Overview
The electrical system in 1973-1987 GM Square Body trucks represents a critical transition period in automotive wiring technology. These vehicles bridged the gap between simple point-to-point wiring of earlier trucks and the more complex computer-controlled systems that followed. Understanding wiring harness replacement and electrical component service is essential for maintaining reliable operation, whether you're dealing with a work truck or a restored show vehicle.
Square Body electrical systems consist of several main harnesses: engine compartment, dash, body, and rear lighting harnesses. The complexity increased significantly with the introduction of Computer Command Control (CCC) systems in 1981, electronic fuel injection options, and various comfort and convenience features throughout the production run.
Proper electrical system maintenance prevents common issues like intermittent starting, lighting failures, gauge malfunctions, and the dreaded "electrical gremlins" that plague aging vehicles. Complete harness replacement is often the most cost-effective long-term solution for trucks with deteriorated original wiring.
Common Issues
Wiring Harness Problems
- Insulation breakdown from heat, oil, and age causing shorts and opens
- Connector corrosion especially at firewall pass-throughs and ground points
- Rodent damage to engine bay and cab wiring
- Previous owner modifications creating voltage drops and fire hazards
- Bulkhead connector failure (1973-1980 models) causing multiple system failures
Component-Specific Issues
- Alternator wiring burning due to inadequate wire gauge on high-output units
- Starter solenoid connections corroding and causing no-start conditions
- Headlight switch failure from high current draw through internal contacts
- Turn signal switch wearing out, causing intermittent operation
- Instrument cluster ground issues creating erratic gauge operation
Year-Specific Problems
- 1973-1980: Ammeter gauge causing dash fires, bulkhead connector overheating
- 1981-1987: CCC system connector issues, ECM ground problems
- 1982-1986: Throttle Body Injection wiring harness deterioration
- 1987: Tuned Port Injection complexity requiring specialized knowledge
Parts & Materials Needed
Complete Harness Replacement
Engine Harness:
- American Autowire Classic Update Kit 500981 (1973-1980)
- American Autowire Highway 15 Kit 500703 (1981-1987)
- Painless Performance 60502 (1973-1980 complete chassis)
- Painless Performance 60508 (1981-1987 complete chassis)
OEM-Style Reproduction:
- LMC Truck engine harness 32-8063 (1973-1980)
- Classic Industries dash harness G1436 (varies by year)
- Brothers Truck Parts rear body harness (model specific)
Individual Components
Switches and Controls:
- Headlight switch: D6240A (1973-1980), D1546B (1981-1987)
- Turn signal switch: D6201A (1973-1980), D1595C (1981-1987)
- Ignition switch: D1470B (1973-1986), D1595F (1987)
- Wiper switch: varies significantly by year and option package
Connectors and Terminals:
- Weatherpack connectors (1-6 cavity) for reliable connections
- Deutsch DT series for high-current applications
- OEM-style terminals from Delphi/Packard for originality
- Metri-Pack 150/280/480 series for 1980s applications
Tools and Materials
Essential Tools:
- Crimping tool set (insulated and non-insulated terminals)
- Wire strippers (10-22 AWG range)
- Multimeter with continuity testing
- Test light (12V automotive type)
- Soldering iron and 60/40 rosin-core solder
- Heat shrink tubing assortment
- Electrical tape (3M Super 33+)
- Wire pulling lubricant
- Panel removal tools
Wire and Cable:
- Primary wire: 10, 12, 14, 16, 18 AWG in multiple colors
- Battery cable: 4 AWG welding cable for starter circuits
- Ground strap: 4 AWG braided copper for engine/body grounds
- Shielded cable: For electronic ignition and CCC systems
Step-by-Step Guide
Complete Engine Harness Replacement
Preparation (All Years)
- Disconnect battery negative terminal and remove battery
- Document existing wiring with photos before removal
- Create wiring diagram noting any modifications or additions
- Remove air cleaner and intake components for access
- Support engine if removing motor mounts for harness routing
1973-1980 Engine Harness Replacement
Removal Process:
1. Disconnect bulkhead connector at firewall (21-pin main connector)
2. Remove alternator connections (BAT, #1, #2 terminals)
3. Disconnect starter solenoid (BAT and S terminals)
4. Remove distributor connector (HEI 4-wire or points ignition)
5. Disconnect oil pressure and temperature sending units
6. Remove A/C compressor clutch connector if equipped
7. Unbolt harness clips from valve covers and intake manifold
Installation Process:
1. Route new harness following original path along passenger side
2. Connect bulkhead connector ensuring proper seating and sealing
3. Install alternator connections with proper terminal orientation
4. Connect starter circuit using supplied heavy-gauge wire
5. Install HEI distributor connector with locking tab engaged
6. Connect sending units with appropriate thread sealant
7. Secure harness with new clips every 12-18 inches
1981-1987 CCC System Harness Replacement
Additional Considerations:
- ECM connector (24-pin 1981-1985, ALDL 1986-1987)
- Oxygen sensor extension (single wire 1981-1984, heated 1985-1987)
- EST distributor 4-wire connection with proper shielding
- Carburetor solenoids (mixture control, EGR, etc.)
