wiring? Have a professional harness builder or tuner do it, or check your work before first power-up. A wiring mistake can damage the ECU, the engine or the car, or start a fire.

Install modifications

Steps on the car itself, outside the harness. Every harness for this car inherits them.

  • Cut the sensor-ground-to-chassis wirerequired

    Cut the wire in the engine bay that ties the ECU sensor/signal ground (2C/2D, BLK/LT GRN) to chassis ground, so the standalone gets a clean sensor-ground reference. After cutting, verify with a meter: sensor ground ↔ chassis ground reads open (OL) unless the standalone manual says otherwise.

    Why: The stock loom bonds sensor ground to chassis ground; a standalone ECU needs its sensor ground isolated from the noisy chassis/power ground, otherwise sensor readings are noisy or wrong.

    Where: Engine-bay ground joint near the ECU (1991 EWD Fig.2 shows the sensor-ground run and its joint connector).

    Correction reported by Justin Barrera

    Unverified — reported by Justin Barrera (Motorsport Wiring Alliance), not yet sourced.

  • Bypass the airflow-meter pump switchrecommended

    The stock pump is run by the flapper airflow meter's pump-switch contact through the circuit-opening relay, not by the ECU. The standalone harness adds ECU control of the pump relay (ignition-switched, fused); remove/bypass the AFM pump switch so the two do not fight.

    Why: There is no stock fuel-pump output at the ECU, so the standalone provides it. Leaving the AFM switch in place leaves a second, unswitched pump path.

    Where: Airflow meter connector (pump-switch contact) and circuit-opening relay.

    Correction reported by Justin Barrera

    Source: 1991 Mazda Miata wiring diagrams (Mitchell reproduction, mellens.net community mirror)

Statusdraft · not yet verified: nobody has reported building and running this harness
Open errors1: read the Findings page before cutting any wire
Size of the job24 wires · 5 connectors · 2 part lines
ToolsCrimp and extraction tools for each terminal family (see Connectors) · wire strippers · multimeter · heat gun + heat-shrink
Order of work1. Check every part against the parts list. 2. Cut and label every wire (labels: Export → Wire labels). 3. Crimp and pin connector by connector, ticking the build checklist. 4. Run the bench test with nothing plugged in and the battery disconnected. 5. Only then connect the ECU and power up.

Build checklist

Tick each box as you go. Continuity: meter from end to end, expect under 1 Ω.

WireFromToGaugeColourCutLabelCrimp ACrimp BPin APin BContinuity
W1OEM-2-ASP-GND-10.85☐☐☐☐☐☐☐
W2OEM-2-BSP-GND-10.85☐☐☐☐☐☐☐
W3SP-GND-1SPEEDUINO-A-100.85☐☐☐☐☐☐☐
W4SP-GND-1SPEEDUINO-A-90.85☐☐☐☐☐☐☐
W5OEM-2-CSP-0V-10.5☐☐☐☐☐☐☐
W6OEM-2-DSP-0V-10.5☐☐☐☐☐☐☐
W7SP-0V-1SPEEDUINO-A-120.5☐☐☐☐☐☐☐
W8SP-0V-1SPEEDUINO-A-230.5☐☐☐☐☐☐☐
W9OEM-2-KSPEEDUINO-A-130.5☐☐☐☐☐☐☐
W10OEM-2-ESPEEDUINO-A-250.5☐☐☐☐☐☐☐
W11OEM-2-GSPEEDUINO-A-240.5☐☐☐☐☐☐☐
W12OEM-1-GSPEEDUINO-A-70.5☐☐☐☐☐☐☐
W13OEM-1-HSPEEDUINO-A-340.5☐☐☐☐☐☐☐
W14OEM-2-USPEEDUINO-A-10.5☐☐☐☐☐☐☐
W15OEM-2-VSPEEDUINO-A-20.5☐☐☐☐☐☐☐
W16OEM-2-YSPEEDUINO-A-30.5☐☐☐☐☐☐☐
W17OEM-2-ZSPEEDUINO-A-50.5☐☐☐☐☐☐☐
W18OEM-1-ESPEEDUINO-A-150.5☐☐☐☐☐☐☐
W19OEM-2-ISPEEDUINO-A-170.5☐☐☐☐☐☐☐
W20OEM-2-NSPEEDUINO-A-210.5☐☐☐☐☐☐☐
W21OEM-2-QSPEEDUINO-A-190.5☐☐☐☐☐☐☐
W22OEM-2-PSPEEDUINO-A-200.5☐☐☐☐☐☐☐
W23OEM-1-CSPEEDUINO-A-180.5☐☐☐☐☐☐☐
W24OEM-1-NSPEEDUINO-A-140.5☐☐☐☐☐☐☐

Bench test (before first power-up)

Pre-power install steps (make these cuts/modifications on the car first, then verify the checks below): Cut the sensor-ground-to-chassis wire.

