2002 xjr
This has happened twice, this week. So, I put my key to the ON position. The "system check" is really dim or not displayed at all. I don't attempt to start the car. Take the key out, put it back in. Go to the ON position, and it's normal. Start it, and it starts up like if you just connected the battery. RPM goes to 2k, and settles at normal. Radio bass, treble, sub etc all at zero. Where as, I always have them at max. Any clue as to what is causing it?
I checked battery voltage right after the "system check" wasn't displayed. It was 12.37v

 

Charge the battery for six hours at 2 to 5 amps and retest. If the battery is still below 12.6 volts, the battery should be replaced.

 

I am charging it now. But I doubt it's due to weak battery voltage. As I have had these cars(X308) start with 11.9v
and show the "system check" just fine.

 

You might check the battery cables and the 'mega-fuse' block in the boot.
The power 'stud' on the false bulkhead in the engine compartment sometimes overheats and melts due to loose connection.(alt needs to use that circuit for charging)
See if the ECM has P1000 or P1111. P1000 will be present if the ECM has lost battery power. (or someone erased/cleared DTCs)

 

When it happened both times, was this the first time you were attempting to start the car for the day? Or had you already driven it as an example, you drove it to the store and 10 minutes later you came back out and the lights went dim? I discovered late last year before going on a long trip, that when I was warming up the engine, I went to go make sure the battery cables were clean and tight. When I put my hand on the negative terminal I found it to be quite warm due to excessive resistance. I had a good spare cable and put it on. When I went to go fill up, I felt the cables again and that seemed to take care of a potential problem before it started.

That may or may not be your issue, just runnin some ideas.

 

Same happened to my (original) positive battery cable in a similar situation except that my cable was pretty hot. I was prompted to check the cables by slight flickering of the dash lights while the engine was idling. The heat appeared to be generated from the crimp of the cable to the battery terminal (it was hottest there). I replaced it with a length of "100% pure copper" heavy gauge welding cable to which I attached a new battery terminal (high quality, USA made - https://www.ebay.com/itm/1-Ga-Positi...wAAOSwvCtfGHci) and a suitable new "eye" terminal. Perfect now.

 

Both times, I had already driven. But very little. 10 minutes max. I'll see how my cables feel, thanks.

 

Might have found the issue? Not sure if you guys can see it, but there's corrosion around it. Mostly on the bottom.



It's the post/ground(I believe that's what it is called) on the right side, by the shock tower.

 

IT IS NOT GROUND!!!!!!!!!!!!!! (it is POSITIVE directly to the battery)
That is the positive post to the Starter and Alternator.

 

Thanks for clearing it up. So does that look suspect? Just clean it and go?

 

I was following thread . lots of good advice as usual .
I just wanted to agree sounds it like a weak cable connection
building up resistance

What you found looks exactly like what I would expect you to find.
It may not be your only poor contact point though.

And yes, as Motorman so kindly reminded , that is the starter cable stud.
obvious proper precautions suggested when cleaning.

 

Oh yeah, I'll definitely disconnect the battery before any cleaning. Should I put dielectric grease to prevent the corrosion? And yes, I'll look at all other similar connections.

I saw the mega fuses in the trunk. Not sure how I would check those. Car didn't act up today. While the car was running, I felt both battery cables/terminals. Neither were hot.

 

There are also main positive supply studs (covered with rubber caps) on each of the engine compartment fuse boxes. Check those as well. It is also a good idea to pull every relay and fuse out and inspect their pins for corrosion.

 

Got it. Will do, thank you.

 

I took out every fuse and relay yesterday. None had any corrosion on them, or anything suspect looking. Here are some pics. There was the SLIGHTEST amount of corrosion on the inside of both battery terminals. Cleaned whatever was dirty in any way. Also looked at the starter and alternator posts. Pics below.

 

Maybe a bad cell in the battery or a bad ground strap (+ terminals look great!).