Overheating and Brake Fade on a Ghat Section
A 2019 TGSRTC express bus with 1.24 lakh km on the odometer reports two symptoms on the same ghat route. The driver states that coolant temperature rose to 105°C on the climb, the low-coolant warning lamp flickered, and a sweet smell came from the engine bay. After a 6 km descent using the service brake, the brake pedal became spongy and travelled almost to the floor; the parking brake still holds. The workshop has one bay free for 90 minutes, one spare bus, and a scheduled departure in two hours. Inspection shows the coolant level below minimum, no external coolant leak, a dark brake-fluid reservoir, front pads at 4 mm, rear linings at 2 mm, and a very hot rear drum. The depot cannot afford to miss the departure, but the route has steep gradients and sharp curves. Decide the diagnostic sequence, the roadworthiness decision and the repair plan.
1. Rank the likely causes of the overheating and the spongy brake pedal, and set out a 90-minute diagnostic sequence.
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Overheating - probable causes in order: low coolant from an internal or concealed leak; radiator cap not holding pressure; thermostat stuck closed; fan clutch or electric fan failure; water pump or belt problem; head-gasket breach. The sweet smell and low level make a coolant leak or cap failure more likely than a gauge fault.
Spongy pedal - probable causes: air or moisture in the hydraulic circuit, old fluid boiling on the descent, a leaking wheel cylinder or caliper, or glazed linings after overheating. A spongy pedal is a hydraulic symptom, not a normal consequence of pad wear, so it must be treated as a safety defect.
90-minute sequence:
- Pressure-test the cooling system and inspect the cap; check for external leaks and coolant in the oil (10 min).
- Check fan operation and belt tension; test the thermostat in hot water if accessible (15 min).
- Pressure-bleed the brake system and inspect all four corners for leaks; measure run-out and lining condition (30 min).
- Replace brake fluid if dark or contaminated, then re-bleed and perform a stationary pedal-hold test (20 min).
- Road-test at low speed in the yard; recheck temperature and pedal feel (15 min).
If the pressure test shows a head-gasket leak, stop: the bus is not roadworthy within the available time.
2. Should the bus be released on the scheduled departure, replaced by the spare, or repaired and sent late? Justify the decision using the workshop findings.
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The bus should not be released on the scheduled departure. The decisive finding is the spongy pedal after a descent, combined with rear linings at 2 mm and a very hot drum. That points to brake fade with possible fluid boil or air in the system. On a ghat route, a second descent could produce total hydraulic failure. The overheating symptom adds a second independent risk: a coolant leak or pressure-cap failure can escalate quickly under load.
The spare bus should operate the scheduled departure if its fitness, brakes and cooling system are checked and a driver is available. The defective bus should be moved to the workshop, not dispatched late with passengers. If no spare is available, the departure should be delayed or cancelled and passengers transferred; a 90-minute delay is preferable to a brake failure on a gradient.
The decision rule is simple: a brake defect that affects pedal travel is a stop-use defect, not a monitor-and-report defect. Schedule pressure cannot override it.
3. After the bus returns from repair, what permanent repairs and records should the workshop complete before it re-enters service?
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Permanent repair should address causes, not symptoms:
- Cooling system: replace the pressure cap and thermostat if tests fail; repair or replace the leaking hose, water pump or radiator; pressure-test again and confirm the fan cut-in temperature. If a head-gasket breach is found, the vehicle stays off road until the head is checked and the job is completed.
- Brakes: replace rear linings and any worn or leaking wheel cylinder; machine or replace scored drums; flush the entire hydraulic system with fresh fluid; bleed in the correct sequence; adjust the parking brake; carry out a loaded road test and a brake-performance check.
- Records: enter the complaint, diagnosis, parts replaced, fluid type, technician name, date and road-test result in the vehicle history card and workshop register. Raise a defect and rectification entry so the next inspection can verify the repair. The depot should also review whether the previous scheduled maintenance missed the brake-fluid change interval or the cooling-system pressure test.