Overview
Proper transmission cooling is critical for the longevity and performance of automatic transmissions in 1973-1987 GM Square Body trucks. These vehicles came equipped with various automatic transmissions including the TH350, TH400, and 700R4, all of which generate significant heat during operation. Factory cooling systems were often marginal, particularly in trucks used for towing or heavy-duty applications.
This article covers factory cooling system components, common failure points, upgrade options, and maintenance procedures specific to Square Body trucks. Understanding and maintaining your transmission cooling system can prevent costly transmission rebuilds and extend service life significantly.
Common Issues
Square Body transmission cooling systems are prone to several well-documented problems:
Radiator-Mounted Cooler Failures
- Internal cooler leaks causing coolant contamination in transmission fluid
- Restricted flow due to sediment buildup in aging radiators
- Inadequate cooling capacity for towing or performance applications
External Cooler Problems
- Corroded fittings and lines on factory auxiliary coolers
- Insufficient airflow through cooler cores
- Improper mounting leading to vibration damage
Line and Fitting Issues
- Deteriorated rubber transmission cooler lines (especially 1973-1980 models)
- Leaking compression fittings at radiator connections
- Cracked hard lines due to age and vibration
Temperature Management
- Lack of temperature monitoring in most factory configurations
- Overcooling in cold climates affecting shift quality
- Inadequate cooling in hot climates or under load
Parts & Materials Needed
Factory Replacement Parts
Transmission Cooler Lines:
- 1973-1980: 15621695 (inlet), 15621696 (outlet) - rubber lines with steel fittings
- 1981-1987: 15621697 (inlet), 15621698 (outlet) - improved rubber compound
- LMC Truck part numbers: 32-1436 (early), 32-1437 (late)
Radiator Cooler Fittings:
- 14044613 - 3/8" NPT to transmission line fitting (brass)
- 25518064 - Radiator cooler inlet/outlet fittings
Factory Auxiliary Cooler (when equipped):
- 22512794 - External transmission cooler assembly
- 22512795 - Mounting bracket set
Upgrade Components
Aftermarket External Coolers:
- B&M SuperCooler 70264 (tube and fin, 11" x 6" x 2")
- Hayden Automotive 677 (plate and fin, 11" x 7.5" x 1.5")
- TruCool 4590 (plate and fin with fan, 12" x 7.5" x 1.75")
High-Performance Lines:
- Earl's 632006ERL - 6AN braided stainless lines
- Lokar TC-1000LS - Stainless braided with proper fittings
Temperature Monitoring:
- Auto Meter 2451 - 2-1/16" transmission temperature gauge
- Dakota Digital SGI-8E - digital temperature display
Tools Required
- Flare nut wrenches (3/8", 7/16")
- Tubing cutter
- Double flaring tool (for steel line repairs)
- Transmission fluid pump
- Basic hand tools
- Jack and jack stands
- Drain pan (minimum 6-quart capacity)
Step-by-Step Guide
Inspecting Factory Cooling System
-
Visual Inspection
- Check transmission cooler lines for cracks, bulging, or fluid seepage
- Inspect fittings at transmission case and radiator for corrosion
- Examine external cooler (if equipped) for damage or restricted airflow -
Pressure Testing
- With engine cold, remove transmission dipstick
- Look for coolant contamination (milky or pink-tinted fluid)
- Check coolant overflow tank for transmission fluid (reddish color) -
Flow Testing
- Start engine and allow to reach operating temperature
- With transmission in Park, observe fluid flow through clear section of cooler line
- Flow should be steady and pulsing with engine RPM
Replacing Transmission Cooler Lines
1973-1980 Models:
- Raise vehicle and secure on jack stands
- Locate cooler lines running from transmission to radiator
- Place drain pan under transmission cooler line connections
- Using flare nut wrenches, disconnect lines at transmission case
- Case fittings torque: 20 ft-lbs - Disconnect lines at radiator cooler connections
- Remove old lines, noting routing path
- Install new lines following original routing
- Connect at transmission case first, then radiator
- Add 2 quarts ATF and check for leaks
1981-1987 Models:
- Process similar to early models, but note improved quick-disconnect fittings at radiator
- Press release tabs on radiator fittings before removal
- Ensure new O-rings are installed on quick-connect fittings
- Push connections fully home until tabs click
Installing External Transmission Cooler
Series Configuration (Recommended):
- Mount cooler in front of radiator with adequate airflow
- Minimum 1" clearance from radiator
- Secure with provided brackets to core support - Cut transmission cooler line at midpoint
- Route line from transmission to cooler inlet (bottom connection)
- Connect cooler outlet (top connection) to radiator cooler inlet
- Maintain radiator cooler in circuit for thermostat function
- Secure all lines away from exhaust and moving components
Parallel Configuration (Cold Climate Option):
- Install cooler as above
- Connect transmission output directly to external cooler
- Connect cooler output directly back to transmission
