Good evening all,

1966 f100 352, bone stock.

As of today, when the truck is started and driven the generator/alternator light in the dash stays dimly lit, very dim.

I went ahead and pulled the negative battery terminal while running and truck stays on, so thats good.

Would the dim light mean a current draw of some sorts?

Belt is tight.
Had alternator tested 3 times recently and is good.
Starter solenoid is new.
Starter is new.

Any ideas what to check?

Thanks,
Aaron

 

Check your grounds and regulator connector. Just went through this. Clean the mounting surface of the regulator and add a dab of dielectric grease. Next, pull the regulator connector and spray with contact cleaner. Make sure the connector is a snug fit.

Check your battery ground and make sure it's good and clean as well.

 

Thanks, will do.
Grounds are all good.
Just made and replaced all grounds.

-gauge cluster to cab
-cab to block
-alternator/block to frame

Going to check volts at battery when truck is running, triple check alternator is working.

 

Light on means charging isn't. First check voltage of battery itself, since it may not be charging or only partially charging. Use charger as necessary.

Check charge voltage at idle spec, and at 2000 RPM with headlights and heater blower on. Report back.

 

Truck off = 12.81 volts at battery
Truck on at idle = 14.21 volts
Truck on, 2000 RPM, headlights and blower on = 14.37 volts

Ambient temp is 74 degrees.

 

Those numbers don't look too bad. Wondering if a diode failure will cause alternator light to illuminate. Hm. Just thinking out loud. The manual mentions "battery won't charge" but not "alternator light on, everything fine".

Is the light on when truck is OFF?

 

Light is nice and bright when key is in the on.

Goes off when key is turned on to start truck.

Then stays dimly lit while truck is running.

Light is off when key is off.

Weird.

 

Everything looks perfect, except the dimly lit part. Hm. Maybe for grins, connect a heavy jumper wire from negative battery post to a clean, unpainted spot on the alternator case. Measure charge voltages again, see if charging voltages go up (or light goes out) Try same on positive side too.

Sure seems like a fault in the alternator of some kind though. Gotta be. Maybe a diode? There are six total. Should be a way to test for backfeeding or an AC component over the DC voltage. The shop manual I have has all the different resistance tests, regulator tests, but doesn't really mention the alternator dash light itself, as part of the troubleshooting.

 

I think after all your tests and work done that Tedster9 is right about one of the diodes in the trio is/has failed.

We had a dedicated charge rate tester with no gauge, just 4 lights. Undercharge, Normal, Overcharge, and Diode. If the diode light light came on we replaced the Alternator.

Might a system tester at an Auto parts store that will come out the door to test your vehicle have a Diode test on it, I don't know but worth asking. Or online search of how to test Diodes in an alternator like this.


There are usually 3 diodes in and alternator and a failure of one may still allow the alt to charge, but failure is looming

 

Well the alternator bearings are getting kind of loud, it may be the original alternator, stamped with "motorcraft" on the backside.

I recon it's served its time.
$45 for a new alternator from local parts store.

Maybe it's just time to replace?