Good morning all, my '95 SC300 has been doing something I don't understand. The battery light comes on while I'm driving it, but it's not on when I start it. Just after I've been driving a while. The voltage is good when I check it parked. I've driven it several times and it keeps 12.3 volts. What exactly does that light mean? Is there more than one thing it can mean?

 

Hello,

12.3V while the car is Started and Running..? That's the Battery Voltage, not the Charging Voltage, which means that the Alternator is not charging the Battery. When the Engine is Running, the Voltage should be at least 13.5V, or ideally closer to 14.2V.

Here is a thread on how to diagnose Battery Light On. Seeing that the light is intermittent, start by inspecting the Control Pigtail on the Alternator, see if the connector is intact and the Wiring is not damaged, then Disconnect the Battery, take the B+ Wire off the Alternator and make sure that it's clean and tightened securely.

If all that is good, put the car back together, then get a Multimeter and measure the Voltage between the B+ Terminal on the Alternator and its Housing, if it's 13.5V or higher, you have Wiring Damage between Alternator B+ Terminal and the Fuse Box, similar to what happened in the thread linked above. If, however, it's just as low as on the Battery, the Alternator is likely on its last legs and in need of a replacement, Denso Remanufactured units are the only option I would trust at this point, whatever you do, make sure to not get any of the Store Brand replacements, here is why.

Hope this helps and best of luck!

 

Check your alternator and battery connections.

I was having issues and realized I didn't fully tighten the power wire coming off the altenator stud, and it was arcing out which was causing the fluctuations in voltage and giving me a battery light.

 

Saw the same/similar symptoms on my ‘92 several years back (…., now maybe 10 &#128513 and the alternator was failing. Replaced it and it was like driving a new car! Advice on checking and/or replacing the cables is worthwhile, too.

 

Thanks all. I have of course considered the alternator and will investigate that, and I now have some direction for that. The 12.3 volts is parked, nothing on. I checked that a couple of days apart without driving it, so see if it was losing voltage and it didn't. Tomorrow I'll measure the voltage with the engine running, and do some of the other checks recommended. I'll let you know what I find.
Update: I started her up and the battery measures 14.48V on my Harbor Freight multimeter. Is that too high? Anyway the alternator appears to be strong. I'll drive around some more in the next few days and see if anything happens.

 

Does this look like the correct Denso part, if I do need one? I don't see an amperage rating.

 

Ok, I see now there's also a 210-0291. It doesn't have a Denso label but it claims 100 amps while I found the rating for the -0287 is 80 amps.

 

From reading it seems the auto cars ran the 100amp, and manual cars called for the 80 amp. Your voltage at 14.2 is great, thats charging. It could still be failing, or you could have a loose connection somewhere.