The Stovebolt Site Forums The Shop Area The Electrical Bay Charging Issues

I have a '52 panel with a 350. It has a 105 amp alternator built by a reputable alternator/starter shop. When the truck is started, the alternator is charging at 14 plus volts (at the gauge and at the alternator. As the engine warms up, the gauge shows (at idle) <13 volts. As the truck is driven more (30 + minutes in city traffic), the gauge shows 9 to 10 volts at idle and 10 to 12 volts (close to 13, sometimes) at speed. I had it home and checked the battery with engine off. It was showing 12.16 volts. When running, it was 9 or 10 at the battery and at the alternator it fluctuated between 10 and 12 volts. Belt is tight. The engine has an electric fan, no stereo, just regular lights, etc. Sometimes after driving for 20 minutes, the battery won't start the truck (starts up quickly with a jump from a deep cycle battery), sometimes it will start again, no problem. Battery? Battery/starter/alternator wiring? Any suggestions to deal with this gremlin greatly appreciated.

Battery or connections. Alternator seems normal from what you describe, factoring in the gauge/needle tolerance and voltage drop. Easiest thing to do is have battery checked, but it is VERY hard to find someone who can do it right. Flaps people may tell you it's OK when it's not. Their little hand held tester in the hands of a person who forgot the training gave me fits. I just kept going to different flaps til I found someone who knew what they were doing. It was a bad battery, it always is.
My opinion comes from :
1. Alternator shows 14V after start, normal. Seems to settle down after a while. Normal.
2. Alternator trying to keep battery "topped up" during driving because regulator sees low volts...... but 12.16V after getting home. NOT normal. So guessing battery. But another warning, bad cell and etc. needs a person to LOAD TEST with a good tester and know-how to use it. I think they hold the load for 15 seconds.
3. Of course 12.16V can be caused by bad alternator also. But rebuilt you say. So battery! sulfation, bad cell, hard winter ....or CONNECTIONS. Corrosion, loose, broken cable.....

If you have done anything recently let us know. Wiring, new parts, noises under hood, age of battery. Just be sure that fan/relay is not doing some over-draw or intermittent shorting out. You could disconnect and test drive maybe.

Try installing a different battery temporarily- - - -quickest way to eliminate that as the source of the problem. There's also a possibility of a diode and/or internal regulator problem with the alternator that only happens when the unit is warmed up. If the problem persists with a known good battery, it's time to dig deep into the alternator/regulator circuits- - - - -maybe use a heat gun to stress the alternator and see if operating temp has an effect on the output.
Jerry

Update

Had the battery checked/load tested by a battery shop. Bad battery. (Around 6 years old so it was time to replace.) Replace the battery cables and cleaned connections. On start, 14.4 volts showing. When fan kicks in, 12.6 at back of alternator and at battery. I'm suspecting the fan, or something with the fan as bartamos mentioned. Or the wire from the alternator to the starter. Or the alternator. I will check with the alternator guy as well and have him check it out.

What output is your alternator? And how much current does the electric fan draw?

One bad diode will cut an alternator output to less than 1/3 of its rated capacity. I've seen a "65 amp" alternator produce less than 10 amps max while still producing normal regulated voltage (14.5+) It would keep the battery charged during daytime operation, but drain the battery with the headlights on. An electric fan will do the same thing- - - -draw more amperage than the alternator can produce, and grab the excess current from the battery. Voltage is only one leg of a 3-legged stool- - - -the others are amperage output and load. Have your spark chaser do a "full current" check on the alternator if he knows how to do that test.
Jerry

Alternator is 105 amp. From what I can gather, the fan (Champion Turbo Series) draws 14 amps. It's basically a stock 350. New Vintage Woodward Series gauges. No stereo. Will get 'the spark chaser' to have a look at it.

It's possible the belt is slipping. 105 amp alt. is a lot for a standard v-belt that is running the water pump & fan also. Try tightening the belt some more & check the voltage.

George