Overview
The fuel system compatibility between TH400 and 700R4 transmissions in 1973-1987 GM Square Body trucks is a critical consideration for owners performing transmission swaps or rebuilds. While both transmissions can physically bolt to the same engines, the 700R4's lockup torque converter and overdrive functionality require specific fuel system considerations that differ significantly from the simpler TH400 setup.
This article covers the essential fuel system modifications, wiring requirements, and compatibility issues when swapping between these transmissions, ensuring proper operation and preventing costly damage to your Square Body's drivetrain.
Key Applications:
- 1973-1987 C10, C20, C30 pickup trucks
- 1973-1987 K10, K20, K30 4WD trucks
- 1973-1987 K5 Blazer and GMC Jimmy
- 1973-1987 Suburban and GMC Suburban
Common Issues
TH400 to 700R4 Conversion Problems
Throttle Valve (TV) Cable Issues:
- Incorrect adjustment leading to harsh shifts or transmission failure
- Missing TV cable bracket on carburetor or throttle body
- Wrong TV cable length for specific engine/transmission combinations
Lockup Converter Wiring:
- No 12V switched power source for lockup solenoid operation
- Missing brake light switch integration causing lockup during braking
- Improper grounding of transmission case and wiring harness
Fuel Economy Expectations:
- Unrealistic MPG expectations without proper tuning and TV cable adjustment
- Overdrive hunting due to improper rear axle ratio combinations
700R4 to TH400 Conversion Problems
Throttle Linkage Mismatch:
- Kickdown switch elimination when removing 700R4 TV cable system
- Throttle bracket compatibility with older carburetor setups
Parts & Materials Needed
TH400 to 700R4 Conversion
Essential Components:
TV Cable Assembly: GM Part #24230309 (1982-1987)
TV Cable Bracket (Quadrajet): GM Part #14081834
TV Cable Bracket (TBI): GM Part #25534346
Lockup Wiring Harness: GM Part #12102747
Electrical Components:
- Brake light switch (if not present): Standard Motor Products SLS-123
- 12V relay for lockup circuit: Standard Motor Products RY-54
- Inline fuse holder (10-amp) and appropriate wiring
- Heat shrink tubing and electrical connectors
Tools Required:
- TV cable adjustment tool or fabricated gauge block
- Digital multimeter for electrical testing
- Basic hand tools (wrenches, screwdrivers)
- Wire strippers and crimping tool
700R4 to TH400 Conversion
Components to Remove/Modify:
- TV cable assembly (entire system)
- Lockup wiring harness
- Transmission cooler lines (different routing)
Potential Additions:
- Kickdown switch for performance applications: Holley Part #12-804
- Vacuum modulator if converting to vacuum-operated shifting
Step-by-Step Guide
Installing 700R4 in Place of TH400
Phase 1: Mechanical Installation
- Remove TH400 transmission following standard procedures
- Install 700R4 ensuring proper torque converter seating
- Connect transmission cooler lines - 700R4 uses same fittings as TH400
- Install TV cable bracket on carburetor or throttle body:
- Quadrajet carburetors: Mount bracket to secondary throttle lever
- Rochester 2GC/2GE: May require aftermarket bracket adaptation
- TBI systems (1987): Use specific TBI bracket
Phase 2: TV Cable Installation and Adjustment
- Route TV cable from transmission to throttle body/carburetor
- Connect cable to throttle lever ensuring smooth operation
- Perform initial adjustment:
- Engine OFF, throttle closed - Push TV cable slider forward (toward cable) - Open throttle to wide-open position - Release slider - should click into position - Road test and fine-tune based on shift quality
Phase 3: Electrical Connections
-
Locate 12V switched power source:
- 1973-1980: Tap into ignition switch accessory terminal
- 1981-1987: Use existing ECM power feed or ignition coil positive -
Install lockup relay circuit:
Relay Pin 30: Battery positive (constant 12V) Relay Pin 85: Ground Relay Pin 86: 12V switched (ignition on) Relay Pin 87: To lockup solenoid -
Connect brake light switch integration:
- Wire normally-closed contact in series with lockup circuit
- Ensures lockup disengages during braking
Installing TH400 in Place of 700R4
Phase 1: Removal of 700R4 Components
- Disconnect TV cable at transmission and throttle body
- Remove lockup wiring harness completely
- Disconnect transmission cooler lines
- Remove 700R4 following standard procedures
Phase 2: TH400 Installation
- Install TH400 with appropriate torque converter
