2002 7.3L Charging Problem - Ford Powerstroke Diesel Forum
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Old 02-28-2014, 04:59 AM
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2002 7.3L Charging Problem

Hello all,
This is my first post, so I apologize if I leave out any critical info. I am new to the Powerstroke world! My truck is a 2002 7.3L, 217,000 miles.

I am having a problem where my alternator is not outputting the correct voltage. While driving the other day, after plugging in my block heater for the first time since I got the truck last week, my lights became dim, gauges died, ABS and airbag light turned on then finally all interior electronics died. The batteries were 5 years old and failed a load test at Advanced Auto Parts, so I replaced those. I also replaced the alternator with a brand new (not re-manufactured) one from Advanced Auto Parts. My batteries still will not charge while the truck is running.

With the truck off, I measure about 12.5v at the batteries after charging with a wall charger. With the truck running cold, I measure around 10v (I'm assuming because the glow plugs are on). Once it warms up, voltage goes back up to around 11v, after the glow plugs are off, and then steadily decreases as I drive. I should be seeing somewhere around 14.5v with the truck running and glow plugs off. I can't figure out what is going on. When I disconnect the red terminal on the alternator with the truck running, I measure around 3.2v to the negative terminal on the battery and chassis ground. Open circuit voltage should be 14.5v to my understanding.

Does the alternator ground through the mounting bolts to the chassis ground? Because I do not see a ground wire to go to the battery negative. Also, there is a three wire plug on top of the alternator, of which, only two wires are connected. It is like the alternator is not turning on. Do the wires on the 3-wire plug control the alternator turning on? Could I have fried a ground wire or something with the block heater? Any suggestions on what to check next would be greatly appreciated!! Thanks in advance!
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Old 02-28-2014, 10:59 AM
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Have you checked voltage at the red terminal on the alternator while the truck is running?

There is always the possibility your new alternator is bad. I got a waterpump from Advance Auto a couple years ago that only lasted 2 weeks.
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Old 02-28-2014, 11:04 AM
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Yes, it measures the same as the batteries when running, never more than 12v and it decreases the longer the truck is running with the electronics(lights, gauges, etc.) on. When I pulled the heavy wire off the red terminal on the alternator and measured the red terminal on the alternator, it measured around 3.2v, which was really confusing... The only reason I don't think the new alternator is bad is because it did the same thing with the alternator I replaced, which now I also don't think is bad.

Last edited by dstep2007; 02-28-2014 at 11:13 AM.
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Old 02-28-2014, 11:09 AM
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Sounds like you have a bad alternator to me.
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Old 02-28-2014, 11:23 AM
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Quote:
Originally Posted by Peter@DP-Tuner View Post
Sounds like you have a bad alternator to me.
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Old 02-28-2014, 11:29 AM
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I ran into the same problem a couple of years ago. I went thru three alternators before I read a post somewhere that said to be absolutely sure to charge both batts to 100% before even attempting to start the truck with a new alt. The post said bad or low charged batts will kill a new alt almost immediately. The next alt I put on, which just happened to test bad at Vatozone, lasted 4 years until I totaled the damn truck.

I have NO idea where I read this, it's on the internet somewhere. Any time the batts come out of my truck they go on the charger.
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Old 02-28-2014, 11:30 AM
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also a member of our chapter had a gpr stick closed and that for some reason tells the pcm to kill the alt, so check that just for grins
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Old 02-28-2014, 11:37 AM
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Quote:
Originally Posted by dannanw View Post
also a member of our chapter had a gpr stick closed and that for some reason tells the pcm to kill the alt, so check that just for grins
So is there a wire that allows the PCM to control the ouput of the alternator? Any chance that wire could be bad? I'll try having the new alternator tested at Autozone and replaced if necessary. This cold whether isn't helping much either, it's -2* F here
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Old 03-03-2014, 06:29 AM
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I had both alternators tested, new and old, both tested good at Advance Auto Parts. I found my block heater plug was really corroded and getting hot so I replaced that, which to my understanding should not have anything to do with the chassis electrical system, since it's an AC heater. It still didn't charge after that. I drove it about 15 miles to the mechanics, checked the voltage while I was there, and now it's reading 14.5V and charging the batteries!! I am so confused. Still going to let the mechanic look at it and see what he says. I'll keep you all posted. Thanks for the help!
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Old 03-12-2014, 07:25 AM
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Well, I discovered the problem. Advance Auto Parts sold me the wrong alternator for the truck. It turns out there are two different models, one that is computer controlled and one that is not. They had sold me the computer controlled version which requires a signal from the computer to turn on. Since my truck is not set up for this it did not turn on. Also, found that the wire to the charge warning light on the dash was broken....that's why I didn't have any warning lights. Thanks for the help!
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alternator no charging, batteries dying, charging

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