What a hub unit is now
Most cars built in the last twenty years use a bolt-on hub assembly: the bearing, the hub, the wheel flange and the ABS trigger, all in one sealed unit. Nothing inside it is serviceable and nothing is meant to be — no packing, no races to press, no adjustment.
That is genuinely better than what it replaced. The catch is a component nobody used to have to think about.
The encoder is a magnet
On older systems the ABS trigger was a toothed steel ring and the sensor was a coil that produced a voltage as the teeth passed. You could see the teeth, and if one was damaged you could see that too.
On most modern hubs it is a magnetic encoder: a ring of alternating magnetic poles moulded into the elastomer of the bearing's inboard seal, read by an active sensor. It looks like nothing — a plain dark seal face. There are no teeth to inspect and no damage to see.
Which means every way of breaking it is invisible.
The five ways a new hub arrives faulty and was not
1. Fitted the wrong way round. The encoder must face the sensor, which is inboard. On designs where the hub is symmetrical enough to go in backwards, this is an instant ABS light and a straightforward fix.
2. Demagnetised. A magnetic pickup tool, a magnetised extension bar, a magnetic tray. Keep the hub away from all of them, and keep the encoder face covered until it goes in.
3. Contaminated. Grinding sparks and wire-brush swarf near an open hub. The particles stick to the pole faces and stay there, and the sensor reads them.
4. Impact damage to the bearing. Hammering a seized shaft out through the hub brinells the races. The bearing is silent on the day and rumbles two months later, which reads as a defective part.
5. Torqued wrong. The axle nut sets the preload on many designs. Too loose and there is play; too tight and the preload is destroyed. Either way the bearing is short-lived, and neither is visible on inspection.
Doing it without any of that
- Read the axle nut spec before you start, and have a new nut. On staked or deform-lock nuts it is single-use, and reusing one is not a shortcut worth taking.
- Loosen the axle nut with the car on the ground and the brake applied — not with the wheel in the air.
- Clean the knuckle bore back to bare metal. A hub sitting on a rust ridge is a hub sitting crooked, and crooked is runout.
- Keep the new hub in its packaging until the moment it goes in, encoder side covered.
- Do not press, hammer or lever on the outer flange. If the assembly is tight, use the correct puller against the correct surface.
- Clean the sensor bore and the sensor tip. A film of rust in the bore holds the sensor a fraction of a millimetre out and the air gap is not adjustable.
- Torque the axle nut with a torque wrench to the exact spec. Stake it if the design calls for it.
- Clear the codes and road test above the speed at which ABS self-tests — a stationary code clear proves nothing.
When the sensor is the part and not the hub
Plenty of ABS faults are not the hub at all.
- Harness fatigue at the knuckle. The wire flexes every time the suspension moves and every time the wheel steers. Conductors break inside insulation that looks perfect. Wiggle-test it with the meter connected.
- Corroded connector. Especially where the connector sits low and forward, in the spray from the front wheel.
- A sensor tip that was never seated. Rust in the bore stops it going fully home and the gap is too large.
Read the live wheel-speed data from all four wheels while the car rolls, rather than reading a fault code and buying the part it names. A wheel that reads correctly at walking pace and drops out at speed is a signal-strength problem — encoder or air gap. A wheel that reads nothing at all is more often a circuit.
Fitting in pairs
Not required, and not a scare tactic either way. The honest position is that hubs fail from mileage, water ingress and impact, and the second one on the same axle has had exactly the same mileage and the same water.
If the car is high-mileage and the job requires the same disassembly on both sides, doing both is sensible. If one side failed early from a pothole, it is one hub and a wheel alignment.
Questions people ask
- Which way round does the encoder face?
- Toward the sensor, which is inboard on almost every design. The encoder side is the dark, matte, slightly rubbery-looking seal face — visibly different from the metal seal on the other side. If a hub can be fitted both ways round, fitting it backwards gives an immediate ABS light.
- Can I reuse the old wheel speed sensor?
- If the hub does not include one, yes — inspect the tip and the connector first. If the new hub comes with a sensor integrated, there is nothing to reuse. Where the sensor is separate and the harness has been flexing at the knuckle for a decade, replace it anyway; a broken conductor inside intact insulation is a fault nobody enjoys chasing.
- The ABS light came on right after fitting a new hub. What did I do?
- In order of likelihood: the encoder faced the wrong way, the encoder was demagnetised by a magnetic tool or tray, swarf is stuck to the encoder face, or the sensor tip was not seated and cleaned in its bore. All four are workshop-floor causes and none of them are a faulty part.