Begin repairs by isolating the main fuse block under the left dashboard panel–label each wire before disconnecting. The 7.5A IGN fuse (position 12) supplies power to the ECU, fuel pump relay, and ignition coils; verify continuity with a multimeter set to 200 ohms.
Trace the red/white wire from the alternator’s B+ terminal to the starter solenoid, ensuring no corrosion at the 10mm bolt connection. This circuit carries up to 80A under load; melted insulation indicates an undersized ground path.
For the A/C system, locate the blue/black wire at pin 10 of the climate control module. It triggers the compressor clutch relay–test for 12V when the A/C switch is engaged. If absent, inspect the pressure switch on the accumulator for leaks.
The instrument cluster’s illumination circuit uses a pink/black wire from fuse 20 (10A). Dimmer control flows through the brown wire (pin 4 of the dimmer switch). Replace any frayed segments with 0.75mm² gauge wire to prevent voltage drop.
For ABS diagnostics, probe the green/red wire at the hydraulic unit’s pin 36. Pulsing voltage confirms wheel sensor operation. If steady, check the reluctor ring for missing teeth–the gap should measure 0.5–1.2mm.
When rewiring the aftermarket stereo, bypass the factory amplifier by splicing the gray/black (left) and gray/red (right) wires directly to the head unit’s output. Ground the chassis with a 14-gauge wire no longer than 18 inches to avoid hum.
Electrical Blueprint for the MKIII: Hands-On Troubleshooting
Locate the main fuse block behind the driver’s kick panel–labelled “F/L,” “IGN,” and “EFI”–and verify continuity with a multimeter. Each fuse should read under 0.5 ohms; anything higher indicates corrosion or a broken link. Replace blown fuses with exact amp ratings: 10A for tail lights, 15A for ignition, 20A for fuel injection.
Trace the engine bay harness from the relay box to the distributor connector (white 6-pin plug). Pin 1 supplies 12V during cranking; if voltage drops below 10.5V at pin 3 (signal return), clean the contact points with dielectric grease and check the ignition coil resistance (0.4–0.6 ohms primary, 10–15k ohms secondary).
Common Failure Points & Fixes
| Symptom | Diagnosis | Solution |
|---|---|---|
| Intermittent stalling | Cracked fusible link (thick red wire near battery) | Splice in new 12-gauge wire with heat-shrink tubing |
| Dim headlights at idle | Voltage drop across alternator “B” terminal | Clean terminal and secure with star washer; test output (13.5–14.5V) |
| Fuel pump whine | Loose ground on frame rail (black wire near rear shock tower) | Sand surface rust, apply anti-seize, torque to 10 ft-lbs |
For A/C clutch engagement, follow the blue/red stripe from the compressor to the pressure switch (mounted on accumulator). Bypass the switch temporarily–if the clutch clicks, replace the switch (normally closed, opens at ~45 psi). If no click, check the 10A fuse under the dash labelled “A/C.”
Dash illumination issues often stem from the rheostat (knob adjacent to headlight switch). Remove the instrument cluster bezel, test the rheostat with an ohmmeter (10k ohms off), and resolder cold joints if necessary. Replace the bulb socket if corrosion is present–use GE #161 bulbs, not LED substitutes.
When tracing the rear defogger grid, use a test light to probe the vertical elements. A steady glow at both ends of a single line confirms continuity; no light at one end indicates a break. Repair with conductive paint, ensuring the patch overlaps the grid by at least 5mm. For persistent blower motor failures, swap the resistor pack behind the glovebox (three-stage resistance values: 0.5 ohms, 1.2 ohms, 2.5 ohms).
Finding the Right Electrical Blueprint for Your MA70 Chassis
Begin with the vehicle’s service manual–specifically the electrical section. Factory schematics for the MA70 are divided into distinct segments: engine control, lighting, instrumentation, and auxiliary systems. The manual’s page numbers vary by publisher, but Chrysler’s 1986-1988 performance models (including this coupe) share identical wiring layouts for the JZ-GTE variant. Look for the fold-out pages at the back; they contain full-system diagrams labeled with connector pinouts and wire gauge specifications.
Key Identifiers on Schematics
Locate the harness routing illustrations first. These show wire colors next to their function–red/black for constant 12V, green/yellow for ignition-switched circuits, and black/white for grounding points. The left-hand page typically maps the main harness while the right side details sub-harnesses like the ECU, alternator, and rear lighting. Each wire is marked with a two-letter code corresponding to its destination (e.g., “IG” for ignition, “ST” for starter). Cross-reference these codes with the component legend to avoid confusion between similar-looking circuits.
Third-party repair guides often reprint these charts but may omit critical details like splice locations or relay box pin assignments. Haynes and Chilton manuals, while useful for general procedures, rarely include the full pinout matrix needed for diagnosing intermittent faults. For precise tracing, use the Toyota Technical Information System (TIS) microfiche archives or original dealership-issued supplements–these documents preserve the exact wire routing through the firewall, door jambs, and undercarriage.
If physical copies are unavailable, digital repositories like ERG or MA70 enthusiast forums host scanned versions. Verify the file’s origin: OEM schematics use a consistent line weight and dot-matrix shading for connectors, whereas amateur redraws often distort scale or merge separate circuits. Focus on diagrams that label relay R4 (fuel pump control) and sensor ground G1–these are common failure points and require accurate representation to troubleshoot voltage drops or parasitic draws.
