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newSchoolWorks.com › VoTechWorks › Transportation › Diesel & Heavy Truck › Preventive Maintenance for Diesel & Heavy Trucks › Lesson 5
TRN-DIES20-06 · Lesson 5 · 18 clock hours · lab

Cooling System Maintenance and Inspection

Objective. Students will test coolant concentration and pH, inspect all cooling system hoses and clamps, pressure-test the cooling system cap and system for leaks, drain and refill with OEM-specified coolant type, and collect a coolant sample for analysis.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Cooling System Maintenance Matters

The cooling system keeps a diesel engine within its designed operating temperature range. Diesel engines produce enormous heat loads, and a cooling system that is neglected even briefly can cause head gasket failure, liner cavitation, cracked heads, or complete engine seizure. Preventive maintenance on the cooling system is one of the highest-value services a technician performs because the cost of a coolant change or hose replacement is a small fraction of the cost of an in-frame overhaul.

Safety First

Before touching any part of a cooling system, confirm the engine is cold and fully depressurized. A hot pressurized cooling system can release scalding coolant under pressure if the cap is removed. Never loosen a cap on a warm system. Turn the cap slowly to the first stop and allow pressure to bleed off before removing it fully.

Wear chemical-splash safety glasses and nitrile gloves when handling coolant. Ethylene glycol is toxic to humans, animals, and the environment. Collect all drained coolant in labeled, approved drain pans and transfer it to the shop's coolant recycling station — never pour it down a drain or onto the ground. Verify your shop's compliance with applicable environmental regulations for coolant disposal.

When pressure-testing, do not exceed the cap's rated pressure. Over-pressurizing can rupture a hose or damage the radiator.

Coolant Types for Heavy Diesel Engines

Heavy-duty diesel engines use coolants that differ from typical passenger-car antifreeze. The two most common types are:

Always verify the OEM-specified coolant type in the service manual before adding or refilling. Mixing incompatible coolants neutralizes inhibitor packages and can cause rapid corrosion.

Supplemental Coolant Additives (SCAs)

Many heavy-duty diesel cooling systems require Supplemental Coolant Additives (SCAs). SCAs replenish nitrites and other inhibitors that deplete over time. SCA concentration must stay within the OEM-specified range: too little leaves liners and metal surfaces unprotected; too much can cause silicate gels to form, plugging the system. SCA concentration is tested with SCA test strips — a quick colorimetric check that compares a wetted strip to a reference chart.

Testing Coolant with a Refractometer

A refractometer measures how much light bends (refracts) as it passes through a liquid, which correlates to the concentration of dissolved material — in this case, ethylene glycol or propylene glycol. The reading gives you freeze protection in degrees Fahrenheit or Celsius.

To use a refractometer:

1. Place one or two drops of coolant on the prism.

2. Close the cover plate and point the instrument toward a light source.

3. Read the scale through the eyepiece at the line between the light and dark fields.

4. Compare to the OEM freeze-protection specification (commonly −34 °F or lower for most North American climates).

5. Record the reading and note pass or fail.

Also test pH using appropriate test strips. Most heavy-duty coolants should fall within a pH range that the OEM specifies (generally alkaline). A pH outside that range indicates inhibitor depletion or contamination.

Hose and Clamp Inspection

With the system cold and depressurized, inspect every hose in the circuit: upper and lower radiator hoses, bypass hoses, and heater hoses. Look and feel for:

Replace any hose that fails inspection. Tighten clamps to OEM torque specification using a clamp driver or torque screwdriver. Worm-drive clamps on heavy-duty hoses typically have a torque specification — using feel alone risks undertightening (leaks) or overtightening (hose damage).

Pressure Testing the System and Cap

A cooling system pressure test kit attaches to the radiator filler neck. The kit includes adapters for different neck sizes and a hand pump with a gauge.

Cap test: Attach the cap to the tester's cap adapter and pump to the cap's rated relief pressure (stamped on the cap). The cap should hold that pressure and then release at approximately that value. A cap that leaks below its rating or fails to release must be replaced.

System test: With the cap removed, attach the tester to the filler neck. Pump to the cap's rated pressure. Observe the gauge for five full minutes. Any pressure drop indicates a leak. Inspect the water pump weep hole, hose connections, heater core, and radiator for wetness or residue.

