Hey all,

About two months ago, it looked like my alternator kicked the bucket. The warning light came on and the thing would only charge above 2000 RPM or so. Obviously, it looked like the exciter wasn't able to garner enough charge at idle. Alright, cool. I changed it out today--quite the PITA as far as alternators go, though I loosened the A/C compressor and braced it against my forearm while undoing the last bolt, so I didn't have to remove any suspension or the radiator. Got the thing out and took a look at the label. Sure enough, "Botched" sticker right on it. I installed a rebuilt Lucas unit in its place. Here's where the head scratching starts.

The alternator now charges perfectly at 14.3V at idle, and the car is happier. However, the battery light stays on. When the exciter connector is removed from the alternator, the light goes off. This looks normal. Thing is, I checked that wire for continuity to ground and got weird fleeting readings, kind of like when you check continuity on a capacitor (beeps for a split second, then stops). Occasionally, it would beep for longer, which makes me think the wire is cracked somewhere and grounding out occasionally; however, I was very careful not to let this happen, and I was double checking that my probe was always firmly inserted into the spade connector each time, so it's not like it was falling out. Now, my question is: would this point to a faulty gauge diode? More specifically, could the old alternator burning out (I drove it for about a week with the problem) have caused the diode to fail? Furthermore, will the new alternator be at risk of premature wear due to the exciter wire being grounded somewhere, or would that wire burn up before that could even happen?

The battery holds 12.6V when shut off, and reads a perfect 14.3 when the car is idling. Some more curious details: shortly before all this, the car experienced some excess humidity, which I promptly took care of; I don't think this can explain the alternator failing, and clearly, the exciter circuit is still working, with the exception of the light never going out--perhaps this could have affected the diode, though. The grounding on the car should be in excellent shape as I had to recheck every single point when doing the head gasket. Also, when I was juggling the A/C compressor on my arms, I did drop my socket wrench right onto the alternator contacts, and of course, sparking ensued. Come to think of that, that may have popped a fuse somewhere--I will check that first thing tomorrow. I would love some input on this, as I've found old threads where other owners had similar problems, but none exactly like mine.

 

I intend to keep this thread updated so that the solution and steps are archived. I know it seems like a problem that doesn't matter/is easily diagnosed, but I think it's worth it; these cars are prone to gremlins, after all.

I grounded out my "low coolant" light, and I took note that the battery light as about half as bright. This tells me that whatever it is, it's likely a small fault; the alternator isn't messed up internally, for example, and the wire is clearly not getting fully grounded. I checked the fuses and found none were damaged. I'm very suspicious of the gauge diode, but I won't have the time to look into it until possibly the weekend

 

Stays on even with the key "off"?

Or stays on only with the key "on"?



Quite possible. As I recall the exciter wire goes 'round, and a positioned against, the front of the cylinder head. I've never seen one that wasn't totally baked to a crisp. Double check yours


Not sure what you by "gauge diode"

Cheers
DD

 

Thanks for the reply, Doug! This is only with key ON. The light operates perfectly fine otherwise; it comes on when I turn the key ON with the engine off, but it stays lit when the engine is running, which it shouldn't. I've checked the wires running in front of the head and they are indeed wrapped in brittle electrical tape, but there are no wires exposed. It would be one hell of a coincidence if those wires were to chafe in the same spot and ground together at the same time that the old alternator went bad, so I'm thinking it's some damage caused by the alternator failing or when I dropped the wrench on the contacts. However, there's nothing out in the open at all.

As for the diode: check out this schematic for the alternator gauge. I highlighted the diode.