It's right below the keyboard socket. I have used this trick and it worked on a system I had. In my case, I assumed that the power button was bad, but I was able to see the button contacts closing using a ohm-meter on the keyboard connector. The only thing I could figure was the socket or circuit on the logic board had an open. I soldered a pair of wires from the power button directly to the contacts on the motherboard. When you find the contacts, just use a small screwdriver to jump the contacts.
Is the replacement LCD new or used? It sounds like the replacement LCD is bad. I've bought used LCD's on ebay that were not good. What was wrong with the old LCD. Is good enough you can put it back in and see if has the same result? I have a spare LCD for an A1181. Let me know if you need it. We can check compatibility.
So if you insert the OS disk in the optical drive, you should be able to turn on the computer while holding down the C key. It sounds like you are saying that won't even work? Do you get any kind of display? Do you hear the startup chime? Is it possible your LCD went out and you just can't see what's happening? If you are getting some kind of display, but the computer won't boot, try to start removing components until the computer boots. Start with the memory. Then unplug the optical drive, the the hard drive. Anything to see if you can get the computer to respond at startup. I've never worked in the IMac G4, but if there are ethernet cards, airport cards, anything like that might be able to be disconnected. The idea is, remove all devices possible until the only thing left is the logic board. If you still can't get any response, then you might have a bad logic board.
This is interesting. I had a friend ask me to look at her computer with the same problem reported here. I replaced the hard drive, reinstalled OSX 10.5 and all was well. I ran the software update, which downloaded 10.5.8 combo update. After the update installed, the computer failed to boot again. Is it possible a recent update could have hosed the system. Mine is an iBook A1054. I'm currently re-installing the OS again. We'll see what happens.
The problem was that the phone wouldn't charge. So I was trying to determine if the battery, charger, or phone was bad. I took a chance and connected a battery from a different phone using jumper wires to the pins on the phone. I connected positive to the pin corresponding to the + sign on the battery (obvious enough, right?) and the negative to the other outside pin (this was a bit of a guess). I left the middle pin disconnected. Wowza! The phone turned on! This proved the phone worked. Since the battery wouldn't charge through the phone, I wanted to try to charge it another way. I connected a 5V AC/DC adapter to the battery pins using tape to hold the wires on the battery. I let it charge for about 20 minutes and then installed the battery. Yippee, it started up. So the phone is good and the battery is good. There must be something wrong with the charging circuit. Sure enough, I took apart the phone and found the charging port was broken loose. Anyway, we bought another phone with a broken screen to use...
It looks like the whole problem was the main battery. I have finally received two new batteries and this computer is working well. The original battery appeared good from everything I could see. The LED indicators on the back showed all lights lit. The charger shows green. The coconut battery program shows everything looks good on the battery. However, when I tried to use this battery in a second computer, that computer exhibited the same symptoms as this computer. So, the final solution appeared to be the main battery. In the process, I reset the PMU and replaced the PRAM battery. These other steps may have contributed the final solution. (Note: The old PRAM battery measured something like 4.1 volts on a voltmeter. I'm guessing the original PRAM battery was not bad) (Note 2: Remember the second computer I tried using with this computer's battery? It's working fine with a new battery installed) Thanks for those who contributed.
I reset the PMU and that has made a world of difference! It's been working pretty good for about a week. However, it has still shut off without warning or any apparent reason about 3 times. I have ordered a PRAM battery. We'll see if that makes a difference.