This article will show you how to diagnose and repair common faults with your Anti Lock Braking System (ABS), based on my own experience repairing my Mk2 MX5 ABS.
Soon after taking possession of my much previously unloved Mk2, my ABS light refused to extinguish. At first this was intermittent, with the ABS indicator light appearing to stay on when the car was cold, but go off if restarted once warm. This soon moved onto a constantly illuminated ABS warning light, within less than a hundred miles.
With the cars MOT around the corner, the ABS system will need to be repaired in order to pass. If you have been advised to simply remove the bulb from the dashboard I would use caution. The ABS is a valuable safety feature that you should keep in good running order, and secondly, the latest testing regulations state that; All parts of the braking system must be in good working order, including ABS, and that if ABS is removed ALL parts must be removed apart from where integral to another part of the car. It is pretty clear that simply removing the bulb will no longer suffice with regards to testing, but i guess a lot will depend upon how vigilant the tester is.
When trying to locate the source problem for an ABS fault, the first task is to ask the ABS ECU for the error code (or codes) that caused it to illuminate the indicator light. You can do this yourself without any special equipment, all you need is a short piece of wire or a paperclip. Check out my article on Reading MX5 ABS Fault Codes for more information.
How does the ABS system work?
The Anti-Lock Braking System on an MX5 (Mk1&2) consists of four Wheel Speed Sensors located near the hub of each wheel, an ABS Electronic Control Unit located under the carpet in the cabin (or attached to the ABS Pump in later models), and an ABS Pump & Valve Unit located under the bonnet close to the fire wall.Based on information provided by the Wheel Speed Sensors, The ABS ECU will briefly release, and then re-apply, the individual brake located on any wheel it detects is about to lock up. This operation can be felt as a pulsating in your brake pedal.
In most cases the cause of the fault will be with one of the Wheel Speed Sensors. With faults located in the ABS ECU or ABS Pump, it would probably be more cost effective to simply replace these entire units with used or reconditioned parts. In the case of my own ABS fault, the faults codes indicated that my front right sensor was open circuit, with a second code indicating my rear left sensor disagreed with other sensors. For now, this second code I am going to put down to the open circuit detected on the front right.
Checking the ABS Wheel Speed Sensors
Before we start throwing costly parts at the problem, it is a good idea to get some form of verification of a fault, outside of that reported by the ABS ECU.A meter is used to measure the resistance and continuity of both the front Wheel Speed Sensors, the suspect sensor to check it is indeed open circuit, and the good sensor so that we know what a "good" resistance is. Both front sensors have a plug located under the bonnet and above the inner wing. Disconnect these plugs and measure the resistance of the sensor via the plug pins at the sensor end. The pictures below show these connectors on the left and right sides of the engine bay.
An acceptable resistance here would be 1-2k ohms. The good sensor provided a 1.6k ohm reading, which is within range. The suspect front right sensor was open circuit and confirms that this Wheel Speed Sensor is bad and needs to be repaired/replaced. Should this sensor have given a good reading, I would have moved onto checking the wiring from the socket back to the pump, before looking into problems related to the pump itself.
Replacing an ABS Wheel Speed Sensor
A new sensor is our only realistic option here, used sensors are almost impossible to find as they almost always break when attempting removal. This was certainly my experience, having to smash up the old sensor and remove it fragment by fragment! New sensors can be costly but please shop around! With an average price of around £60 I was happy to locate one for a mere £22 online. I will update to let you know if it lasted until the MOT, or if it was cheap for a reason! (Update: Still working fine after 2 years when i sold the car).The ABS Sensors on the MX5 are a Hall Effect & Reluctor Ring type configuration whereby a ring containing magnetic teeth is passed in front of a small sensor, inducing a small AC current. The ABS ECU translates this current into wheel speed. The Reluctor Ring is located behind the wheel bearing assembly and the sensor itself passes through a hole in the upright where it is fixed by a small bolt, as shown in the picture below.
New sensor in hand, I gathered my tools and jacked up the front right of the car, ready to repair my ABS. Removed the wheel and all the brake components to give myself some room to work. See Replacing Brake Pads and Rotors for information on removing brake disks, pads and caliper.
While attempting to remove the sensor mounting stud the head broke straight off, as did another of the wiring bracket(s) bolts, an eventuality you should be prepared for. The main fixing stud needed to be drilled out.
Using a screwdriver as a wedge lodged between the sensor and wiring bracket, a few stern taps released the back end of the sensor, but only the back end, the remaining parts of the sensor were still present in the upright as the sensor had snapped. Using a screwdriver and hammer to dig around I was able to free the central metal core, and once that was free, removal of the remaining plastic and copper wire was far easier.
Before the new sensor would fit it was necessary to use sandpaper to clear out rust from the sensor mounting hole in the upright until the new part could be inserted, ensuring it had first passed through the large hole on the wiring mounting bracket attached to the sensor lead. This can now be passed through the hole and secured with the mounting stud. Rotate the hub after fitting to ensure the correct air gap (1mm) and that no contact is made between the ABS sensor and the reluctor ring.
Taking care to follow the original route of the old sensor wiring, the cable is attached to the suspension with two brackets, and another attaches to the inside wing, where it is fed through the car body into the engine bay, and plugged into the wiring harness. A quick test at this point reveals the ABS light extinguishes correctly after turning the ignition key to the on position.
Refit the braking components, re-attach the road wheel, and lower the car back onto the grounded. A quick road test verified that replacing the front right Wheel Speed Sensor had repaired my ABS system, with no further fault codes logged.
Did you find this article on ABS Repair useful? Do you have a great tip to help when replacing an ABS Wheel Speed Sensor? Leave a comment below.