ABS

An Engineer Rambles
Recently my 1990 Corvette began displaying a “Service ABS” light on the DIC (Driver Information Centre). That is the panel just above the radio, which displays various messages in red or yellow lights.

By now all C4 Corvette owners would be aware of the standard diagnostic routine to adopt when an error light appears.. Bend up a paper clip and use it to ground the terminal on the ALDL connector. In this instance, one uses the “H” terminal, which is the ABS (Anti-Lock Brake System) diagnostic. If you don’t know what I am talking about, you have not been paying attention and you need to re-read my earlier articles.

The result of my test was a Code 76, which means “Lateral Accelerometer fault - signal out of range or incorrect”. “”What is a lateral accelerometer?” I hear someone in the back row asking.

The ABS consists of the following elements:-
Modulator Valve and relays. Located under the left rear storage compartment
Control Module. The box full of electrical circuits. Also in storage compartment
Lateral Accelerometer. Located behind the instrument pane, under the radio.
Wheel Sensors and toothed rings. One at each wheel.
Wiring harness and module relay.
Fuse
Anti-lock Warning Lamp
Recently my 1990 Corvette began displaying a “Service ABS” light on the DIC (Driver Information Centre). That is the panel just above the radio, which displays various messages in red or yellow lights.

By now all C4 Corvette owners would be aware of the standard diagnostic routine to adopt when an error light appears.. Bend up a paper clip and use it to ground the terminal on the ALDL connector. In this instance, one uses the “H” terminal, which is the ABS (Anti-Lock Brake System) diagnostic. If you don’t know what I am talking about, you have not been paying attention and you need to re-read my earlier articles.

The result of my test was a Code 76, which means “Lateral Accelerometer fault - signal out of range or incorrect”. “”What is a lateral accelerometer?” I hear someone in the back row asking.

The ABS consists of the following elements:-

  1. Modulator Valve and relays. Located under the left rear storage compartment
  2. Control Module. The box full of electrical circuits. Also in storage compartment
  3. Lateral Accelerometer. Located behind the instrument pane, under the radio.
  4. Wheel Sensors and toothed rings. One at each wheel.
  5. Wiring harness and module relay.
  6. Fuse
  7. Anti-lock Warning Lamp

The ABS functions by monitoring the speed of each wheel using the toothed rings and sensors. Basically the electronic control module takes as input, the frequencies of pulses generated as each tooth passes by its stationary inductive sensor. In the case of the Corvette system, the two rear wheels are treated as a pair, taking the lower wheel speed of the two rear wheels as the input for processing. This is known as a three channel system. In addition, the lateral accelerometer provides an output voltage proportional to the vehicle’s lateral acceleration.

The control module uses the information gathered from the wheel sensors and the lateral accelerometer to control the braking action of the wheels to prevent wheel lock. When the control module see that a wheel is about to lock (ie it is turning slower than the others), it calculates what action is required, adjusts the result for lateral acceleration (ie how fast the car is cornering) and then sends a signal to the appropriate solenoid. The solenoid moves an hydraulic valve, which cuts off the circuit from the driver’s brake pedal and prevents any increase in brake hydraulic pressure coming from the drivers foot. If this action is insufficient to restore traction, the control module sends a further signal which moves the valve even further, allowing the brake line drain fluid back to an accumulator, thus reducing the pressure to that brake. When the wheel stops slipping (ie attains the same speed as the other wheels), the valve is released, allowing normal pressure between foot pedal and brake. This control process is repeated at a rate of fifteen control cycles per second. The episode is completed by the operation of a pump which evacuates the accumulator chamber and returns fluid back to the master cylinder reservoir.

The ABS goes through a self diagnostic routine the first time the vehicle passes through 5 kph after turning on the ignition. You have probably heard the pumps operating during this BITE test. (Built In Test Equipment operation) The lateral accelerometer will give a voltage of about 1.2V at 1g right, 4.2V at about 1g left and about 2.6V when the car is running straight ahead. If the control module reads in excess of 0.6g either way for more than 120 seconds, an error is indicated. Either the lateral accelerometer is faulty, or the car is lying on its side.

As near as I could determine, the car had not fallen over, so a replacement lateral accelerometer was called for. A mere five hours of contortions and a few phone calls to fellow club members and I had the offending unit out from behind the instrument panel. I sourced a replacement part from Vette Connections in Melbourne. It only took thirty minutes to pull the instrument panel apart a second time and install the replacement unit.

Col