Right before Christmas, at night with the headlights, radio, etc all on, the battery light came on. Other warning lights were acting weird and flashing on and off.
I assumed that the alternator was going bad.
I thought my suspicion was confirmed when, the next morning, the car wouldn't start.
Since I didn't want to drive it in the snow and salt and crap...and was planning to not use it until Spring, I figured.."perfect timing". I'll just cover it up and replace the alternator when the weather's better.
Well, I couldn't wait so I put a charger on the battery and got it running. No warning lights. Voltmeter gauge on the dash.. over 13 volts. Battery holding at around 12.65 Volts.
Not great, but I drove it around with all the lights, A/C on full blast, radio, foglights...all the accessory load I could put on it.
Didn't die. Started back up when I shut it off.
So, here's thing ....
I thought I would disconnect the battery and see if it would run off the alternator.
When I disconnected the NEG. cable, the engine started racing and "hunting" like...AND the speedo was going wild...up and down(?)..and warning lights were going on and off....
What's up with that?
I know I should probably rebuild the alternator and replace the crappy Walmart battery that came with the car. Get the best, most powerful battery that will fit...
I just don't understand why all that stuff happened when I disconnected the negative from the battery.

 

Not sure, but I think I've read that you shouldn't try that on modern cars as they all just run directly from the battery and the alternator's only role is to charge the battery.

I used to "test" the alternator that way too with cars of the sixties and seventies but now I leave it all connected and use a voltmeter to check the alternator.

 

Your battery terminal could have been loose or slightly corroded/bad connection when the original fault occurred.

Possibly you fixed it inadvertently by charging/moving the terminal. When you took of the neg terminal you disconnected the ecu so all sensor inputs were ignored and calibrations went haywire ..that's what I THINK happened, and there's a good possibility your battery and alternator are just fine.

just my

Larry

 

Error

 

Give me some time to developing the big picture of the X300 electrical system and some least intrusive approaches to your solution based on your observations

On paper by lifting the battery terminals or a bad battery the sub system grounds will still provide a current path for devices to function , car should run in all aspects like engine or window lifts

And vice versa if the alternator goes out the battery will power the car until the battery charge goes too low , 11.4 volts

But there is a sudden voltage change in doing so , does the ECU reference voltage adjust quickly or does the return sensor signal get rejected so the ECU reverts to a less then optimal set of maps

Tips Bucket :

Loose terminal nuts on the fuse link at the battery

Loose terminal nuts on the fuse link under the rear seat pan

Streached positive terminal clamp on the battery post , battery post shims

Intermittent shorting noise suppressor module ( a capacitor )

Loose terminal nut on the starter solenoid , 15 mm



And on the area of providing quality power to the voltage regulator so the alternator can provide quality power to the ship

Tips Bucket :

counterfeit fuse limiting the current to the VR

other devices attached to the fuse acting up , usually blows the fuse - but

loose pin grip on the round connector at the alternator causing current shortfall to power the VR

dirty or wet round connector

Fuse socket dirty

The fuse box itself has a connector on the underside if you unbolt the box

The VR is part # IN435 and is a high response rate

Unstable VR voltage regulation at steady state RPM

Unstable VR voltage regulation at changing RPM





Editing

 

Nice work..
Thanks alot.
I can actually read this..

 

L - I'm beginning to think that's the case.
When I got this car last Sept. , the guy I got it from didn't seem to know much about it but he said he had given it new tires (crappy Chinese snow tires (?), new shocks (in the front, only, and, most likely; crappy) and a new battery (from Walmart, so, crappy and probably too small).. .
I'm going to probably rebuilt the alternator to be safe and get a proper battery. This most powerful one that will fit.

Thanks,

James