Battery disconnected, ECU and every sensor unplugged. Meter on resistance. Each pair below must read open (OL): a reading means a pinched wire, a stray strand or a mis-pinned cavity. Then check continuity of every wire (last column of the build checklist).

#Probe AProbe BExpectWhyReading
1SPEEDUINO-A-10SPEEDUINO-A-13open (OL)power ground must not touch 5 V sensor reference
2SPEEDUINO-A-12SPEEDUINO-A-13open (OL)sensor ground must not touch 5 V sensor reference

If any reading doesn't match, stop and get help before powering up.

Revisions

RevLabelByDate
No revisions yet.

Schematic

OEM-1OEM-2SP-0VSP-GNDSPEEDUINO-A

Connectors

OEM-1

CavityFunctionSignalWireColourGauge (mm²)DestinationTerminalSealPlug
AConstant +12V————————
BECU power +14V (main relay)————————
CStart signal—W23—0.5SPEEDUINO-A-18———
EMalfunction indicator lamp (MIL)—W18—0.5SPEEDUINO-A-15———
GIgnition 1 (coils 1+4, wasted spark)—W12—0.5SPEEDUINO-A-7———
HIgnition 2 (coils 2+3, wasted spark)—W13—0.5SPEEDUINO-A-34———
NWide-open throttle switch—W24—0.5SPEEDUINO-A-14———
PPower steering pressure switch————————
QA/C request————————
VNeutral/clutch switch————————

OEM-2

CavityFunctionSignalWireColourGauge (mm²)DestinationTerminalSealPlug
APower ground—W1—0.85SP-GND-1———
BPower ground—W2—0.85SP-GND-1———
CSensor ground (E2)—W5—0.5SP-0V-1———
DSensor ground (E2)—W6—0.5SP-0V-1———
ECrank position (CAS NE)—W10—0.5SPEEDUINO-A-25———
FNC————————
GCam position (CAS G)—W11—0.5SPEEDUINO-A-24———
ITachometer output—W19—0.5SPEEDUINO-A-17———
K+5V sensor supply—W9—0.5SPEEDUINO-A-13———
LIdle switch (TPS)————————
NO2 sensor—W20—0.5SPEEDUINO-A-21———
OAir flow meter (AFM)————————
PIntake air temperature (IAT)—W22—0.5SPEEDUINO-A-20———
QCoolant temperature (ECT)—W21—0.5SPEEDUINO-A-19———
UInjector 1 & 3 (batch)—W14—0.5SPEEDUINO-A-1———
VInjector 2 & 4 (batch)—W15—0.5SPEEDUINO-A-2———
YInjector 3 (California sequential)—W16—0.5SPEEDUINO-A-3———
ZInjector 4 (California sequential)—W17—0.5SPEEDUINO-A-5———

SP-0V

CavityFunctionSignalWireColourGauge (mm²)DestinationTerminalSealPlug
1——W5—0.5OEM-2-C———
1——W6—0.5OEM-2-D———
1——W7—0.5SPEEDUINO-A-12———
1——W8—0.5SPEEDUINO-A-23———

SP-GND

CavityFunctionSignalWireColourGauge (mm²)DestinationTerminalSealPlug
1——W1—0.85OEM-2-A———
1——W2—0.85OEM-2-B———
1——W3—0.85SPEEDUINO-A-10———
1——W4—0.85SPEEDUINO-A-9———

SPEEDUINO-A

CavityFunctionSignalWireColourGauge (mm²)DestinationTerminalSealPlug
1Injector 1—W14—0.5OEM-2-U———
10Ground—W3—0.85SP-GND-1———
11MAP sensor (0-5V)————————
12Ground—W7—0.5SP-0V-1———
135V—W9—0.5OEM-2-K———
14Proto area 1 (flex fuel)—W24—0.5OEM-1-N———
15Proto area 2 (fan)—W18—0.5OEM-1-E———
16Proto area 3 (fuel pump)————————
17Proto area 4 (tachometer)—W19—0.5OEM-2-I———
18Proto area 5 (clutch)—W23—0.5OEM-1-C———
19Coolant temp (CLT)—W21—0.5OEM-2-Q———
2Injector 2—W15—0.5OEM-2-V———
20Inlet air temp (IAT)—W22—0.5OEM-2-P———
21O2 sensor—W20—0.5OEM-2-N———
22TPS input————————
23Ground—W8—0.5SP-0V-1———
24Cam input / VR2+—W11—0.5OEM-2-G———
25Crank input / VR1+—W10—0.5OEM-2-E———
26VR2-————————
27VR1-————————
285V————————
29Idle stepper 2B————————
3Injector 3—W16—0.5OEM-2-Y———
30Idle stepper 2A————————
31Idle stepper 1A————————
32Idle stepper 1B————————
33Ignition 3————————
34Ignition 2—W13—0.5OEM-1-H———
35Boost————————
36Idle 2 (3-wire idle)————————
37PWM idle————————
38VVT————————
5Injector 4—W17—0.5OEM-2-Z———
7Ignition 1—W12—0.5OEM-1-G———
8Ignition 4————————
9Ground—W4—0.85SP-GND-1———

