Check the Ignition Switch
For your 1988 GMC Truck C3500 1 ton P/U 2WD 5.7L TBI 8cyl
See this picture http://www.autozone.com/.../N-ihzgpZ9n859
REMOVAL & INSTALLATION
Remove the column shroud halves.
Remove the column-to-dash attaching bolts and slowly lower the steering column, making sure that it is supported.
Make sure that the switch is in the Lock position. If the lock cylinder is out, pull the switch rod up to the stop, then go down one detent.
Disconnect the actuating rod from the switch.
Remove the two screws and the switch.
Before installation, make sure the switch is in the Lock position.
Installation is the reverse of removal.
NOTE: Ignition starter switch directs current from the fuse panel to other systems in the vehicle
When not replaced, vehicle may experience no-start, intermittent shut down and/or current will not flow to all systems.