I have tried to find an answer on fte but I can't. I had a battery light come on, took the alternator to the shop, had it tested and he replaced the brushes and regulator. so when I put it back on the truck I was shocked to see the battery light was still on. Took it back off and took it to the parts store had it bench tested and was good they said. so then I took the truck up to Napa and had the batteries tested. They said the batteries were fine but needed charged. I chared the batt. for about 1 and half days on 2amp charge. I still have a battery light, but it only goes out when I unplug that 2 wire plug on top of the alt. The batt. are 3 years old but don't want to spend 200 buck on new ones if that is not the problem. The truck starts and runs fine. It is a 2000 psd auto 4x4 w/ 141,000 miles on it. any help would be great

 

I had a new alternator bench test good but I insisted on a new one because it was only about a week old. The light went out.....I know this doesn't help. It is just more of a point I don't trust autoparts store alternator testers

 

What voltage are you reading at the battery with the truck running? It should be around 14.0-14.3V.

 

I actually had a new Auto Zone alternator that tested good at the store, but the voltage would spike up to 18v avery so often. I had them replace it and no problems since. Apparently it's possible to get a bad alt or regulator.

 

The diodes in the rectifier circuit can go bad and I don't think they test this. It's possible that you could have a short somewhere else.

 

Check your wiring from the alternator.. especially the two small wires out of top.. Mine shorted to the alternator housing and caused the battery light to come on.. Another area to check is the main wiring harness that goes over and lays on top of the driver's side valve cover.. From rubbing, you can have bare wires..

 

Get a measure on the voltage when running. That will really tell you if the alternator is actually going out or not. Also, what is the voltage on the output post of the alternator when not running?

 

i totally agree with clint above, have seen this problem. but most likely its the alt. use a dvom and measure the voltage at the alt , when off and the running, also measure at the batts.
if you have access to an alt tester, probably at napa or like store then test the alt while on the truck.
hope that helps.
cheers.

 

When the truck is running its only showing 11 volts at the battery I will go check what it is when it is not running at the post on the alt. I don't know what a dvom is, I just used a multi-meter

 

I also have 11 volts at the post when not running. I won't be able to check for a bare wire untill tommorrow.

 

I just went through this my self, not saying to rule out a short in a wire however I replaced my original alt with a remanufactured alt and it was bad as well. Showed about 14.3 volts at the battery and tested good at NAPA's fancy tester but my battery light kept coming on. After about a week I took it to my local hole in the wall NAPA and had it retested on a 1950's or so tester and he said it could be bad. I spent a little more money and upgraded to a new Alt and my light has been off since. I am told it's pretty common to get a bad reman alt although the parts stores don't like to fess up to it right away.

 

well mine is the same alt. that has always been on the truck. I took it to a electric shop and he tested it and said it was not doing anything. He said give him an hour and he would have it fixed. He said that a snap-ring broke and caused one of the brushes to wear out. But the brush holder is also the regulator, that must be replaced. It cost me 50 bucks. But it may be still bad.

 

digital volt ohm meter, sorry man, its a multi meter. 11v when running is why your light is on. if its not the alt, and it most likely is, then the excite wire isnt getting a signal to the alt at start up. id say try another alt if you can. but if you jump from the "b+" terminal to the excite wire terminal while the truck is running and the alt starts charging to 14+ volts the alt is ok. most mechanics will be able to tell you how or maybe even the napa store. check that the terminals on that plug are not moved out alittle. but i doubt they are as you said that unplugging it makes the light go out.
it probably the alt.

 

Had this happen to me as well. Checked all wires, replaced the connector on the alt and still lit up. I took it to AZ and they tested it on the truck and said it was good. Two days later I read somewhere where they said take it off and let them test it. Well AZ couldnt mount it in their tester and I said screw it and went with a BRAND NEW alt and the light hasnt come back on since! It was well worth the extra 30 bucks!

 

is the light on with the glow plugs on or off or both? the gp draw considerable amperage, easily the largest draw on a powerstroke short of a snow plow or similar hydraulic accessory. If you keep having trouble check the draw of your glow plugs.