Pin map

OEM pointOEM functionOEM familyTarget pointTarget functionTarget familyWire colourEvidence
OEM-1-AConstant +12Vpower_in———single_source
OEM-1-BECU power +14V (main relay)power_in———single_source
OEM-1-CStart signaldigital_inSPEEDUINO-A-18Proto area 5 (clutch)digital_insingle_source
OEM-1-EMalfunction indicator lamp (MIL)low_side_outSPEEDUINO-A-15Proto area 2 (fan)low_side_outsingle_source
OEM-1-GIgnition 1 (coils 1+4, wasted spark)ignition_outSPEEDUINO-A-7Ignition 1ignition_outsingle_source
OEM-1-HIgnition 2 (coils 2+3, wasted spark)ignition_outSPEEDUINO-A-34Ignition 2ignition_outsingle_source
OEM-1-NWide-open throttle switchdigital_inSPEEDUINO-A-14Proto area 1 (flex fuel)digital_insingle_source
OEM-1-PPower steering pressure switchdigital_in———single_source
OEM-1-QA/C requestdigital_in———single_source
OEM-1-VNeutral/clutch switchdigital_in———single_source
OEM-2-APower groundgroundSPEEDUINO-A-10 + SPEEDUINO-A-9Ground + Groundgroundsingle_source
OEM-2-BPower groundgroundSPEEDUINO-A-10 + SPEEDUINO-A-9Ground + Groundgroundsingle_source
OEM-2-CSensor ground (E2)sensor_groundSPEEDUINO-A-12 + SPEEDUINO-A-23Ground + Groundsensor_groundcorroborated
OEM-2-DSensor ground (E2)sensor_groundSPEEDUINO-A-12 + SPEEDUINO-A-23Ground + Groundsensor_groundcorroborated
OEM-2-ECrank position (CAS NE)trigger_inSPEEDUINO-A-25Crank input / VR1+trigger_insingle_source
OEM-2-FNC————corroborated
OEM-2-GCam position (CAS G)trigger_inSPEEDUINO-A-24Cam input / VR2+trigger_insingle_source
OEM-2-ITachometer outputtacho_outSPEEDUINO-A-17Proto area 4 (tachometer)tacho_outsingle_source
OEM-2-K+5V sensor supplyvrefSPEEDUINO-A-135Vvrefsingle_source
OEM-2-LIdle switch (TPS)digital_in———single_source
OEM-2-NO2 sensorlambda_nb_inSPEEDUINO-A-21O2 sensorlambda_nb_insingle_source
OEM-2-OAir flow meter (AFM)analog_in———single_source
OEM-2-PIntake air temperature (IAT)temp_inSPEEDUINO-A-20Inlet air temp (IAT)temp_insingle_source
OEM-2-QCoolant temperature (ECT)temp_inSPEEDUINO-A-19Coolant temp (CLT)temp_insingle_source
OEM-2-UInjector 1 & 3 (batch)injector_outSPEEDUINO-A-1Injector 1injector_outcorroborated
OEM-2-VInjector 2 & 4 (batch)injector_outSPEEDUINO-A-2Injector 2injector_outcorroborated
OEM-2-YInjector 3 (California sequential)injector_outSPEEDUINO-A-3Injector 3injector_outcorroborated
OEM-2-ZInjector 4 (California sequential)injector_outSPEEDUINO-A-5Injector 4injector_outcorroborated