- TBI fuel injectors (1982-1986) requiring specific connector types
CCC-Specific Steps:
1. Disconnect ECM with ignition OFF (prevent damage)
2. Remove ALDL connector from dash area
3. Disconnect oxygen sensor at exhaust manifold
4. Remove carburetor/TBI connectors noting wire positions
5. Install new harness maintaining original routing
6. Reconnect ECM ensuring clean, tight connections
7. Perform ECM relearn procedure after installation
Dash Harness Replacement
1973-1980 Dash Harness
- Remove instrument cluster (4 screws, speedometer cable)
- Disconnect printed circuit from cluster housing
- Remove headlight switch and turn signal switch
- Disconnect fuse block from harness
- Route new harness through dash structure
- Install switches with proper connector orientation
- Connect instrument cluster ensuring gauge accuracy
1981-1987 Digital Dash Systems
- Additional complexity with electronic displays
- Pulse generators for speedometer operation
- Computer interfaces requiring specific connectors
- Climate control integration on equipped models
Individual Component Replacement
Headlight Switch Replacement
1973-1980 Models:
1. Pull headlight knob to full ON position
2. Depress release button on switch body
3. Pull knob and shaft assembly out
4. Remove switch bezel nut from front
5. Disconnect harness connector from rear
6. Install new switch reversing removal process
1981-1987 Models:
1. Remove left dash panel for access
2. Disconnect harness connector
3. Remove switch mounting screws
4. Install replacement ensuring proper engagement
Turn Signal Switch Replacement
- Remove steering wheel using proper puller
- Remove lock plate with compressor tool
- Disconnect harness connector at base of column
- Remove switch mounting screws
- Pull switch and harness through column
- Install new switch with harness routing
- Reassemble steering column components
Year/Model Differences
1973-1976: Early Square Body
- Simple wiring with minimal electronics
- Ammeter gauge instead of voltmeter (fire hazard)
- Points ignition or early HEI systems
- Single-speed wipers on base models
- No computer controls or feedback systems
1977-1980: Mid-Production
- HEI ignition standard across all engines
- Voltmeter gauge replaces ammeter
- Improved bulkhead connector design
- Intermittent wipers available
- Early emissions controls (EGR, AIR systems)
1981-1984: CCC Introduction
- Computer Command Control system integration
- EST ignition timing control
- Oxygen sensor feedback systems
- ALDL diagnostic connector
- Electronic carburetor controls
1985-1987: Advanced Systems
- Tuned Port Injection (1985-1987 5.7L)
- Throttle Body Injection refinements
- Improved diagnostic capabilities
- Electronic transmission controls (700R4)
- Computerized climate control options
Model-Specific Variations
C10/C20 (2WD Models):
- Simpler wiring without 4WD components
- Standard alternator mounting
- Conventional power steering pump location
K10/K20 (4WD Models):
- Transfer case wiring and controls
- Front axle disconnect systems (1981+)
- Additional ground straps for front axle
- Modified alternator mounting brackets
K5 Blazer:
- Rear wiper/washer systems
- Tailgate window controls
- Additional interior lighting circuits
- Convertible top wiring (early models)
Suburban:
- Extended body harness requirements
- Third row lighting and power
- Rear A/C controls (if equipped)
- Auxiliary battery provisions (some models)
Tips & Best Practices
Installation Best Practices
- Use dielectric grease on all electrical connections
- Maintain proper wire routing away from heat and sharp edges
- Secure harnesses every 12-18 inches to prevent vibration damage
- Test all circuits before final assembly
- Update ground connections with new straps and hardware
Upgrade Considerations
- Convert ammeter to voltmeter (1973-1976 models)
- Upgrade alternator wiring for higher output units
- Install relay-controlled headlights for improved performance
- Add modern fuse block with blade-type fuses
- Consider complete rewire for show-quality restorations
Common Mistakes to Avoid
- Never assume wire colors match between years
- Don't reuse old connectors with new harnesses
