Warning Light Does Not Illuminate During Bulb Check
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 1991 GMC Pickup, 1991 GMC Cab & Chassis, 1991 GMC C3500 HD, 1991 Chevrolet Pickup, and 1991 Chevrolet Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.
If warning light does not illuminate when ignition is turned on with engine off and parking brake NOT applied, check for burned-out bulb. If bulb is okay, check for open in circuit by removing wire connector from differential pressure switch on brake combination valve and touching connector to ground. If warning light DOES NOT illuminate, repair open in circuit. If light illuminates, circuit is okay. Replace differential pressure switch.