F350 Diesel Alternator Question

UP Oliver

Member
Hello:

I have a question. I have been having problems with my F350. I got new batteries, but have been running them down when I plow. I got a Boss V Plow. I did a test on the alternator as described in my Chilton book a couple weeks ago and it passed. The truck has been fine until yesterday when it died when I lifted the plow. I checked the batteries and I think they were at 11.6 I charged them and had 12.8 or so when I went out there today. After start up it was at 11.9 and only got a little higher than that, stayed under 12, which to me means my alternator is shot. I plowed snow in the yard for 5 minutes and shut it down. I had 11.75 volts then. I went back out 2 hours later and had 12.4 volts. Can anyone explain that to me? Where does that voltage come from to go from 11.75 to 12.4?

I started the truck and it went down to 11.8. Ran it for 10 minutes and shut it off. It immediately went a little over 12 and now two hours later it is about 12.3. I don't know how I am gaining voltage, but I see a pattern here. I would guess I have a few more starts and will be in the same boat where it doesn't start.

Any advice on where to get a good alternator?

Thanks in advance for any help.
 
Check and make sure that you did not blow the fuse that powers the voltage regulator on the alternator. It also powers the charge LIGHT. Smart move by Ford. I had the alternator go out on my 2000 F350. When I installed the new alternator it would not charge. I fooled around for a good while figuring out it did not have any power to the voltage regulator. Checked the fuse block and sure enough one was blown.

Your truck is not charging as it should be up to around 13.5-14 volts when running. The voltage is rising after you shut it off because your not putting any load on it and the chemical activity in the battery is making it rise a little after it is shut off.
 
Several items: The system is not keeping up with the load of the plow draw. If the new batteries can be charged using a charger to 12.6 and then they work well to start the truck they may be OK. New batteries can be bad. Is the cranking amps of the new bats the same or larger than the original? If they are appropriate, the charging system may be at fault. The battery voltage should be 14.2 to 14.6 volts with nothing turned on but the engine. The voltage may be 13.5 or so just after the startup. and get to 14.2 within 5 minutes. Is the drive belt tensioned correctly? Is the belt worn and bottoming in the multi grooves? Is there a voltage drain in the circuit when the truck is off? Is the electric motor on the plow hydraulic system drawing more than allowed. I also believe an amp meter that is hand held and pressed against the wire, (cheap) will allow a clear measure of alternator utput in real amps. Jim
 
you need to have the batteries load tested. you didnt say how old they are. a battery can be old and just have enough amps. to start the truck but will not handle any extra load on it. and a dead battery will gain charge just by giving it a rest. with the truck running you should have at least 14.5 volts showing on your meter. for the extra load of the plow its best to get new batteries if those show borderline.
 
Plows put a heavy load on the electrical system. The extra load could have taken out the alternator, or could have blown a fuse or fuse link. Check the large battery terminal on back of the alternator. It should have battery voltage.

One thing that can be confusing in cold weather, the glow plug circuit stays on longer. If the glow plugs are on, the battery voltage will be drawn down, giving a false indication the alternator is not working.
 
First check your battery cables where they hook up to the battery, mine did the very same thing after the guys put all new brakes on it. They did not have the positive cable tight.
 
I have a live data reader on my 6.0. The system is in exellent condition "BUT" for 90 to 200 seconds after start up,voltage hoover's around 11.5 then jump's to 13.9+ when glow plugs go off. I suggest you plug a ScanGuageII into the OBD port so you know what's going on all the times. You can keep it and use on next truck plus it will tell you soooo much about your truck. If batteries and alternator are good,you may have to install dual alternators if voltage never get's above 13v. You should have a tender on batteries 24/7 while sitting idle in cold weather.
 

Battery voltage increasing after shut down is a good sign the bats are good..

Pull up a wiring diagram on the net and go from there... The bat terminal on the alt feeds directly to the bat positive but has at least one splice in it. I spec the alt is feeding a relay on the fender the relay is feed direct battery voltage to one stud on the relay on that stud is were the alt ties into the battery. From that stud on the relay to the alt you will find several fusible links, two of those fusible links will have a splice were the alternator ties into them I am going to guess about a foot from the relay.I have seen the splice corroded into.

The tip off is no power on the bat stud at the alt and a good place to start... You will have to open up the wiring harness to find the open if no power at the alt and power on the bat stud/fusible links
the fusible links will be the big ones attached to the bat stud at the relay/solenoid. If that's your problem start you surgery on the relay/solenoid side to find the open..

Once you pull up a wiring diagram you will find the system is simple...
 
Batteries are likely gaining voltage because they are stabilizing to their nominal voltage after current draw ceased. Normal for any battery system. My 94 Ford diesel did the same last year. Starter an glow plugs bring down batttery voltage pretty fast. I use Chinese alternators and starters. I've had 100% success with them. I see no reason to rebuild the old ones anymore. My 130 amp alternator is "Pure Energy" brand and cost $85 brand new.

At least when my diesel F250 had trouble, I knew it via the voltmeter.

My 2001 Chevy Tracker only has a charge-light and it died recently due to low voltage and no charge - yet the no charge indicator never came on. I was going down the highway unaware when the voltage got so low - the fuel injection stopped working and it died.
 
Hello.

Thank you for all the replies. To answer a few questions.

The batteries were purchased and installed in August, Motorcraft from the Ford dealer like the ones I took out.

A few weeks ago when I tested the alternator the voltage jumped up to over 14 a couple minutes after I started the truck, and then I turned everything on and still had voltage well up above 13, which was over what the voltage read before I started the truck for that test. Yesterday is was still under 12 volts 10 minutes after I started the truck.

I have checked the drive belt. I should replace that since it is original on a 2004, but from what I can tell it looks good. I need to check the tensioner today. Have not done that. And from what I can tell there is no voltage draw when the truck is off, I tested that a couple weeks ago also.

Thanks again for the help.
 
I did. and I mentioned that the belt can look good and be worn so much that the ribs touch the bottoms of the grooves. they will slip when this happens. I will ad that Gates company has a plastic handle with a sprig of plastic sticking out of it that fits between the belt and the bottom of the grooves. If the sprig of plastic fits the belt is OK if not it needs replacement. A 1/16" dia. welding rod can be used in its place. Jim
 
When you're troubleshooting a problem and you see something that doesn't make sense, back up and re-check.

First off, are you always checking the voltage at the same point? e.g. from positive to negative battery post? The voltage is NOT the same everywhere in the system; when the batteries are charging there will be a significant voltage drop from the alternator to the batteries and possibly from each battery to ground. Look for voltage drop in the system; a marginal ground connection can cause a lot of aggravation.

If the alternator is "weak" and not charging at capacity, it might have a blown diode. If one phase of the alternator is knocked out it will still charge but at a greatly diminished capacity. Unfortunately, it's not easy to check the diodes without disassembling the alternator; it may even be necessary to desolder some connections.
 

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