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What Causes a Wheel to Lock Up? This is serious

A locking road wheel is a serious concern, and you are correct to deal with this immediately. A locked wheel renders the car unstable; if it’s a front wheel, your car’s steering is also compromised, and if it’s a rear, it could cause the tail to spin out. I’m a mechanic, and very shortly, we’ll get this figured out. 

A vehicle road wheel commonly locks up because the brake rotors and pads are contaminated. However, there are other possible causes, which include:

  • Brake caliper issue
  • Collapsed brake line
  • Faulty ABS controller
  • Faulty wheel speed sensor
  • Faulty proportional valve

In this post, you’ll learn why your vehicle’s road wheel locks up and what you can do to fix it.

BMW wheel with Michelin tire

Index

Contaminated Rotors & Pads

Rotors and pads are the business end of stopping your vehicle, and even a small issue will make itself known under braking. 

Common brake contamination symptoms include:

  • Unusual noises like squealing or grinding under braking
  • Pulling to one side under braking
  • Smoke from brakes
  • Excessive wheel heat
  • Aggressive brake bite
  • Excessive brake dust on one wheel

Brake contamination usually occurs from road grime like oil and grease etc. Rotors and pads are good at self-cleaning; every brake application removes a micro surface layer from the pad’s surface.

However, in some cases of contamination, especially where the driver uses the brakes sparingly (a ton of highway driving) or uses feather-light braking force, the contamination can build up and penetrate both the pads and the rotors.

Pad contaminates

Brake pads are made from a mixture of metal materials bounded into a composite. While pads look pretty solid, they will absorb contaminants such as oil or grease, etc. If the contaminants penetrate the pads, it’s best to replace them. 

Rotor contaminates

Rotors are metal; they suffer from surface rust; day-to-day driving is sufficient to clean the surface rust away. However, rotor corrosion is problematic in vehicles that sit idle for extended periods, especially in the colder salted roads of the northern states.

The surface rust can cause pitting in the rotor surface. The pitting is a form of contamination, and any type of contamination on the surface of the rotors or pads will affect the braking quality and can easily cause wheel grab or lock up.

How to diagnose – A brake inspection is needed. Remove the wheel and the brake caliper on the wheel that’s affected. Examine the pads and rotor surfaces for any signs of corrosion or oil. Check the brake pad is undamaged and that the back plate is corrosion free.

How to fix – If in doubt, go ahead and clean or replace the components.

It’s possible to clean both if the contamination isn’t bad. 

To clean brake pads, lay a flat sheet of rough grit paper on a flat surface and run the pad across the grit several times to remove a layer of surface material. Use a mask, and best to perform this outdoors, pads no longer contain asbestos, but the dust is still harmful. Spray them with brake cleaner and use the anti-squeal paste on the pad arms.

The rotors may be sanded with grit paper also and a brake cleaner to remove grease and oil. Pitting, however, will require a date with a lade. Turning the rotors removes a layer of metal from the surface and will work so long as your rotors are not below the min spec for turning. 

Brake Caliper Issue

Three main brake caliper issues can lead to a wheel lock-up, they are:

  1. Brake caliper piston partially frozen
  2. Brake caliper slide pins frozen
  3. Brake caliper carrier rail corrosion

Let’s take each one in turn

1 Brake caliper piston partially frozen

A brake caliper, as you know, houses the piston, and the piston is responsible for forcing the pads against the spinning rotor to slow and stop your vehicle. As the brake pedal is released, the piston retracts slightly and allows the pads to move away from the rotor.

However, when a caliper’s piston becomes partially frozen, it only retracts very slightly when the brake pedal is released. On the next brake application, the affected caliper’s piston has less distance to travel to engage the pads and rotor. This results in a more immediate wheel grab and often a locking up of the affected wheel.

How to diagnose – A brake inspection will be required. The process looks like this:

  • Open the fluid reservoir one turn
  • Remove the wheel and remove the caliper
  • Using channel locks or brake tools, push the piston home (into the caliper)
  • A partially frozen piston will put up a ton of resistance

You should know that a blocked Felix line, although rare, will prevent a piston retraction and should be eliminated before ordering calipers. You can check that quickly, and we cover exactly that in this post; check it out here.

How to fix – Replace the caliper, and it is advisable to replace both calipers on the same axle, i.e., both front or rear, at the same time.

Brake caliper slide pins frozen

Most modern passenger cars employ floating calipers. You’ll recognize floating calipers as they’ll have piston(s) on the inner side of the rotor only. Anyhow, floating calipers are designed to well… float, and they do so on two slide pins.

The problem is the pins are often neglected, meaning they like to be relubricated on every brake job but sometimes don’t. It’s common for slide pins to seize or partially seize. This results in brake grab and wheel lock-up.

