About 5 years ago, when my G4 and ACD were two years old and still under AppleCare, it suddenly wouldn't "Sleep". After repeated visits to the Apple Store dragging both G4 tower and ACD back and forth, the Geniuses decided it was the fault of the ACD. It was sent back to Apple and repaired - under AppleCare. If memory serves, I was told it was an inverter in the ACD that was bad and was fixed (or replaced, but I doubt that as it failed again).
Now, about 5 years later, the same problem has occurred. I don't know if it's the inverter (?) or not, but it IS the ACD. I removed it from the system and left the secondary ViewSonic in its place. I had transferred the tool bar to the ViewSonic before disconnecting the ACD. Hallelujah, the computer now goes to sleep. Unfortunately, I got used to the ACD and am not as happy with the ViewSonic. As a secondary, it was fine - no complaints. As a primary, not so fine.
My next primary monitor will be an S-IPS. Anyone have an opinion on a good 24" S-IPS monitor?
BTW, I spoke to an Apple rep on the phone. He suggested I take the monitor to a local Apple Service Provider as it would be cheaper to go that route. Apple would charge a lot more to fix it. I had thought that since Apple had originally fixed the ACD and that was probably the part that failed, Apple would give me a break. I didn't expect free as it has been 5 years. But NOOOO break. Apple must be on the ropes financially (LOL) and has to charge exorbitant fees to stay in business. No breaks for loyal Mac enthusiasts.