Cut list

WireFromToGauge (mm²)ColourLengthSignalIdent
W1OEM-2-ASP-GND-10.85no routePower ground—
W2OEM-2-BSP-GND-10.85no routePower ground—
W3SP-GND-1SPEEDUINO-A-100.85no routeGround—
W4SP-GND-1SPEEDUINO-A-90.85no routeGround—
W5OEM-2-CSP-0V-10.5no routeSensor ground (E2)—
W6OEM-2-DSP-0V-10.5no routeSensor ground (E2)—
W7SP-0V-1SPEEDUINO-A-120.5no routeGround—
W8SP-0V-1SPEEDUINO-A-230.5no routeGround—
W9OEM-2-KSPEEDUINO-A-130.5no route+5V sensor supply—
W10OEM-2-ESPEEDUINO-A-250.5no routeCrank position (CAS NE)—
W11OEM-2-GSPEEDUINO-A-240.5no routeCam position (CAS G)—
W12OEM-1-GSPEEDUINO-A-70.5no routeIgnition 1 (coils 1+4, wasted spark)—
W13OEM-1-HSPEEDUINO-A-340.5no routeIgnition 2 (coils 2+3, wasted spark)—
W14OEM-2-USPEEDUINO-A-10.5no routeInjector 1 & 3 (batch)—
W15OEM-2-VSPEEDUINO-A-20.5no routeInjector 2 & 4 (batch)—
W16OEM-2-YSPEEDUINO-A-30.5no routeInjector 3 (California sequential)—
W17OEM-2-ZSPEEDUINO-A-50.5no routeInjector 4 (California sequential)—
W18OEM-1-ESPEEDUINO-A-150.5no routeMalfunction indicator lamp (MIL)—
W19OEM-2-ISPEEDUINO-A-170.5no routeTachometer output—
W20OEM-2-NSPEEDUINO-A-210.5no routeO2 sensor—
W21OEM-2-QSPEEDUINO-A-190.5no routeCoolant temperature (ECT)—
W22OEM-2-PSPEEDUINO-A-200.5no routeIntake air temperature (IAT)—
W23OEM-1-CSPEEDUINO-A-180.5no routeStart signal—
W24OEM-1-NSPEEDUINO-A-140.5no routeWide-open throttle switch—

Bill of materials

CategoryPart numberManufacturerDescriptionQtyUnit
connector174515-6TEAMP Multilock .040/.070 hybrid 22-pin female harness plug (Miata NA/NB ECU)1.000ea
connector174516-6TEAMP Multilock .040/.070 hybrid 26-pin female harness plug (Miata NA/NB ECU)1.000ea

ECU setup sheet

RoleEngine refEngine signalSensorECU pointECU functionECU classSetup flag
No configured channels.

Findings

Open (2)

SeverityCodeSubjectMessage
errorREQUIRED_UNMAPPEDOEM-1-BRequired OEM function "ECU power +14V (main relay)" (power_in) is not connected to the new ECU.
infoUNMAPPEDOEM5 optional OEM pins are not mapped: OEM-1-P Power steering pressure switch; OEM-1-Q A/C request; OEM-1-V Neutral/clutch switch; OEM-2-L Idle switch (TPS); OEM-2-O Air flow meter (AFM)

Accepted (1)

CodeSubjectReasonBy
REQUIRED_UNMAPPEDOEM-1-ABattery-constant (memory) feed of the stock ECU: the new ECU has no such input, so it is not needed.mapping-proposal

Notes

Auto-mapped by auto_map() from the library pin data on 2026-10-09. Review every wire against both manuals before building. Open items: - Main-relay feed: no ecu_main_power pin on the stock or the new ECU - OEM-1 B: ECU power +14V (main relay): no rule for class ecu_main_power; wire by hand - Fuel-pump control: Speeduino Speeduino needs to drive the fuel pump, but the stock ECU has no fuel-pump output (an airflow-meter contact or a separate module runs it). Add a fuel-pump relay on a spare low-side output — see the install notes for this car. Not wired: - OEM-1 A: Constant +12V: Speeduino Speeduino has no battery-constant input; not needed - OEM-2 O: Air flow meter (AFM): no free maf/analog_0_5 on Speeduino Speeduino - OEM-1 P: Power steering pressure switch: no free switch_in/frequency_in/analog_0_5 on Speeduino Speeduino - OEM-1 Q: A/C request: no free switch_in/frequency_in/analog_0_5 on Speeduino Speeduino - OEM-1 V: Neutral/clutch switch: no free switch_in/frequency_in/analog_0_5 on Speeduino Speeduino - OEM-2 L: Idle switch (TPS): no free switch_in/frequency_in/analog_0_5 on Speeduino Speeduino

Sources

ReliabilityKindTitleURLArchiveCapturedSHA-256
4vendor_docLink G4X MX5X install manual (NA 1.6/1.8)https://linkecu.com/documentation/MX5X.pdfarchive2026-10-05b7f730de9f6e
4vendor_docSpeeduino Manual (08/04/25) — V0.4 board IDC40 pinout & featureshttps://speeduino.com/Speeduino_manual.pdfarchive2026-10-0521806bd29ef0
2community_copy1991 Mazda Miata wiring diagrams (Mitchell reproduction, mellens.net community mirror)https://trull.org/alex/pubmirror/www.mellens.net/mazda/Mazda-Miata-1991-1993/1991_wiring.pdf—2026-10-086f957a38ed49
2expert_reportJustin Barrera — Miata NA/NB wiring corrections————