April 2019

A n anti-lock braking system was developed for use on aeroplanes as far back as 1929. However, it wasn’t until the 1950s that a workable system was developed for a road vehicle. This was a prototype fitted to a Royal Enfield Meteor Super. However, with a lack of foresight typical of the British bike industry, bosses thought the consumer would continue to do just fine with standard drums. The first production car (albeit in very limited numbers) fitted with ABS was the Jensen FF – which was also the first performance car with four- wheel drive. However, it wasn’t until the introduction of electronic control in the 1980s that ABS became a popular option. ABS consists of the following components: Speed Sensors Pump Vales Electronic controller Anti-lock brakes are now standard equipment on every new car sold in the UK. The systems are generally trouble-free, but when problems do occur, most ABS system faults are related to wheel speed sensor issues. An important point to remember is that there are three main types of diagnostic trouble codes (DTC) related to wheel speed sensors. These are: Open-short Range performance intermittent Invalid (no signal) There are two main types of wheel speed sensors. The first is known as ‘active’ or ‘hall effect’ and the second is called ‘passive’ or ‘inductive’. Here are some basic tips and things to remember when checking sensors and their circuits. 54 AFTERMARKET APRIL 2019 TECHNICAL www.aftermarketonline.net TOP TECHNICIAN FLASHBACK: ANTI-LOCK BRAKES Below: There are two types of ABS sensor: Passive and inductive Back in 2011, ABS expert and Top Technician semi-finalist, John French looked at one of the most common reasons for system failure Sensors First of all a few things about hall effect sensors. Remember these are powered sensors and they normally have two wires for power and a signal wire and ground through the hub. However, they can also have three wires and have a specific ground wire. These can be a little tricky to test because generally the first thing the control unit normally does when it detects a circuit malfunction is raise a DTC open-short to switch off the power supply to the sensor, to protect itself and ignore any wrong speed signal. Obviously when this happens, you cannot check the waveform with a scope as the power supply has been cut. Also in this situation, hall sensors do not generally have a set resistance so you cannot check accurately using a multimeter (although that test may highlight a dead short or open circuit in the sensor.) In this instance, all you can do is check the wiring in-between the sensor and control unit by disconnecting the sensor and control unit and checking for open and short on all wires. Scope it out If this is ok, then most likely it is a sensor fault (and of course you can always substitute the sensor for a known good unit). If you have a DTC indicating ‘range performance intermittent’ you may still have a power supply. Remember when checking with a scope, you are looking for a nice square wave or digital signal normally ranging between 0.5 to 1.5V. If the waveform is not good, it could mean a faulty sensor but remember to check the gap to the tone wheel. Also the wheel is not split or damaged, as this can also cause this trouble code to appear. Check the wheel bearing as this can cause similar issues – especially if it has been installed the wrong way around! Remember, if the control unit has not switched off the power supply it thinks the circuit is good. Also remember after repairing