I drove a lovely little girl in a Home Coming parade last Tuesday evening. Car was running great. Still had oil in the engine and I'm 50 miles from home, at night, on the Natchez Trace Parkway. Somewhere, before I got home with the engine running well, the alternator light failed. If the light fails while the engine is running you have no way of knowing. About a mile from home the headlights dimmed...nearly went out. I got to my neighborhood and the car began to buck and snort, obviously the battery was about to give up. I turned off the lights and was able to drive into my garage.

Next day I installed a new alternator, started the engine and no juice. Everything is wired correctly, must be a bad alternator. Exchange it and installed the new new alternator; no juice. By now  I'm really frustrated. I had that alternator in and out 4 times checking the wiring ( what is the definition of insanity?)

I woke up this morning at 6:00, that's 4 a.m. California time, and knew what the problem had been all the time. The idiot light was out. I pulled the light and it was not burned out, but had been displaced in the socket enough that a good connection was not made. Now the idiot light is on; start the engine and I have 14.5 volts.

This may not apply to all alternators. My car has the 911 shroud ans run a Delco CS130 alternator. The mounting brackets are machined off to fit inside the 911 fan. 

I was really dreading the failure of the alternator because I suspected that the installation would be a bitch, but it wasn't. I could have been finished in an hour.

Beware of the Alternator light!



Original Post

“This may not apply to all alternators.”

Yes, it does apply to all alternators.  If the dash “charge” light is burned out or not making adequate contact to be functioning properly, the alternator (any alternator) will not charge.

Technically, the dash light wire (labeled “D+” on top of the alternator) provides an initial electrical path for the alternator field coil to begin charging once the alternator starts spinning.  If the light is defective, no field charge.  The alternator internal magnets are not adequate to maintain charge on their own.  They need the field winding operating to start or maintain the charging sequence.

Glad you found the culprit.  Most people wouldn’t look there, especially if the light is out!

Those little 4W bulbs in those little brass sleeves are notorious for corroding over time and failing to work (the solder button on the bottom of the bulbs, and where the sleeve inserts into the gauge, too).  If you swap an alternator and still no charge, first place to look is the dash bulb.

I used a 50-ohm 10-watt power resistor in parallel with a 12V rated red LED lamp assembly to replace the alternator lamp when I upgraded the dash lights to LED. It functions much like the original lamp. A word of caution... the resistor gets rather warm when the key is on and the engine is not running, or if the alternator is not making power. Position it so it hangs in free air, not tied to a harness. https://www.ebay.com/itm/PLB12...p2057872.m2749.l2649

Since I have the Speedhut gauges, they have all LED lights inside them. I chose to use the LED meant for something else for my alternator light. It did NOT have the resistor wired with it to give enough resistance(load to the alternator) to work. So, I used a relay to trigger it with a 500 ohm coil. Now it works properly!

If you use a relay, it won't matter if the light ever burns out.

I've never actually had an idiot light burn out in ANYTHING. This problem came out of a loose connection between the light and holder.  While being intimately involved with replacing the alternator; every time I turned on the ignition an idiot light (wonder they call them that) came on, but it was the oil light! I wasn't paying attention to the color; just the light.

I've been repairing mechanical things all my life; well for 55 years anyway. If it's on a car, motorcycle or personal airplane, I've touched it. I'd bet that average self taught, but highly skilled, mechanic is unaware that the idiot light out would not allow the alternator to work. I sure wish I had known that.

This thread has only had 118 views. Our other friends really need to know about this.

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