How to diagnose – A brake inspection will be required. The process looks like this:

  • Open the fluid reservoir one turn
  • Remove the wheel and remove the caliper
  • Manulipiate the slide pins by moving them latterly
  • A problem pin will be difficult or impossible to move

How to fix – Remove the pins and lubricate using silicone grease and replace the rubber moisture and dust booth if damaged. If they are frozen solid, you’ll need to replace them and the caliper carrier.

Brake caliper carrier rail corrosion

The caliper carrier rail has two important jobs; it provides a stable perch for the caliper and also holds the brake pads in place. The carrier employs a rail upon which the pads sit; the rail serves as a guide allowing the pads to move latterly with the floating caliper.

Problems arise when corrosion builds o the carrier rail and prevents the pads from moving away from the rotor as the brake pedal is released.

How to diagnose – A brake inspection will be required. The process looks like this:

  • Open the fluid reservoir one turn
  • Remove the wheel and remove the caliper
  • Remove the brake pads
  • Inspect the rail for corrosion

How to fix – Using a wire brush, clean the rails. Many carriers employ a pad spring or clip. Check the springs are in good shape and replace them as necessary. Apply copper grease or anti-squeal paste to carrier rails and pad lugs.

Collapsed Brake Line

As flexible (known as Flexi lines) brake lines age, they degrade, and sometimes from the inside. The flexi lines are twin walled for safety, and it’s not uncommon for a strip of internal wall to come away and act as a valve, meaning it blocks the return flow of brake fluid from the caliper to the reservoir as the brake pedal is released.

This, as you can imagine, holds pressure in the line, almost like it’s precharged and primed. Applying the brakes causes the affected caliper to bite faster and harder than the others.

How to diagnose – A brake inspection will be required, and note we’ll need to open the caliper bleed screw, but you should know when you do, so you’ll need to bleed the brake system. I’ve covered that previously and you’ll find that here.

The process looks like this:

  • Remove the wheel and remove the caliper
  • Open the bleed screw
  • Use channel locks or brake tool to push back piston

Two outcomes are likely:

  1. The piston retracts, and fluid shoots from the bleed screw – Which indicates your flexi line doesn’t suffer from internal blockage.
  2. The piston doesn’t retract, and no fluid from the bleed screw – This indicates you may have an issue with the flexi line, but it could also mean you have a frozen caliper. To know for sure, remove the flexi line from the caliper end and use channel locks to attempt to retract the piston again. If the piston now retracts, your flexi line is blocked.

How to fix – Replace all Flexi lines, I know it’s a pain in the hiney, but if you only do one, you know you’ll be back before doing another, and so on.

Faulty ABS Controller

All modern vehicles are fitted with ABS (Anti Lock Braking), and as you know, the whole point of ABS is to prevent wheel lock-up under braking. As we have lock-up, we know something is a miss. ABS is a major safety component with its own dedicated integrated computer (Known as an ABS modulator or ABS controller). Even minor issues with the ABS system will cause the controller to generate and register a fault code, illuminating a dash warning light.

When the ABS goes down, you’ll also receive a fault or message to indicate traction control is offline also.

How to diagnose – Since your ABS system is a motor with valves and an integrated computer makes sense to integrate it and read its computer’s fault codes. Without a fault code reader, your testing is limited; you can still check for wiring integrity, loose, water damage, or corroded wiring are all common in older or accident-damaged vehicles.

How to fix – Faulty wiring may be repaired without any special tools, but best to use solder and proper water-tight wiring shields.

If your ABS modulator is faulty, you may need to visit the dealer. Used ABS controllers, in most cases, can’t be used as the controller must be recognized by all the other controllers in your vehicle, and as such, they are all coded to each other from the factory.

Fitting a used model and bringing it to a dealer won’t work either, as controllers can’t be re-coded.

The best solution is to remove your ABS controller and have it repaired; that way, it retains its coding and it’s plug-and-play when repaired. You will need to bleed the whole system, however.

Faulty Wheel Speed Sensor

Your ABS controller relies heavily on information supplied by your car’s wheel-mounted speed sensors (WSS). The sensors work in tandem with a reluctor ring coupled to the wheel, which signals wheel speed to the ABS controller directly.

The ABS controller compares the speed information from each sensor which allows it to recognize lock-up events and kick the modulator into action. The ABS system uses information from an onboard Yaw sensor to allow for differences in wheel speeds on turns.

Wheel speed sensors live in a hostile environment; heat, vibration, road, and debris impact all take their toll.

How to diagnose – A faulty WSS will set a code in the ABS controller. And so the place to start is controller interrogation with a Scan tool. That said, here are a few visual checks that very often yield gold.

Check for:

  • Loose wiring
  • Corroded wiring
  • Broken sensor
  • Corroded sensors or sensor mounting plate
  • Corroded damaged reluctor ring
  • Loosed worn out wheel bearing

How to fix – Make repairs as necessary. WSS speed sensors do not require coding; it’s a plug-and-play deal.

You may also find the following posts helpful:

New brakes smoking on one side

No brakes after changing pads

Rotors won’t come off