- Connect cooler lines - same routing as 700R4
- Connect shift linkage - typically no modifications required
- Install vacuum modulator line (if equipped)
Year/Model Differences
1973-1980 Models
TH400 Standard Equipment:
- C10/K10: TH400 standard with V8 engines
- C20/K20: TH400 standard equipment
- K5 Blazer: TH400 with 350 and 400 engines
- Suburban: TH400 standard with V8s
700R4 Conversion Considerations:
- No factory ECM integration - requires standalone lockup control
- Carburetor TV cable brackets may require modification
- Speedometer drive gear may need changing for accurate readings
1981-1987 Models
Transmission Applications:
- 1981: TH400 still standard, 700R4 optional
- 1982-1987: 700R4 becomes standard with most V8 engines
- Heavy-duty applications: TH400 remained available through 1987
Fuel System Integration:
- 1981-1986: Carburetor with Computer Command Control (CCC)
- 1987: Throttle Body Injection (TBI) on many models
- ECM integration: 700R4 lockup controlled by ECM on CCC-equipped trucks
Model-Specific Considerations
K5 Blazer/Jimmy:
- Shorter driveshaft may require modification when swapping transmissions
- Transfer case compatibility - NP208 and NP241 bolt patterns identical
Heavy-Duty Trucks (C20/K20, C30):
- TH400 preferred for towing applications
- 700R4 HD version (4L60) not available until 1990s
- Cooler capacity may need upgrading for 700R4 conversions
Tips & Best Practices
TV Cable Adjustment Critical Points
Proper Adjustment Technique:
- Never force the slider - should move freely when pressed
- Verify full throttle travel - cable should not bind at WOT
- Check cable routing - avoid sharp bends or heat sources
- Test drive immediately after adjustment to verify shift points
Common Adjustment Mistakes:
- Too tight: Causes harsh shifts and premature wear
- Too loose: Results in delayed shifts and overheating
- Binding cable: Creates inconsistent shift timing
Electrical System Best Practices
Lockup Circuit Reliability:
- Use proper gauge wire (14 AWG minimum for power feeds)
- Install inline fuse protection (10-amp recommended)
- Ensure solid ground connections at transmission case
- Use weatherproof connectors for underhood connections
Testing Lockup Operation:
- Monitor with scan tool (CCC-equipped trucks)
- Feel for RPM drop at steady cruise (45+ MPH)
- Check engagement with brake pedal test
Cooling System Considerations
Transmission Cooler Upgrades:
- 700R4 generates more heat than TH400 due to lockup converter
- Consider auxiliary cooler for towing applications
- Verify proper fluid flow through cooler circuits
- Use appropriate transmission fluid (Dexron III or equivalent)
Troubleshooting
TV Cable Problems
Symptoms: Harsh Shifting, Late Shifts
Diagnosis Steps:
1. Check TV cable adjustment at idle and WOT
2. Inspect cable for binding or damage
3. Verify throttle lever travel matches cable movement
4. Test transmission line pressure with gauge
Solution:
- Readjust TV cable using proper procedure
- Replace damaged cable assembly
- Modify throttle bracket if binding occurs
Lockup Converter Issues
Symptoms: No Lockup Engagement
Diagnostic Process:
1. Check 12V power at lockup solenoid connector
2. Test solenoid resistance (20-40 ohms typical)
3. Verify brake light switch operation
4. Check ECM lockup control (CCC systems)
Solutions:
- Repair power feed circuit
- Replace faulty lockup solenoid
- Adjust or replace brake light switch
- Diagnose ECM lockup control logic
Shift Quality Problems
Symptoms: Erratic Shifting, Slipping
Troubleshooting Sequence:
1. Verify proper fluid level and condition
2. Check TV cable adjustment
3. Test transmission line pressure
4. Inspect for vacuum leaks (TH400 modulator)
Common Fixes:
- Adjust TV cable per specification
- Service transmission fluid and filter
- Repair vacuum modulator circuit (TH400)
- Address internal transmission wear issues
Speedometer Accuracy Issues
Symptoms: Incorrect Speedometer Reading
- 700R4 overdrive ratio affects speedometer calibration
- Different final drive ratios require gear changes
Solutions:
Speedometer Drive Gear Selection:
- Calculate: (Tire Revolutions/Mile × Axle Ratio) ÷ 1000
- Round to nearest available gear tooth count
- Common 700R4 gears: 17, 19, 21, 23 tooth options
Electronic Speedometer Conversion:
- Dakota Digital and Classic Instruments offer conversion kits
- Programmable ratios eliminate gear change requirements
- GPS-based calibration available on premium units