The ICM is located on the passenger side, up under the dashboard. Remove the glove box and you can see it. It will be bolted to the side panel It is held on by 2 bolts, one on top, one on bottom. Disconect battery before removing!
The above is the location of the engine control module or computer.
If your car has the V6:
The ignition control module is mounted to the lower part of the engine block on the radiator side of the engine. If you follow the spark plug wires down they will lead you to it.
The coils are bolted to the ignition control module.
If your car has the 4 cylinder engine:
The ignition control module is below the intake manifold towards the rear of the engine.
The spark plug wires will lead you directly to it. The coils are mounted on the ignition control module.
It is in a awkward spot to get to.
On the V6 the ignition module is under the distributor cap.
The ignition module on a 1988 Chevy Celebrity with 2.5 engine is on the side of the block beneath the intake manifold. You can simply follow the spark plug wires to it. As information the crankshaft position sensor is behind the ignition module.
All 1987 to 1990 Chevy Celebrity engines use distributorless ignition. Timing is controlled by the engine control module in conjunction with the ignition module. There is no provision for adjustment.
Which engine does your car have?
On a 2.8l v6 it is at the front of the engine behind the ignition coils.
I take mine to Autozone, they offer free testing of ignition modules.
The 1988 Chevy Celebrity uses distributorless ignition. There is no provision to alter timing, the engine control module takes care of that.
The 1990 Chevy Celebrity engine is distributorless, the ignition is handled by a combination of the engine control module(computer) and the ignition control module. You did not mention which engine the car has. If your car has the 4 cylinder engine the ignition control module is below the intake manifold at the rear of the engine. You can follow the spark plug wires to it. The coils are mounted to the ignition control module. If your car has the V6 the ignition control module is near the starter and again you can follow the spark plug wires to it. Again the coils are mounted to the ignition control module.
I would suspect a failing ignition control module.
The coils are bolted to the ignition control module, there is no coil wire as such.
To replace the ignition module in your Celebrity: Unplug the wires, take note of where they go so you can put them back in the correct place. Remove the screws that hold the ignition module in place. If the old module was coated with dielectric grease on the mounting surface put fresh dielectric grease on the replacement ignition module. Put the new ignition module in place and attach with the screws. Plug the wires back in their respective places.
You can replace the ignition control module and the crankshaft position sensor as required.