99 C70 loses all electrical power intermittently

[MaxIcon]

My 99 C70 hardtop, 90k miles, has started dying intermittently and unpredictably due to loss of electrical power. It can happen a few times a week, or not for a week or two.

I haven't seen it happen - the kids have been driving the car, and aren't too tuned in to the details, but here's what I've gotten from them.

Before it dies, the lights on the dash dim, sometimes a few times, and the radio/CD stops playing. Sometimes the radio comes back on after a few seconds, other times it requires the code to be entered. It doesn't always die when this happens; sometimes it keeps on going when the power recovers. Often, it happens when the brakes are hit, but not always.

This can happen when it's been driven for a minute, a while, and once it just wouldn't turn over after it had been sitting all day at school. In this case, the door was opened with the key (no remote fob on the spare keys), and the trunk release switch in the door was pulled, but nothing happened, and it wouldn't start after that. When the switch was pulled, nobody had opened the other door, sat in either seat, inserted the key in the ignition, or anything.

When it dies, if it sits for a short time, it starts right up as if nothing has happened. This can be right away, after just a minute, or longer when they thought there was a dead battery. It's not blowing any fuses or generating error codes. I haven't had a chance to see if any of the breakers are getting hot.

Will a bad key chip cause loss of power as part of the theft prevention? The spare key being used is a beater, and every time I go help them out with my own key, it starts right up, no problem. We've swapped spare keys to see if there's any difference, just in case, but it's only been 3-4 days now, with no fails.

Other problems this car has had (like many of its era) are bad window switches (the passenger switch is marginal right now, but there's never a passenger when this happens) and bad bulb sockets (mostly fixed under recall, but not all). My wife's 97 850 was recalled for seat heater problems, but I never heard about that in the C70. I'm wondering if there's a loose connection somewhere that's shorting out intermittently.

Any thoughts on troublesooting directions would be greatly appreciated!

[BEJinFbk]

If you wiggle the ignition key while this happens and power returns,
you probably need to replace the electrical part of your ignition switch.
The good news is that it's a dead simple job for these cars.

The other likely issue is the 105 relay that provides power to many
of the cars systems. But, try the key jiggle option first!

[jblackburn]

Check for corrosion on your battery cables as well - both at the fuse box end and the battery.

[FCPEURO]

+1

I had an issue with the ground wire on my 850. The battery would run down and the car would just die on me. I replaced the ground cable and all was good again.

[850tech]

+1 to the ground cable, specifically where it mounts to the trans, have seen a few break right off.

[MaxIcon]

Thanks for the ideas.

Both battery cables seem fine at both ends. No amount of wiggling or tugging would make it skip a beat, and they appeared solid where I could see them at the battery and main fuse box. The ground cable goes to 2 places from the battery terminal, and the one to the frame was fine. The other one went under the battery somewhere I couldn't follow (the trans?), but it seemed solid, and it seems as if one good ground connection would be sufficient. Let me know if I'm off base on this, and I'll crawl under to check it out.

When it dies, the battery is always well charged, and it starts quickly when it recovers.

I wiggled and thumped the key (using the suspect beater key), knocked on the steering column, and couldn't get any flickers. It seems like this wouldn't kill the entire electrical system, though would cause the car to die, but it was worth a try.

The main relay/breaker was a bit warmer than the one next to it. Not real hot, but I didn't have it running long. I've heard problems with it overheating and tripping, then resetting. This doesn't fit all my symptoms, but maybe I'll replace it just for grins.

Any other thoughts or areas to check?

[Pauloil]

do voltage drop test on all cables

http://www.aa1car.com/library/voltage_drop_testing.htm

you can test ground by running motor at 2K with things on and then connecting neg jumper cable from batt. neg to motor. If voltage at batt goes up much more than 0.2V, then bad ground cable

also suspect the "B cable"

[MaxIcon]

OK, I'm pretty sure I found the problem, so I'm updating this old thread.

It finally died for more than a few minutes (and close to home), giving me time to get to where my son was with a meter.

I found no voltage at all at the main fuse box, and a 12v drop between the fuse box input and the battery clamp. After a minute or two, the fuse box voltage started rising, eventually getting up to a full 12v with no drop across the cables.

Now, there's nothing between them but 4 large gauge cables in parallel, so I started taking stuff apart. The cable at the fuse box end was fine, but at the battery clamp end, the connector that clamps the 4 cables to the main battery cable was slightly discolored where it connected to the main cable. It was plenty tight (no wiggle at all), there was no corrosion or damage, but it wasn't as shiny as I expected. I scrubbed it down, tightened it up, and so far, no more problems.

I'm an old hand with high-current connections, and I'd never have suspected a problem that looked this minor would cause it to drop out so thoroughly (no dash lights, no key buzzer, no mileage display, nothing), and then recover and work fine for weeks.

It'll take another month for me to be confident it's gone, but after getting to measure it, there's nothing else it could have been, as far as I can tell.