- Bypass radiator cooler completely
- Note: May cause slow warm-up and poor cold-weather shifting
Adding Temperature Monitoring
- Install temperature sender in transmission pan
- Drill and tap 1/8" NPT hole in pan side
- Use thread sealant rated for ATF - Route wiring through firewall using existing grommet
- Mount gauge in instrument panel or A-pillar
- Connect power to switched 12V source
- Ground to chassis
Year/Model Differences
1973-1980 Models
Transmission Applications:
- C10/K10: TH350 standard, TH400 with big block or towing package
- C20/K20: TH400 standard
- Blazer/Suburban: TH350 or TH400 depending on engine
Cooling System Characteristics:
- All models used radiator-mounted cooler as primary cooling
- External auxiliary coolers rare, mainly on heavy-duty packages
- Rubber cooler lines throughout system
- No factory temperature monitoring
1981-1987 Models
Transmission Applications:
- 1981: Continued TH350/TH400 usage
- 1982-1987: 700R4 introduction (C10/K10, Blazer, Suburban)
- C20/K20: Continued TH400, some late TH700R4
Cooling System Improvements:
- Quick-disconnect fittings at radiator (1982+)
- Improved rubber compounds in cooler lines
- Factory auxiliary coolers more common with towing packages
- 700R4 transmissions particularly sensitive to overheating
Model-Specific Notes:
- K-Series 4WD: Cooler lines routed around transfer case, more complex routing
- Suburban: Longer wheelbase allows better cooler line protection
- Blazer: Shorter cooling lines but less airflow area for external coolers
- C20/K20: Standard auxiliary coolers with 454 engine option
Tips & Best Practices
Installation Best Practices
- Always use series configuration with radiator cooler unless in extremely cold climates
- Route lines with smooth bends - avoid sharp angles that restrict flow
- Secure lines every 18 inches to prevent vibration damage
- Use proper torque specifications - over-tightening causes leaks
- Install temperature gauge - most critical upgrade for any cooling system
Maintenance Recommendations
- Inspect cooler lines annually for deterioration or damage
- Change transmission fluid every 30,000 miles with external cooler
- Clean external cooler fins annually with compressed air
- Monitor operating temperatures - normal range 160-200°F
Common Mistakes to Avoid
- Don't bypass radiator cooler completely unless specifically required
- Don't use compression fittings on transmission case connections
- Don't mount external cooler behind radiator - reduces effectiveness
- Don't ignore temperature monitoring - most transmission failures are heat-related
Vendor Recommendations
Parts Sources:
- LMC Truck: Complete OEM-style cooler line sets, good quality reproductions
- Classic Industries: Wide selection, competitive pricing
- GM Parts Direct: Original GM parts when available
- Summit Racing: Performance cooling components and gauges
Performance Upgrades:
- B&M Racing: Proven SuperCooler line, excellent for towing
- Hayden Automotive: Good value, reliable performance
- Earl's Performance: Premium braided lines and fittings
Troubleshooting
High Transmission Temperatures
Symptoms: Temperature gauge reading over 220°F, burnt fluid smell, harsh shifting
Diagnosis Steps:
1. Check fluid level and condition
2. Verify cooler line flow and restrictions
3. Inspect external cooler for blockage
4. Test radiator cooler for internal restrictions
Solutions:
- Clean or replace external cooler
- Flush transmission cooling circuit
- Upgrade to larger capacity cooler
- Add auxiliary electric cooling fan
Fluid Contamination
Symptoms: Milky transmission fluid, coolant loss, sweet smell in transmission fluid
Diagnosis:
- Internal radiator cooler failure allowing coolant mixing
- Pressure test cooling system
- Check transmission fluid for glycol contamination
Solutions:
- Replace radiator or repair internal cooler
- Flush transmission multiple times
- Install external cooler to prevent recurrence
Poor Cold Weather Performance
Symptoms: Delayed engagement when cold, harsh shifts until warm-up
Diagnosis:
- Transmission running too cool
- Oversized external cooler
- Bypassed radiator cooler
Solutions:
- Install thermostatic bypass valve
- Relocate external cooler for less airflow
- Ensure radiator cooler remains in circuit
Line Leaks and Fittings
Symptoms: Fluid drops under vehicle, low fluid level, visible seepage
Diagnosis:
1. Clean all fittings and lines
2. Run engine and observe leak points
3. Check fitting torque specifications
4. Inspect lines for age-related deterioration
Solutions:
- Retorque fittings to specification
- Replace deteriorated lines
- Use proper thread sealant on NPT fittings
- Replace O-rings on quick-disconnect fittings
Flow Restrictions
Symptoms: High temperatures under load, poor performance, delayed shifts
Testing:
- Remove cooler lines and check individual flow rates
- Pressure test each component separately
- Inspect for internal blockages
Solutions:
- Flush cooling circuit with appropriate solvent
- Replace restricted components
- Upgrade to larger diameter lines
- Install bypass filtration system