- Avoid automotive store terminals - use quality components
- Don't skip the test phase before buttoning everything up
- Never work on electrical with battery connected
Money-Saving Strategies
- Repair sections instead of complete replacement when possible
- Buy quality terminals in bulk quantities
- Reuse good connectors from salvage harnesses
- Consider used ECMs for CCC system repairs
- Join Square Body forums for group purchases and advice
Troubleshooting
Diagnostic Approach
- Visual inspection for obvious damage or modifications
- Voltage testing at key connection points
- Continuity testing of suspected circuits
- Load testing under actual operating conditions
- Ground verification at all major components
Common Electrical Problems
No-Start Conditions
Symptoms: Engine cranks but won't start
Testing:
- Check ignition voltage at coil or HEI module
- Verify fuel pump operation (1982+ with electric pumps)
- Test starter signal at solenoid S terminal
- Check ECM power and ground circuits (1981+)
Solutions:
- Replace ignition switch if no voltage at START position
- Repair starter circuit wiring or connections
- Clean ECM grounds and power connections
- Test fuel pump relay and associated wiring
Charging System Issues
Symptoms: Battery not charging, voltmeter shows discharge
Testing:
- Check alternator output at BAT terminal
- Verify field circuit voltage at #1 terminal
- Test ground circuit continuity
- Check belt tension and pulley alignment
Solutions:
- Replace alternator if output is below 13.5V at idle
- Repair field circuit wiring or connections
- Install new ground strap from alternator to engine
- Upgrade wiring for high-output alternators
Lighting Problems
Symptoms: Dim lights, intermittent operation, blown fuses
Testing:
- Check headlight switch voltage drop
- Verify ground circuits at light assemblies
- Test dimmer switch operation and connections
- Measure current draw of lighting circuits
Solutions:
- Install headlight relays to reduce switch load
- Clean and tighten all ground connections
- Replace headlight switch if voltage drop exceeds 0.5V
- Upgrade to sealed beam or LED lighting systems
Gauge and Instrument Issues
Symptoms: Erratic readings, dead gauges, flickering displays
Testing:
- Check instrument voltage regulator (5V supply)
- Verify sending unit resistance values
- Test printed circuit continuity
- Check cluster ground connections
Solutions:
- Replace voltage regulator if output is incorrect
- Install new sending units with proper resistance ranges
- Repair printed circuit with conductive paint or jumper wires
- Clean cluster ground connections and install star washers
CCC System Troubleshooting (1981-1987)
ECM-Related Issues
Symptoms: Poor performance, high emissions, check engine light
Diagnostic Steps:
1. Connect scan tool to ALDL connector
2. Retrieve trouble codes following GM procedures
3. Check ECM power and ground circuits
4. Verify sensor inputs (TPS, MAP, coolant temp, etc.)
5. Test actuator outputs (solenoids, injectors, etc.)
Common Solutions:
- Clean ECM connector with electrical contact cleaner
- Repair sensor wiring using OEM-specification wire
- Replace failed sensors with AC Delco or equivalent parts
- Update ECM calibration if available for specific application
Oxygen Sensor Circuit Problems
Symptoms: Poor fuel economy, failed emissions test, rough idle
Testing Process:
- Check sensor voltage (0.1-0.9V switching)
- Verify heater circuit operation (1985+ heated sensors)
- Test signal wire continuity to ECM
- Check ground circuit integrity
Repair Procedures:
- Replace oxygen sensor every 60,000 miles
- Repair signal wire with proper gauge and shielding
- Clean sensor ground connection at exhaust manifold
- Update wiring harness if using aftermarket sensors
This comprehensive guide provides the foundation for successful electrical system maintenance and repair on Square Body trucks. Remember that electrical work requires patience, proper tools, and systematic troubleshooting approaches. When in doubt, consult factory service manuals and seek advice from experienced Square Body enthusiasts and professionals.