GM Testing - Snap-on

Chapter 8 <strong>GM</strong>
<strong>Testing</strong> ABS Systems
ECU___BB 33599 BRAKE LIGHT SW___ON
** ABS BRAKING IS OFF! OK TO DRIVE. **
NO CODES PRESENT
LF WHEEL (MPH)____45 RF WHEEL (MPH)____45
Figure 8-54 Sample Bosch 2 Codes & Data
Select CODES & DATA on a Bosch 2 system and the EBCM enters a diagnostic state. The
EBCM remains in the diagnostic state after exiting Codes & Data. The ABS is disabled until
the ignition is switched off.
z To restore the ABS to normal operation:
1. Switch the ignition off.
2. Disconnect the scan tool.
The Bosch 2 EBCM enters the diagnostic state only once for each ignition cycle. During ABS
testing in Codes & Data mode, other vehicle components may seem to operate abnormally
(i.e., the instrument panel may appear to stop working or the gauge readings may change
randomly). This is normal and stops when the EBCM is taken out of the diagnostic state.
NOTE:
i ABS testing may cause false DTCs to appear on the engine code list. After ABS testing,
reenter vehicle ID for engine testing and check the engine code list. Clear any codes that may
be present and recheck to be sure they were false.
Bosch 2 Trouble Codes
The 1992–94 Corvette uses a Bosch 2 ABS that provides code history information separately
from the ABS data. For these vehicles, CODE HISTORY is a MAIN MENU <strong>GM</strong> ABS selection.
New codes cannot be set while the system is in diagnostic mode. If codes are present, the
third line changes to read “scroll down for ABS codes.” Scrolling reveals a code number and
description, along with certain conditions that relate to the code (Figure 8-55).
35 LEFT REAR WHEEL SPEED SENSOR
CONDITIONS AT TIME OF ABOVE FAULT CODE:
BRAKE SW________OPEN STARTS SINCE CODE_16
ABS STATE________OFF VEH SPEED(MPH)____47
Figure 8-55 Sample Bosch 2 display with codes
When a code sets, the EBCM records three operating conditions:
• Brake switch state to indicate if the brakes were applied.
• ABS state to indicate if ABS was operational.
• Vehicle speed.
Additionally, the EBCM records the number of times the vehicle was started since the fault
occurred. If the fault is an intermittent problem, or soft code, the EBCM counts the number of
starts since the code last occurred. If the problem is continuous, a hard code, the code resets
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