Thanks again to all those who helped me rebuild my 6.0. It is doing great except...
I have to jump it off quite often. Both batteries pass a load/stress test, but I've noticed that if I drive it at night, I almost certainly will have to jump it off the next morning, so I suspect one or more of the alternators isn't quite up to snuff. Driving along, the voltage jumps back and forth between about 12.5v and 13.3v. I'm guessing that is PCM switching alternators. This may be a dumb question, but how do you test them on a dual alternator system?

 

2003 Dual Charging

 

Thanks, Jack! Just what I needed.

 

Thanks also..since i also have a dual alt setup..mine are fine now 13.5-14.2 normally..i will save for later

 

Mine are 13.5 and 13.7 at 2000 rpm which is lower than I would like. I'm thinking of replacing the upper one at least. Leaning toward the USA Diesel 170A from FICMRepair. XDP has the 200A Mean Green at the same price but the reviews don't sound too good. But then the USA Diesel doesn't have any reviews so can't really compare on that.

 

I would check the voltage off the alternator's post to the alternator body and then compare downstream at multiple places to see if you have a bottleneck somewhere, both from a positive and negative perspective.

13.5v to 13.7v is not that far off what would be expected with hot underhood conditions. The thermistor in the voltage regulators will drop voltage when hot, so you don't boil off the batteries by design. You can also check the amps your truck pulls under different conditions. With all the electronics on with my truck, it pulls ~100a at the positive cable heading to the CJB. If your two alternators have the capability of over 200a, especially at 2,000rpm, you will not get a higher voltage by going to another 40 to 60 amps; there is already enough headroom. You are either reacting to the designed voltage (if heat soaked), or you have some connection resistance in the positive OR grounding bolted locations.

Some of the aftermarket alternators do not use a voltage regulator that compensates for underhood temps (SAE advisement). So you might get your achieved goal, by buying batteries more often.

 

Now I'm confused (well, more confused than usual). I thought for sure I read that both alternators are never "on" at the same time, that the PCM switches back and forth between them for load sharing purposes.
Edit: Never mind. That's only during the glow plug cycling. But would still be an argument for having a more powerful upper alternator, no?

But your point is well taken regarding a bottleneck. My Edge gauge in the cab never shows more than about 13.3v while my voltmeter was showing 13.7 as mentioned.

 

You didn't read the functionality section. The lower alternator is turned off during glow plug operation as high voltage will burn them out. Once the glow plug circuit is turned off, the PCM allows the secondary alternator to activate, so both are on. The entire point of the Dual Alternator Package is to provide a high amount of amps (upper 140a and lower 120 amps for a total of 260 amps) for situations like emergency service vehicles which draw huge amounts of amps with lights and other equipment. It's not there as a secondary alternator in case one goes bad.

With my single 230a alternator, the Glow Plug Controller constantly jumps in and out because the high amp output (180a) has enough headroom that the voltage goes above the 14.5v threshold at startup. The aftermarket alternator voltage regulator has a higher set point for voltage than the Ford specification.





This is further explained in the first section of the Charging System description, which I didn't include, saying the same as the functionality section with more illustrations.








All OBD2 PCM monitors typically show a lower voltage than alternator output. It's a quirk of the PCM of about 0.3 to 0.5v.

 

You can use the voltage drop test to check both the positive and negative cables.
Also good for checking any cable the carries a heavy current. Like the fusible links.

 

Thanks, y'all, for pointing me in the right direction. I thought I had checked the wiring thoroughly but didn't think to check this spot.


Voltage at the top of the clamp was 12.8, at the bottom was 12.3. Loosening and re-tightening the 4 bolts fixed everything. I guess that's why I'm not much good at electrical problems. I don't understand how there couldn't be continuity with 4 bolts connecting the clamp that is in direct contact with bare wire. It was pretty much a fluke that I even found it. This is the ground side of the left battery.

 

All of those are terrible connections. It's not unusual to have the copper wires compress and move over time and heat. Electrical contact is dependent on the compressive load of the joint. They are sometimes needed as an emergency fix, but should be replaced when they can be. There is enough lay with that cable that a crimped or soldered terminal or lug could be used. Lug if you are using something like a military terminal.

 

The very first time I got my truck to take home on an over the weekend test drive. I had stopped at the local hardware store and when I went to go it would only click.
All the lights seemed to be bright and everything other than the starter worked. Popped the hood and took a look at the batteries. Driver side still had the factory
connector on it. Went to the passenger side and found that the cable had been "FIXED" with a 2 bolt style connector, and it was loose and had white fuzz on it.
Went back into the store after trying to call the sales man. It was too late in the evening they had already gone home. Went back into the store and picked up a cheep
socket wrench set and a brush. Cleaned it all up and in doing so found both of the bolts were only finger tight. Got it all back together and turned the key, and it roared
to life. So when I got back to the dealer on Monday, one of the things in the pre-sales deal was that they would replace the power cables and the batteries.

I would bet that over the years all of us have been at one time dealing with battery terminals that needed to be cleaned.

 

Heh, for all I knew, it was stock with 20 years of wear and tear. 3 of the 4 terminals are those clamp types. Easy enough to replace if I knew what size lugs to get. If the numbers on the wiring diagram are the wire gauges then the engine ground is 0 gauge and the frame ground is 8 gauge. Does that sound right?