Step-by-Step Wiring Harness Tracing for Engine Components
Locate the main fuse block under the dashboard or near the battery terminal. Identify the thick cables leading from the fuse block to the ignition switch–these are typically 10-gauge wires clad in black or red insulation. Trace the primary power feed from the switch to the starter solenoid using a multimeter set to continuity mode. Verify voltage at the solenoid’s “B” terminal (30+ volts during cranking) before proceeding further. If readings deviate, inspect the solenoid’s plunger and brushes for corrosion or wear.
Mapping Sensor and Actuator Circuits
Disconnect the engine control module (ECM) connector to isolate sensor loops. Pinpoint the throttle position sensor (TPS) harness by following the 4-wire bundle from the throttle body–white/black (signal), red (power), black (ground), and green/white (reference). Test resistance across the TPS terminals (closed throttle: 0.5-1.5kΩ; wide open: 3.5-6kΩ). For the coolant temperature sensor (CTS), probe the yellow/black (signal) and black/white (ground) wires at the thermostat housing junction–cold engine readings should hover around 2.5kΩ, dropping to ~300Ω at operating temperature. Replace sensors if values exceed ±10% of specified ranges.
Examine the fuel pump relay circuit by back-probing the relay socket’s terminals: 85 (trigger), 86 (ground), 30 (constant power), and 87 (load). Engage the relay manually; a functioning unit will emit an audible click and deliver 12V at terminal 87. If silent, swap with a known-good relay (e.g., A/C or horn relay) before condemning the ECM’s pump driver. For injector circuits, pulse each cylinder’s 12V feed (typically pink or orange) with a noid light–steady flashes confirm ECM command signals, while flickering indicates partial short-to-ground. Clean or replace corroded injector plugs if resistance exceeds 16Ω per coil.
Troubleshooting Electrical Gremlins in Your Classic Turbocharged Coupe
Check the ignition system’s low-tension circuits first–start with the ignition coil’s primary resistance, which should read 0.7–1.0 ohms between the positive and negative terminals. If resistance exceeds 1.2 ohms, inspect the coil’s internal windings for corrosion or heat damage. Replace the coil if readings remain inconsistent after cleaning terminal connectors with a wire brush and contact cleaner. The reluctor pickup inside the distributor must output a clean 2–4V AC waveform; a distorted signal indicates worn reluctor teeth or faulty ignition amplifier. Swap the amplifier before condemning the distributor–it’s a common failure point.
Fusible Links and Chafed Harness Repair
Locate the fusible link bundle near the battery’s positive terminal–its red, yellow, or white braided wires melt at 80–120A. Probe each link with a multimeter: zero resistance confirms integrity; infinite resistance signals burnout. Splice in a new fusible link segment (gauge-matched to the original) using crimp connectors and heat-shrink tubing, avoiding solder–it’s brittle under vibration. For chafed harnesses, expose the damaged section and wrap it with self-fusing silicone tape (not electrical tape), overlapping by 50% to restore insulation. Prioritize areas behind the dashboard and near suspension components, where flex causes short circuits.
Color-Coded Wire Reference for All Major Systems
Always verify connector C100 (main harness under dashboard) before assuming wire integrity–corrosion at pin 18 (black/white) disrupts ignition signals without warning. Replace entire segments if brittleness exceeds 3mm fraying.
Engine control module terminals rely on precise color standards:
- Red/black (IG+): 12V switched ignition, max 4A draw
- Black/yellow (STA): starter trigger, verify continuity sub-0.5Ω
- White/red (INJ1): fuel injector pulse, scope for 4.5ms duration
- Green/white (THW): coolant temp sensor, voltage drops below 0.5V at 120°C
Measure across connectors, not harness, to isolate phantom resistances.
Lighting circuits obey reversed polarity warnings:
- Tail lamps: green/red wire carries ground; brown is chassis return–crossing them blows fuse 14
- Headlamp high beams: blue/white switches to blue at relay R8; 1.2mm² wire gauge minimum
- Turn signals: light green/black and light green/white toggle at 1.5Hz; check flasher unit for stuck contacts if pulsing erratically
Inspect bulb sockets for copper oxidation–sandpaper grit 400 removes it without damaging plating.
Audio and instrumentation share a critical pink/blue wire (illumination dimmer signal). If gauges flicker at night, trace this line from dimmer resistor to cluster plug P7; 1.5kΩ resistance confirms proper function. Installed aftermarket alarms often splice here–red flag for parasitic draws.
Chassis grounds differ by location:
- Battery negative: pure black 8-gauge braid, must clamp directly to frame rivet
- Engine block: black/white striped, 6mm bolt torque 8-10Nm
- Trunk lid: black/yellow, often neglected–pry insulation for solid contact
Shielded twisted pairs (gray/white) protect sensor signals–never untwist more than 2cm during repairs.
HVAC blend door actuators use orange/black (recirculation) and white/black (temperature). If blend doors click but don’t move, backprobe actuator connector with 5V reference; 2.5V output indicates internal gear failure. Replace entire servo unit–not the motor alone.
Anti-lock braking module wires carry colored sleeves: orange (wheel speed FL), blue (FR), green (RL), violet (RR). Each signal varies 0.5V–4.5V during rotation; flatline at 2.5V means sensor air gap exceeds 1.2mm. Clearances above spec require tone ring alignment, not just sensor replacement.