JCB 3CX / 4CX Rear Axle Overheating: Causes and What to Check

When a hot rear axle is normal

The manual states it plainly: the brakes generate a lot of heat in operation, and an axle casing that is hot to the touch is normal (Section G, page G-32). The axle oil both lubricates the components and cools the brake plates, so an axle with oil-immersed brakes will always run warmer than a front axle with no brakes. The brake design and all the test procedures are covered in detail in the article JCB 3CX / 4CX brakes.

These are the signs that should worry you:

• the axle is noticeably hotter than usual after just a short run with no braking;
• the machine accelerates poorly and rolls as though the brakes are on;
• one side of the axle runs hotter than the other;
• the oil is dark and smells burnt, or leaks from the seals or the breather;
• the brakes have become noisy and lose effectiveness when hot.

Cause 1: brakes not releasing

If the brake plates stay clamped, the axle heats up continuously while the machine is moving. The manual gives the following causes for this fault (page G-34):

• broken or weak pedal return spring, so the pedal does not return fully to its stop;
• incorrect pedal free play, so the pushrod keeps pressing on the master cylinder piston;
• blocked vent hole in the reservoir cap, so the fluid cannot return to the brake fluid reservoir;
• master cylinder plunger sticking in its bore;
• annular brake piston sticking in the axle. A common cause is the wrong fluid, which makes the piston seals swell. A serviceable piston should turn freely in its housing with the seals removed, and the seal should retract it by about 0.5 mm;
• brake pipes trapped or crushed;
• brake plates not sliding freely on their splines or pins; check them and renew if necessary on both sides of the axle.

Start with the simple checks: make sure the pedals return fully to their stops and have free play, and clear the vent hole in the reservoir cap. Only then move on to the master cylinders and axle strip-down; dismantling the brake packs is described in the separate transmission service manual (publication 9803/8610). Master cylinders and pistons can be found in the brake cylinders and brake pistons categories.

Cause 2: the wrong brake fluid

The brake system on the JCB 3CX and 4CX does not use ordinary brake fluid but a mineral oil, JCB Light Hydraulic Fluid to ISO VG15; the system holds 1.4 litres (Section 3, page 3-12). The manual specifically warns that ordinary brake fluid must not be used: the wrong fluid damages the seals and can lead to brake failure (page G-37). Swollen annular piston seals are one of the main reasons why the brakes stop releasing and the axle overheats. If the reservoir has been topped up with whatever was to hand, the system must be flushed and all the seals renewed (pages G-33, G-34). JCB brake fluid is listed under JCB oils and fluids.

Cause 3: low, worn or incorrect axle oil

The axle oil lubricates the brakes and cools the brake plates, and wear debris from the plates gradually degrades it, which is why it must be changed strictly at the service intervals (page 3-71). For the rear axle of all-wheel-steer machines the manual specifies JCB Gear Oil HP Plus to API GL-4, suitable for oil-immersed brakes and limited slip differentials; the centre casing holds 16.5 litres plus 2.0 litres in each hub, while the rear axle on front-wheel-steer machines holds 21 litres (page 3-11). Worn or incorrect oil, or loss of oil, results in brake noise and overheating (page G-34).

Always check the level with the machine on level ground, otherwise the reading will be wrong: remove the filler plug, and the oil should be up to the bottom of the filler hole (pages 3-71, 3-73). The drain plug is magnetic, and the amount of metal particles on it gives an indication of wear. How to check the levels on every unit of the machine is explained in the article how to check the oil levels on a JCB 3CX.

Cause 4: blocked axle breather

During braking and long road runs the pressure inside the axle rises, and the breather vents it. If the breather is blocked, the manual warns of seal leaks and brake problems (page G-45). Keep the area around the breather clear. The breather is located on the side opposite the crown wheel, and its outlet should point outwards, towards the wheel.

Cause 5: park brake dragging

On machines with the Synchro Shuttle gearbox the park brake is a disc brake with its caliper on the rear axle drive head (page G-3). If the caliper is over-adjusted, the brake will not release fully, and the manual warns about this specifically (page G-47). The disc then heats up continuously and the heat passes into the axle. Check that the park brake lever lies horizontal in the off position and that the cable is not pulling on the caliper lever (Section D, page D-19). After an emergency stop using the park brake, renew both pads. On Powershift machines the park brake is in the gearbox and has nothing to do with rear axle heating. Park brake pads and discs are listed under brake pads and brake discs.

How to tell if the axle brake cylinders are leaking

If the annular brake piston seal is damaged, brake fluid escapes into the axle. The sign is a falling level in the reservoir with no external leaks; if fluid is lost overnight while the pedal does not sink to the floor under steady pressure, the manual points specifically to a damaged piston seal (page G-35). The test (page G-43) is done only with the axle cold: fill the piston chamber with JCB Light Hydraulic Fluid and use a hand pump with a 0–40 bar gauge to apply pressure, not exceeding 69 bar. A rapid pressure drop means the seal is badly damaged. Small cuts are detected from the fluid level in a vertical clear tube after half an hour. Always bleed the system after the test.

Checking sequence

1. After the same run, compare the temperature of the left and right sides of the axle and of the front axle.
2. Check the oil level and condition in the axle and the hubs, and make sure the oil is suitable for oil-immersed brakes.
3. Clean the breather and check for leaks past the seals.
4. On synchro shuttle machines, check that the park brake releases fully and the caliper is not over-adjusted.
5. Check pedal return, free play, springs, the reservoir cap vent hole and the type of brake fluid.
6. If the reservoir level drops with no external leaks, test the brake piston seals.
7. If the cause has still not been found, remove and check the master cylinders, then strip the brake packs.

Carry out all work on level ground with the wheels chocked and the battery disconnected; never go under the machine with the engine running. Axle parts are listed under rear axle. To choose brake plates, pistons or seals, send us the machine model, serial number and the details from the axle data plate.

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