Today while on a 400 mile trip with only 25 miles left, the check gauges light comes on and the voltage drops to zero. I drove on to destination and plugged in the charger. It showed 12.1 volts and I charged it up to 13.5. Started the truck and now have proper voltage.

A week or so ago, I grounded out the alt while changing the oil filter and sparks flew. I figured I had fried the alt, but I drove it on several 16 mile trips to town and it seemed fine. THe voltage would start out somewhat below the middle of the gauge and then after a few minutes it would be up at "normal". Probably after the grid heaters turned off.

I'm no mechanic, but I would assume if I'd fried a diode it would show up immediately, not after almost 400 miles.

History; the truck has 230,000 miles of which I've put on 150.000. It has only been driven maybe 5000 miles in the past year, but last fall I was having trouble with the voltage gauge only reading about half way to normal and the check engine light coming on. Then sometimes it would read normal. It was during 20 degree weather and the forum concluded that I might have a grid heater relay going bad. I have not replaced it. I did replace the alternator and that seem to take care of it until now.

I have to drive back 400 miles on Sunday. What should I check (or should I say "have checked", since I'm not a mechanic and I have no tools with me anyway).
Thanks.

 

Take it to just about any auto parts store they will test the system for free...most likely you did indeed fry it and the dual batteries held out OK on the short trips until now.

 

You most likely have a blown 140 amp fuse in the PDC. The charge wire goes into the PDC and then to the battery. If the alt is shorted this fuse is most likely blown. Ask me how I know, you should have power both sides of fuse when engine is not running, use a test light.

 

Big iron beat me to it, the usual dead giveaway is "I shorted out the battery lead on the Alt. and sparks flew"

LOL, I know exactly how you know Bigiron, the same reason that I know..............

 

Yes it did happen to me once many years ago. I did install the terminal protector on the alt. stud so it could not happen again. Very easy to do if you are not careful removing oil filter.

 

Thanks for all the replies. I'm reasoning that if a relay or fuse blew that the truck would NOT charge up and show normal voltage on the voltmeter later on as mine did. Possibly the voltage regulator is just working periodically?

 

A new alternator fixed the problem. There was something rattling around inside the old one.

 

You got it, and the big batteries we have on these things will let you run for quite a while without charging.

But I see in your case you probably blew a diode inside the Alt. when it shorted.

If it ever happens again, check the 140 amp fuse first, most times it is what goes.