6.2 Cooling System Tests, Radiators, Hoses & Clamps, Coolant Pumps, Flush & Refill

Why Cooling Service Is on an HVAC Test

The cab and sleeper heater cores are branches of the engine cooling circuit, and the A/C condenser shares the cooling package's airflow. Weak coolant, air pockets, a failing pump, or a leaking hose therefore produce HVAC complaints: no heat, cold sleeper, fogged windows, or a frozen heater core. This section covers tasks C3, C4, C5, C6, and C8. Thermostat operation and the physics of pressure caps and surge tanks are in the diesel cooling integration section, and fans are in the next section.

Cooling System Tests (C3)

Additive balance: conventional and NOAT coolants depend on nitrite (and often molybdate) to protect wet cylinder liners from cavitation. Test with the strips specified for that coolant, then add SCA or install the correct filter per the chart. Do not simply add SCA each service, because over-concentration causes silicate dropout and deposits that plug heater cores. Fully formulated OAT coolants use different test methods and extender products; follow the coolant maker's instructions.

Freeze damage: when freeze protection is inadequate, coolant can freeze in the coldest, highest, and most exposed places. Those are sleeper heater cores, under-cab heater lines, and auxiliary heater circuits. Expanding ice splits heater core tubes and cracks housings. Ruptured heater core tubes after a cold snap point to incorrect coolant protection, not a bad heater valve.

Hoses, Lines, Fittings, and Clamps (C4)

• Inspect hoses by squeezing them near the ends with the engine cool. Replace hoses that are soft or spongy, hard and cracked, swollen, or oil-soaked.
• Electrochemical degradation (ECD): an electrical potential between the coolant, engine, and radiator can attack a rubber hose from the inside, usually within a couple of inches of the ends. You may feel gaps or channels inside when squeezing near the fitting. Replace affected hoses with ECD-resistant EPDM hose and correct the grounding.
• Silicone hoses are common on heavy trucks. They last long but "cold-flow" under standard worm clamps, which can bite into the surface. Use constant-tension (spring or Belleville washer) clamps or lined T-bolt clamps as specified, and torque them to the clamp maker's value.
• Heater lines to the sleeper may be long hoses, steel tubes, or a mix, with quick-connect fittings. Check for chafing along the frame, secure the clamps, and inspect plastic quick-connect latches and O-rings.
• After replacing hoses, pressure test the system and recheck clamp torque after the first heat cycle if the clamp maker calls for it.

Radiator, Pressure Cap, Surge Tank, and Mountings (C5)

• Radiator: inspect for debris packed between the cores, bent or corroded fins, and leaks at tube-to-header joints and at plastic tank crimp seams. With the engine hot, an infrared scan (or a contact thermometer) across the core finds cold bands, which are plugged tubes. A radiator that runs cool at the bottom while the engine overheats under load is restricted.
• Pressure and fill caps: test the pressure valve at its rating and check the vacuum valve, as described in the diesel cooling integration section. Use only the rated cap.
• Surge (expansion) tank: look for cracks, especially at mounts and fittings. Check the deaeration and fill lines for kinks, and confirm the level sensor works.
• Mountings: truck radiators and cooling modules sit on rubber isolators and brackets that absorb frame twist and vibration. Worn or missing isolators, broken brackets, or loose tie rods let the module move, which cracks tanks, chafes hoses, and opens gaps in the air seals. Replace isolators and restore every seal and brace after service.

Coolant Pumps and Drives (C6)

• Engine coolant pumps may be belt-driven or gear-driven. Auxiliary electric pumps are used for sleeper heat, APU coolant loops, and some coolers.
• Weep hole: the seal vents through a weep hole. A small stain may be normal per the OEM, but active dripping or a steady trail means the seal has failed.
• Bearing: check for shaft play and noise. For belt-driven pumps, check the belt as described in the compressor drive section.
• Impeller erosion or slippage reduces flow without external symptoms. Watch for poor heat at idle, especially in the sleeper, with normal-looking temperatures on the highway.
• For electric auxiliary pumps, check voltage and ground under load and the pump's current draw, as covered in the coolant flow control section.

Flushing, Refilling, and Bleeding (C8)

1. When: contaminated coolant (oil, fuel, rust, mixed types), the wrong coolant, or a scheduled change.
2. Drain completely: radiator drain, engine block drains, and low points in heater lines. Capture and recycle or dispose of the coolant as regulations require.
3. Flush with an approved cleaner if there is scale, sludge, or oil contamination, then rinse. Remove heater cores from the flush path if they are plugged, and back-flush them separately at low pressure.
4. Filters: install the correct coolant filter, charged or blank as the coolant type requires.
5. Refill with the OEM-approved coolant at the right concentration, premixed with clean water. Fill slowly or use a vacuum refill tool, with heater valves open and the HVAC set to full heat so the heater circuits fill.
6. Bleed air at the vent plugs and petcocks, including high sleeper cores. Run the engine to operating temperature with heater valves open and any auxiliary pump running.
7. Verify: recheck the level cold, pressure-test, confirm heater hose temperatures, and confirm the protection level with a refractometer.