Descent from the hills: a spongy pedal, a hot smell and an ABS lamp
You are driving a loaded 2016 five-door hatchback down a long mountain descent of about 6 km: five adults plus luggage, outside temperature 31 C. The sign at the top sets 80 km/h. Your automatic gearbox is in D and you have been holding the car back with the footbrake rather than by downshifting, because traffic keeps nudging you back up to about 70 km/h. Halfway down the pedal starts to feel long and spongy, and there is a hot, bitter smell. The ABS warning lamp has been lit since you started the car this morning; the owner said it had been on 'for months'. Before setting off you noticed the right rear tyre at 26 psi against the 33 psi recommended, the front tyres at about 4 mm of tread, and the car pulling slightly right under braking. No other warning lights.
1. What is happening to the brakes, and what exactly do you do in the next minute?
Read the worked answer
Long, spongy pedal travel plus a hot, bitter smell plus continuous light braking on a descent under a full load is the classic picture of brake fade: the discs and pads are overheated, and the fluid may be approaching or past boiling point, which produces gas in the lines and that soft, long pedal.
Next minute:
- Select a low gear now - second, or a manual override - and let engine braking carry the car. On an automatic, move out of D into a numbered low range rather than tapping the brakes.
- Apply the brakes in short, firm applications with cooling gaps between them, not one long continuous drag. Continuous light pressure is what generated the heat in the first place.
- Look for a pull-off or lay-by and stop. Let everything cool with the car in gear and the handbrake on.
- Put something behind the wheels if the gradient is steep.
Do not: coast in neutral to 'save the brakes' - you lose engine braking entirely; pour water on hot discs, which can crack or warp them; or continue the descent hoping it recovers. Fade gets worse, not better, as the hill continues.
2. The ABS lamp is on. What does that change about how you brake and how you finish this descent?
Read the worked answer
An ABS warning lamp means the anti-lock function may be disabled or degraded. The base hydraulic brakes still work - the pedal should still stop the car - but under hard braking the wheels can lock. Once a front wheel locks you lose steering, which is exactly the input you need most when a vehicle stops suddenly in front of you on a bend.
Practical consequences for the rest of this descent:
- More distance, lower speed. Without anti-lock you need a larger margin for everything, and on a hot, fading system the margin should be larger again.
- Brake earlier and more gently, in a straight line, so that any lock-up happens at low demand rather than in a panic.
- Expect the unexpected pull. The car already pulls right under braking; with a locked wheel that pull becomes a swerve.
- Finish early. Take the first safe exit or lay-by, stop, and get the system read. An ABS lamp is usually a wheel-speed sensor, but you cannot diagnose that at the wheel, and a warning lamp is not a cosmetic item to be ignored for months.
If the pedal ever goes to the floor, you still have engine braking and the handbrake acting on the rear wheels - use them progressively, not as an on/off switch.
3. How do the tyre pressures, the tread and the load interact with the brake problem?
Read the worked answer
They compound it, and each one matters for a different reason.
Pressure. 26 psi against a recommended 33 psi on one rear corner, at 31 C with five people aboard, means that tyre is flexing far more than it should. It runs hotter, rolls more, and grips less predictably than the other three. The car's balance under braking shifts, which is consistent with the pull to the right you already noticed. With a full load the handbook usually specifies higher 'loaded' pressures - 33 psi may itself be the light-load figure, so the shortfall is probably bigger than it looks.
Tread. 4 mm is well above any legal minimum but wet and hot grip already degrades noticeably below about 3 mm, and 4 mm is not a comfortable place to be on a hot descent with fading brakes.
Load. Five adults and luggage add several hundred kilograms. Braking energy rises with mass, so the same descent generates more heat in the same discs. That is not a reason to brake more; it is a reason to descend slower, in a lower gear, with longer gaps.
The correct answer to all three is not a set of small compensations - it is to check pressures cold, correct the loaded pressures, and treat the descent as a low-gear descent from the start.