1979 Voltage Spike

[thumpernickle]

I am not much of an electrician and I am puzzled by a problem I am having with my 1979 Chevy pickup. My truck starts up fine in the morning. I drive it about 4-5 minutes with my factory installed voltmeter reading about 13-14 volts. After the 4-5 minutes, my voltmeter jumps up 17-18 volts and usually stays there for the rest of the drive. Sometimes while the engine is running it will drop back down to 13 range and then jump back up, other times it stays pegged at 18. The headlights seem to decrease in brightness when the voltmeter reads 17-18 volts. When I have driven it like this for more than 10 minutes and then immediately try to start it, it is dead electrically except for the lights. If I let it sit for 15 minutes I have electricity again and it will start.

My battery shows signs of overcharging. It is obviously leaking fluid from the top (it is a top post battery). The positive battery cable clamp has turned black.

I had my alternator and battery tested at Kragen and the they tested "normal."

Any suggestions of where to start looking for problems?

[68longhorn]

When the alt. was tested was it acting up? Sounds like a bad regulator.

[Edgel82c10]

Just had the same issue on my 82, took off the alternator had it tested and it tested fine. put it back on the truck drove when it warmed up it jumped backed to 18+volts. Mine has a internal regulator that when the alternator warmed up it was malfunctioning, replaced alt and im back in the 13 to 14volt range.

[thumpernickle]

68longhorn: The Kragen guy said the alternator tested "normal." This test was done with the alternator on the truck, engine running.

Edgel82c10: Ok. I haven't had the alternator tested with it off the truck. I pull it and see what the test results are. But you are saying that your old alternator tested normally but was still "bad", correct? I guess I will get a new alternator and put it on and see what happens.

Thanks for your comments.

[Edgel82c10]

Mine tested good on Autozone's machine they did a total of 3 tests. I put it back on the truck ran it up and down the road and when my motor got to operating temp it started over charging. Took the alternator off and went back to Autozone they checked to see when I purchased it from them and on 09/20/1998 was original purchase date and they said they would exchange it under warranty.

[Goldfinger]

Thats odd mine has period when the voltage won't jump to the correct level even when its running. a spike could be a bad volt regulator in your alternator.