Drain, Flush, and Refill

Drain coolant into an approved container. If directed by the service schedule, perform a flush with clean water or a manufacturer-approved flush chemical. Refill with the correct coolant, either pre-diluted 50/50 mix or a correctly mixed concentrate. Use distilled or deionized water when mixing concentrate — mineral-laden tap water introduces calcium and magnesium that can deposit in passages. Bleed air from the system using the OEM-specified procedure; trapped air causes hot spots and thermostat hunting.

Collecting a Coolant Sample

A coolant sample sent to a fluid analysis laboratory gives a detailed chemical picture of inhibitor depletion, metal contamination (indicating internal corrosion or component wear), and glycol concentration. Use the lab-provided sample vial. Draw coolant from the middle of the fluid in the system, not from the first drain. Label the vial with the unit number, mileage or hours, date, and coolant type. Package in the provided mailer and ship promptly.

Key terms8

OAT (Organic Acid Technology)A heavy-duty coolant formulation that uses organic acid corrosion inhibitors and typically offers a long service life; must not be mixed with conventional or NOAT coolants.
NOAT (Nitrited Organic Acid Technology)A heavy-duty coolant that combines organic acids with nitrites to protect wet cylinder liners from cavitation erosion, commonly specified for many diesel engines.
SCA (Supplemental Coolant Additive)A chemical additive package added to heavy-duty diesel coolant to replenish nitrites and other inhibitors that deplete during service; concentration must stay within OEM-specified limits.
RefractometerAn optical instrument that measures the refractive index of a liquid to determine coolant concentration and corresponding freeze-protection temperature.
Cavitation erosionPitting damage to wet cylinder liner surfaces caused by vapor bubble collapse; nitrite-based inhibitors form a protective film that resists this damage.
Cooling system pressure testerA hand-pump tool with a gauge and filler-neck adapters used to pressurize a cooling system or radiator cap to check for leaks and verify cap relief pressure.
pHA measure of acidity or alkalinity on a scale of 0–14; heavy-duty coolant must remain within an OEM-specified alkaline range for adequate corrosion protection.
Fluid analysisLaboratory testing of a coolant (or oil) sample to measure inhibitor levels, metal contamination, glycol concentration, and other indicators of system condition.

Worked examples4

1. A refractometer reading shows freeze protection to −28 °F. The OEM specification requires freeze protection to at least −34 °F. The cooling system holds 6 gallons. How do you determine whether to add concentrate or drain-and-refill?
  1. Step 1 — Understand what the reading means: −28 °F freeze protection indicates the glycol concentration is lower than required. Adding water to dilute is not the fix; more glycol is needed.
  2. Step 2 — Consider the headroom in the system: if the system is already full, you cannot simply add concentrate without removing some coolant first. Drain enough coolant to make room, then add the appropriate concentrate.
  3. Step 3 — If the system is significantly low and the coolant is due for a change, a full drain, flush, and refill with properly mixed coolant (50/50 pre-diluted, or concentrate mixed with distilled water to achieve the target concentration) is the most reliable corrective action.
  4. Step 4 — After correction, retest with the refractometer. Record the new reading and confirm it meets the −34 °F or lower requirement before returning the vehicle to service.
Answer: The system requires either a partial drain and addition of concentrate or a full drain and refill with correctly mixed coolant; top-off with water alone would worsen the problem. Retest after correction to confirm the reading meets OEM specification.
2. A radiator cap is rated at 15 psi. During the cap test, the gauge reaches 15 psi and then immediately drops to 8 psi within 30 seconds without any audible release. What does this indicate and what is the correct action?
  1. Step 1 — Interpret the result: the cap dropped from 15 psi to 8 psi without venting at its rated pressure. A good cap holds rated pressure and releases at approximately rated pressure through its pressure-relief valve in a controlled way. A drop below rated pressure that is not a normal relief release indicates the cap's seal is leaking.
  2. Step 2 — Conclude that the cap has a faulty lower seal or a spring that has lost tension, allowing it to leak below its rated holding pressure.
  3. Step 3 — The correct action is to replace the cap with a new cap of the same rated pressure as specified by the OEM.
Answer: The cap is defective — its seal leaks below rated pressure. Replace the cap with a new cap matching the OEM-specified pressure rating.
3. You are mixing coolant from concentrate for a 6-gallon system. The OEM specifies a 50/50 mixture of concentrate and distilled water. How many gallons of concentrate and how many gallons of distilled water do you need?
  1. Step 1 — Identify total volume: 6 gallons total.
  2. Step 2 — Apply the 50/50 ratio: 50% of 6 gallons = 3 gallons of concentrate, and 50% of 6 gallons = 3 gallons of distilled water.
  3. Step 3 — Confirm: 3 gallons concentrate + 3 gallons distilled water = 6 gallons total at 50/50 ratio.
Answer: 3 gallons of coolant concentrate and 3 gallons of distilled water, mixed before adding to the system.
4. During a system pressure test, you pump the system to the cap-rated pressure of 15 psi. After five minutes the gauge reads 12 psi. Is this a pass or fail, and where should you look first for the leak?
  1. Step 1 — Interpret the result: the system lost 3 psi over five minutes. Any pressure drop during the five-minute hold period is a failure — the system should hold steady.
  2. Step 2 — Begin the leak search at the most common external leak points: hose connections at the radiator, water pump inlet and outlet, thermostat housing, and heater hose fittings.
  3. Step 3 — Inspect the water pump weep hole for seeping coolant, which indicates a failing pump seal.
  4. Step 4 — Check the radiator tanks and core for wetness, mineral deposits (white or rust-colored residue), and the cab heater core connections.
  5. Step 5 — If no external leak is found, consider internal leakage paths such as a head gasket or cracked head, which may require further diagnosis (combustion gas test of the coolant).
Answer: Fail — any pressure drop during the five-minute test indicates a leak. Inspect hose connections, water pump weep hole, radiator, and thermostat housing first, then consider internal sources if no external leak is found.

