Antilock Brake System for Early Subaru Part 3:
Each time the ignition switch is turned from Off to On, the self-diagnostic function begins to look for fault conditions. These self tests occur in two stages: one at key On and another as soon as the vehicle has been driven at a speed of 6 miles per hour or more for 20 seconds.
Assume that a particular vehicle has no ABS codes stored. If the ECU detects a fault condition, it goes into fail-safe mode and turns on the ABS warning lamp. In fail-safe mode, the ABS system is essentially shut down and completely passive, while the brake system operates conventionally. The system remains in fail-safe mode until the ignition switch is turned Off.
The next time the ignition switch is turned On, the ECU again initializes and looks for fault conditions, first at key On and again after 20 seconds at 6 miles per hour or more. If the fault condition is still there, the ECU simply returns to fail-safe mode.
Note: Even though the ECU can store up to three codes, this can happen only if at least two of the fault conditions are intermittent.
This means the ECU stays in fail-safe mode as long as the first fault condition remains in effect, and will neither detect nor store in memory any additional fault conditions. If the first fault condition clears, the ECU again exercises active ABS control at the next key On.
If a second fault condition occurs, the ECU will then store the second code. To get a third code into memory, the second fault condition must also be intermittent. When it clears, the ECU can come out of failsafe mode at the next ignition “On-Off” cycle. At that point, the ECU can detect, then store the third code. If the ECU detects another fault condition once three codes are in memory, the newly arriving code displaces the oldest stored code. The newest code takes the first place in line for display.
To clear the memory of all stored codes, alternately disconnect and reconnect the jumper between ground and terminal L in the ABS check connector. Do this three times in the span of about 12 seconds. Tip: At the moment the ECU clears its memory, you can hear the relays in the HCU cycle once.