I had an unusual no start problem that had been bugging me for months. It got so bad that I didn't drive the car for like 3 months it just sat in the garage because I was sick of getting stranded and having to get in and out of the car or get a passerby to help.

Here is what would happen. Turn the key and get a loud click and then everything in the car electrical would be dead, no lights, radio or horn nothing. I would then get out of the car and shake the battery and then all the power would come back. Try and start the car and it would either turn over or do the click and no power. I would sometimes have to do this multiple times. Figured it was my POS Optima Redtop acting up again and replaced it with an Excide Orbital. Stopped doing it for a few weeks then started up again. Checked battery voltage and it was low. Bought a batter tender and charged it back up. Still had the problem. The wierd thing was that as soon as i touched the batter or the cables after a no power episode the power always immediately came back.

I was perplexed and somewhat pissed about the whole thing. I figured it was the grounds. Cleaned those and didn't fix it. Decided to replace the battery cables. Let me tell you that is a pain job on the C5. Damn bolts for the exhaust connection under the car are right in the way and one slip is band-aid time plus you have to remove the oxygen sensor which took a wrench size I of course didn't have (23 mm). However it fixed the problem. No more no start and the car is not going dead when it sits for more than a few days.

Inspected the old cables - didn't see anything wrong with them. But it is a hard wire encased in plastic. My assumption is that there was an internal break in the copper.

 

The battery cables would probably be one of the last things to suspect.

To be sure that the problem was caused by an internal break the old battery cables, check each one with an ohmeter while flexing the cable in all directions. A good cable will remain close to 0 ohms resistance while you're flexing it.

 

This drove me nuts for weeks. I was getting all kinds of wierd electrical gremlins. It kept making me think this thing or that thing was wrong. The only reason I thought of the cables was that when you touched them the power came back. I tightened them down, re-tightened them again. I even though that maybe the post for the battery was broken internaly. Can't check them as I thru them out.

 

Hi, I had a similar situation many, many years ago on an old Pontiac Bonneville. The cable ends looked fine, as did the terminals, but the car wouldn't start and the lights would dim . Tested the starter, solenoid, switch, battery and everything I could think of. Finally in an act of desperation I bought new cables on the advice of a very old mechanic that was my mentor so to speak. If you looked at him you thought he didn't know anything, or was even breathing most of the time. But he had forgot more than most of us knew at the time. Anyway to make a long story short, the new cables started her up in a second. Curiously I had to know why, when the old cables looked great from the outside, so I cut the insulation off all the way to the ends, in between the strands was all corroded, looked like battery acid looks but the ends were perfect. Now I know the theory about how electricity is conducted on the outside of the wire strands not the inside, so it makes sense. The corrosion between the strands was blocking the flow of electric. Never can take anything for granted. And thanks for your indulgence
Luck To All

 

Well it looks like you found your problem, and that's what counts!