My Alternator Light is on....

[truxson]

I suddenly have an alternator light on in my 1976 C10.  I pulled the alternator and had it checked....OK.  Battery, not running, checks at 12.5V...That should be OK.  Belt tension is OK.  Does anybody have any other ideas about what could be causing this problem?  My shop manual mentions some vague causes, but none of them have checked out.  I'd appreciate any ideas anyone might have....

Regards, David (Truxson)

[bd]

Have you checked voltage at the battery with the engine running?  Does the dash telltale remain illuminated if you unplug the two-wire connector from the regulator?  Does the heater blower work?  Check the heater fuse.

[truxson]

Thanks for the ideas....I just got home from work so I'll check them in the morning....but the voltage at the battery with the engine running was only about 12.5-13 volts.

Thanks, David.

[hatzie]

Battery voltage with the engine running should be 14.5.  If the alternator is really OK then the wiring has a problem.

Make sure the Ground cable is making good contact with the alternator bracket.

The BAT terminal on the alternator should have battery voltage.

Unplug the two wire plug from the alternator.
The RED wire of the two terminal plug should have battery voltage.
The Sense/Field Brown wire has around 45Ω of resistance or the instrument panel warning lamp resistance...  It should have at least 12v with the ignition switched on.

[truxson]

Hi Folks:
I want to say thanks to the guys who gave me some ideas about the alternator light.  I'm happy to say my problem is solved. 

I started with the first suggestion....check the heater blower.  The blower didn't work, and the fuse looked fine.  But...I've had experience with the old style fuses where the center looks OK, but it's burned out at the ends.  So..I took out the fuse, and the end came off.  I said....I believe we're on to something.  I replaced the fuse and the problem was cured.

But...If it isn't too much trouble, would you offer some insight as to WHY it fixed the problem.  I can't figure out in my head how that could have caused the problem in the first place, or how a new fuse fixed it.....

Again, thanks for the help.

David.

[bd]

Magic!!   



Actually, SI alternators with their integral regulators need a small external voltage signal to turn the alternators on so that they begin to charge.  In trucks manufactured w/o factory gauges, that voltage signal comes through the charge lamp.  It just so happens that the mid-70's trucks powered the charge lamp through the heater blower fuse.  Heater fuse blows, charging system quits charging.  Same thing would happen if the charge lamp bulb blew out, except in that scenario you wouldn't know there was a problem until the engine quit cranking due to a dead battery.  Trucks with factory gauges substitute a resistor wire for the charge lamp.  At least with the factory voltmeter a driver would receive a consistent warning of low system voltage - assuming (s)he monitored the gauges.

When the heater fuse in your truck opened, the charge lamp illuminated, because a small voltage from the regulator backfed the charge lamp and the lamp grounded through the unpowered blower circuit.



Mystery solved!   

[zieg85]

Magic!!   



Actually, SI alternators with their integral regulators need a small external voltage signal to turn the alternators on so that they begin to charge.  In trucks manufactured w/o factory gauges, that voltage signal comes through the charge lamp.  It just so happens that the mid-70's trucks powered the charge lamp through the heater blower fuse.  Heater fuse blows, charging system quits charging.  Same thing would happen if the charge lamp bulb blew out, except in that scenario you wouldn't know there was a problem until the engine quit cranking due to a dead battery.  Trucks with factory gauges substitute a resistor wire for the charge lamp.  At least with the factory voltmeter a driver would receive a consistent warning of low system voltage - assuming (s)he monitored the gauges.

When the heater fuse in your truck opened, the charge lamp illuminated, because a small voltage from the regulator backfed the charge lamp and the lamp grounded through the unpowered blower circuit.



Mystery solved!   

I love still learning, Thanks Rich!!!

[bd]

Y'elcome, Carl!

[truxson]

Thanks for the education!  I'm really happy that there are lots of folks in the world that are smarter than I am.  You saved my day!

Regards, David.

[Stewart G Griffin]

So now i have the same problem.   Battery checks out at 14.2-14.5ish when running IN MY OTHER CAR.  Brand new alternator.  Heater fan does work.   i just noticed it tonight, although it may have been on during the day---it is not as bright as the other warning lights.

The dash lights seem dimmer, and the left turn signal seems slower than normal.

Other than that the headlights seem normal and the truck starts easy.

[Stewart G Griffin]

Super bump.

What i've done so far:

1) Measured battery voltage at 12.5, then started the engine.  No change in voltage.

2) 2-3 hours later, tested alternator output with the engine running with the voltmeter. Red probe to the B+ terminal and then black probe to the body of the alternator. So with the engine off, 12.4ish and with the engine running no change.

3)  i test lighted both wires in the plug that plugs into the top of the alternator (terminals 1 and 2   or "R" and "F"). With the ignition on; Both wires have power, but the smaller brown wire (terminal 1 or "R") is dimmer than the larger red wire (terminal 2 or "F").

So, i used the voltmeter; The larger red wire has 12.4-8 ish while the smaller brown wire, 6.3-5 ish.

4) i jumpered terminals 1+2 while the engine was running and the plug plugged in. The battery stayed at 12.08 V

And as far as i can tell the gen light stayed on, again seems 1/2 lit compared to brake, oil, choke lights.
 

Next?

[bd]

Unplug the two-wire regulator connector.  With ignition on does the charge telltale extinguish?

[Stewart G Griffin]

Thanks for the attention.  i probably can't get to it until next weekend. 

i have another car i can use and in fact have transferred the battery over.  So at least we know the battery is good.