The math in this lesson2

Learners split a 6-gallon system 50/50 into concentrate and distilled water.
6.RP.3N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Using a refractometer and test strips, measure freeze protection and coolant inhibitor (SCA) concentration; compare results to OEM specification and record pass/fail with recommended corrective action. · With the system cold and depressurized, inspect all upper and lower radiator hoses, bypass hoses, heater hoses, and all clamps for cracks, softness, abrasion, or collapse; replace any defective hose and tighten loose clamps to OEM spec. · Attach a cooling system pressure tester to the radiator neck, pressurize to the cap-rated pressure, and inspect for leaks over five minutes; drain coolant into approved container, flush if directed, refill with correct pre-diluted or correctly mixed coolant, and collect a sample vial for fluid analysis lab.
Materials: Coolant refractometer, SCA test strips, cooling system pressure test kit, hose pliers, clamp drivers, torque screwdriver, replacement hoses and clamps, approved coolant (OAT or NOAT as specified), coolant sample vials and mailers, drain pans, coolant recycling station, OEM service manual
Check: Instructor checklist: refractometer reading recorded and interpreted correctly, pressure test held for full five minutes with no drop, correct coolant type installed, sample collected and labeled; all items required to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Safety and Hazardous Materials
1. Why is it essential that a cooling system be cold and depressurized before removing the radiator cap?
Engine Systems — Cooling
2. A technician tests a heavy-duty diesel cooling system with SCA strips and finds the concentration is above the maximum limit on the OEM chart. What is the most likely consequence if left uncorrected?
Engine Systems — Cooling
3. Which coolant property does a refractometer directly measure?
Engine Systems — Cooling
4. During a cooling system pressure test, the gauge holds steady at the cap-rated pressure for five minutes with no drop. What does this result indicate?
Engine Systems — Cooling
5. A technician is refilling a diesel cooling system after a flush. The OEM specifies NOAT coolant. Only OAT concentrate is available in the shop. What should the technician do?
Engine Systems — Cooling
6. When inspecting a lower radiator hose, a technician squeezes the middle of the hose and feels a crunching or crackling sensation from inside. What condition does this most likely indicate?
Engine Systems — Cooling
7. Why must distilled or deionized water be used when mixing coolant concentrate rather than tap water?
Engine Systems — Cooling
8. A coolant fluid analysis report shows elevated iron and aluminum levels significantly above normal baseline. What condition do these findings most likely indicate?
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