Gearbox Whine in Every Gear but Fourth
A rear-wheel-drive light truck with a five-speed manual gearbox and a live rear axle develops a whine that rises and falls with road speed. The driver reports it in 1st, 2nd, 3rd and 5th but not in 4th, and says it is present even when the truck is standing still in neutral with the clutch pedal released; it stops as soon as the pedal is pressed. On the road it is also faintly audible while coasting in neutral at 50 km/h.
Data: 4th gear 1.00:1 (direct), 5th 0.82:1, final drive 4.10:1, tyre rolling circumference 2.055 m. Gearbox oil level is correct but the oil is dark, and there is a little grey sludge on the drain plug magnet; the lubricant was last changed 60,000 km ago. The owner needs the truck for deliveries and asks whether he can keep using it for another month.
1. From the symptoms, identify the most likely failed components and justify each conclusion by reference to which shafts are rotating or loaded in each condition.
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Standing still, neutral, clutch up - whine present. The mainshaft and output are stationary, the final drive and wheel bearings are stationary. What is turning is the input shaft and the countershaft, driven through the closed clutch. So the noise is in the input shaft bearing or the countershaft bearings - not the output bearing, not the final drive, not a wheel bearing.
Clutch pressed - noise stops. The input shaft stops turning, confirming the same conclusion.
Road test, every gear except 4th. Fourth is direct: the mainshaft is locked to the input shaft and the drive passes straight through, so the countershaft gear train is no longer transmitting load. The whine disappearing in direct is the classic sign that the trouble is in the countershaft bearings or the countershaft gear mesh. A mainshaft or output bearing would still be noisy in 4th, and would not be heard at all with the truck stationary - both are eliminated.
Grey sludge on the magnet after 60,000 km is bearing and gear material, not just normal run-in paste. That supports bearing breakdown rather than an oil-quality complaint alone.
2. The driver says the whine is worst at about 100 km/h in fifth at 3,000 rpm. Check whether that is consistent with the gearing, and explain why the truck should not be driven further.
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Speed check at 3,000 rpm in 5th:
- Wheel speed = 3,000 ÷ (0.82 × 4.10) = 3,000 ÷ 3.362 = 892 rpm.
- Distance per minute = 892 × 2.055 = 1,834 m/min.
- Road speed = 1.834 km/min × 60 = about 110 km/h.
So at a true 100 km/h the engine is turning about 2,720 rpm - the driver's "3,000 rpm at 100 km/h" is consistent within speedometer error, and the gearing figures are not in doubt. The noise rising with road speed confirms a shaft-speed bearing fault rather than an engine-speed one.
Why it must not be driven further:
- The bearing is already shedding material into the oil, which is being pumped through the gear mesh and the synchros, damaging parts that are currently sound.
- As the bearing collapses the shaft loses its centre distance; the gears run out of mesh, then teeth break.
- The likely end point is a locked gearbox or a cracked casing at road speed with no drive - a recovery, a replacement gearbox and possibly a road-safety incident.
The repair is cheapest now, with the box out and bearings and seals replaced.
3. The owner asks whether changing to a thicker oil will get him through the month. Answer him.
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No - and say so plainly rather than taking the money.
A thicker oil can quieten a worn bearing for a short while by filling clearance, but it repairs nothing. It also carries the metal already being shed rather than letting it settle, so the debris keeps circulating through the gears and synchros; cold shift quality gets worse and the layshaft gets less oil when cold. The bearing will still fail, and the failure will now come without warning.
What I would offer instead:
- Book the gearbox out and strip it now: replace all the shaft bearings and seals as a set, inspect the countershaft and 5th gear teeth and the synchros, and flush the casing.
- If the truck must work until the slot, it can be used gently: check the oil level daily, keep road speed down, avoid heavy towing, and stop immediately if the whine turns into a rumble, if the lever jumps out of gear, or if the noise changes pitch suddenly.
- Point out the cost comparison: bearings and seals against a replacement gearbox plus recovery plus lost deliveries. Coming in now is the cheap option.