So from what I've read when you start the truck only one alt works till the GP shut off then they work intermittently base on load.

I just put in two new alternators and I still get only 12v after I start it and before the glow plugs shut off. Once they shut off it's 13.5-14v solid.

So I'm thinking that the harness may be chaffed but which alternator should I look at? Top one or bottom?

There isn't much info on dual alts on the web.

 

The dual alternators are a 140 upper and a 100 amp lower. They don't come on commanded by the PCM until after the GPs shut off, that is why the low voltage at start up. They are charging, just not full productions. Ussually you are at idle. Then as the primary gets warm, the PCM cycles the second one on and turns apoff the upper. It cycles back and forth based on load. Sounds like your alts are working properly. If you do a search in hunks section, you will find a lot more information on this. I have upgraded my single 140 amp to a single 230 amp Prestolite model alternator. I can hear it cycling early after start up, but not on full power until the truck warms and RPMs are higher.

Also, how are you measuring voltage, at OBD2 the voltage is often lower by about 0.50 volts or so. What do you get amd the charge lug at start up and from each alternator, when cold, hot, high amd normal idle? The alternator at initial start, idle should be at about 14.4 volts, it will ramp up to 14.8 on full charge. Best way is to measure amps at the alternator, but you need a pretty good DC amps clamp meter to measure. At initial start I get about 80 to 100 amps, I get over 190 at idle after GPs off, but this is my single alt It really depends on how low the batteries are too. My original 149 would show about 120 after GPs off, in amps. 14.4 volts. Rarely seen more amps oops 14.8 volts at the alt pos lug.

 

Elyrain, Yours are working just like mine.....exactly the same and my batteries are 3 years old and in good shape. No worries.

 

In order to see 13+ volts at the OBD port during GP cycle a minimum 230 amp alternator would need to be installed.

Anything less than that struggles with the amperage demand.

Josh

 

Sounds about rightBullit, I see 13.2 at the OBD2. I saw 12.9, rarely 13.0 before with the 140 amp stock. It would take 30 plus minutes to hit 13.1 and never did on short drives, especially in our colder weather. Which is why I did this upgrade. Stock alt made it 7 years though, that's pretty good. Batteries made it 7 too.

 

This is not true for the first alternator, only the second one. The first alt will come on as soon as you start the engine. A lot of people think it's not on because their alternator simply can't keep up with the load.

Add up all the crap that runs in a normal truck:
Glow plugs - 60-80 amps
Lights - 5-10 amps
Injectors - 1-5 amps
Computers - 5 amps
Heated mirrors - 10 amps
Radio - 1-5 amps
HVAC blower - 10-30 amps

And more. Add all that up and you need at least 100 amps, if not more, at idle. If you have less than that your alternator may appear off, but it is not. It's just struggling to keep up. Stock alternator is a joke.

I put a 250 amp alternator in my 04 and voltage goes up immediately, while the glow plugs are still on.

 

In the fleet we found haveing enough battery cut most of the alt. problems out.
We used industrial grade batteries of no less than 950 cca, 1000 when we could get them.
That and cleaning cables and terminals every 90 days. A commercal fleet tho bigger budget.

 

Yes, bigger batteries are good. But a fleet unit is going to run a lot more and be able to charge when it needs to cause you are not paying for fuel. Whereas my truck is a daily driver and it's the short trips in the cold weather that is why you need a bigger alt. my 140 did well, later 4 years.

Yes, the loads are true FS357, that is why the alternator is on, at start up, why yo don't see higher voltage, and because at idle, it's only really able to do about 80 amps. I think this is a reason why the cold strategy brings the idle up to 1200 or so rpm, in which case mipy 149 was able to do about 100 amps, but my 230 amp unit hits 190 or so. I can ussually see my alt switch down asp father as little as 15 minutes run time now with the 230.

 

It doesn't hurt to sometimes throw the battery charger on over night. Especially in cold weather. I do this occasionally, and I think it prolongs alternator life, as well it aids cold starting.