1998 GMC Sonoma Owner's Manual - Page 127

Security Warning Light This light will come on briefly when you turn the key toward START. The light will stay on until the engine starts. Service 4WD Light This light should come on briefly when you turn on the ignition, as a check to show you it is working. SECURITY SERVICE 4WD If the light flashes, the Passlock system has entered a tamper mode. If the vehicle fails to start, see "Passlock" in the Index. If the light comes on continuously while driving and stays on, there may be a problem with the Passlock system. Your vehicle will not be protected by Passlock, and you should see your dealer. The SERVICE 4WDlight comes on to indicate that there may be a problem with the drive system and service is required. Malfunctions can be indicated by the system before any problem is apparent, which may prevent serious damage to the vehicle. This system is also designed to assist your service technician in correctly diagnosing a malfunction. 2-63