Jaime,

You asked what size alternator you need...the answer depends on what accessories you have.
First let me tell what happened to me and this may help answer that question:
I bought my truck new in 2006 and everything was working fine. Within a couple years I started seeing symptoms like:
- speedometer did not return to zero
- Security system would trigger (or no apparent reason) while the truck was sitting.
- Dome light would turn on and off while I was driving. Eventually, the dome light stayed on permanently.
The symptoms got steadily worse until about 3 months ago, when the vehicle would not start after sitting over night. It would click, but not turn over. I could charge the batteries and it would start fine for a day or 2 but then, one day it would only click. This was on a new set of batteries. To get to the chase, it was eventually found to be a short in the gauge cluster. I replaced the gauge cluster, recharged the batteries and it's been starting every time for the last 3 months, whether it sits for a day or even a week.
The reason I bring up this story is because I bought a DC Power 185 amp alternator and intended to install it, but first I fixed the 'minor' short in the dashboard before I installed it. I still have the original 105 amp alternator in the truck (the DC Power is on my bench) and it starts up every time without a hitch.
My truck does not have a lot of accessories on it, but I was having starting problems, similar to yours until I found the minor short in the dash.
Now, having said this I believe the original alternator is too small. The problem with the 105 amp alternator is that it takes too long to recharge the batteries on the diesel trucks, just after starting them. This is not good for 2 reasons. First, it takes so long to recharge that the batteries that they may never get fully recharged if the vehicle is only driven short distances. (I've noticed it takes about 25 minutes to fully charge the batteries). Secondly, the glow plugs on the diesel trucks stay on for about a minute, even after the truck starts. They take a lot of current, even more current than the 105 amp alternator can supply. As a result, the batteries are still being discharged, until the glow plugs turn off.
Both of these issues can be improved upon with a larger amperage alternator. And it's important the alternator put out sufficient current at idle to help offset the large draw of the glow plugs. I believer the DC Power 185 amp alternator puts out over 100 amps at idle, whereas the 105 amp alternator puts out only about 60 amps at idle.
Sorry I don't know how many amps the 140 amp alternator puts out at idle.

Hope this helps.

P.S. If you a lot of accessories you may need a larger alternator. A high output sound system is an example.
Also, if you can wait about 2 minutes after starting the truck, to be sure the glow plugs have turned off, this will help lessen the draw and reduce the stress on the batteries. Installing a voltage meter in your accessory port will help you determine when the glow plugs have turned off...the voltage will go up from less than 12 to over 13.

 

Ok so I did the test bustedknuckles asked me to do this is what I got.
Voltage 1 ...11.60
Voltage 2....11.69
Voltage 3.....11.62
Voltage.4 ....9.56
Voltage 5.....9.31

 

I jump started the truck took the jump cables off the batteries slowly start to drain till the truck shuts off. I just can't figure out what is draining the batteries my alternator output is 140 could that be the reason why.

 

That's exactly what this truck does it'll run about 2 days about the third day my truck is dead no click nothing I can barely unlock the truck with the keyless. My speedometer seems to be working fine, the dome lights flicker a bit but that's when the trucks about shut off. I have no stereo system in it just the original it came with, no other accessories have been added to my truck. I appreciate the help on this I'll look into getting the gauge cluster checked or replaced.

 

OK,
Let's make sure we nail the problem for you. I know it's frustrating, but even worse would be not hitting the nail on the head.

So, sorry to harp on this, but, were voltage 1 and voltage 2 measured with the power cable removed? It's important.....

And, just to get you going on the next steps of the debug process.
Disconnect passenger battery positive lead, then connect a charger (to this battery only) for at least 4 hours.
Reconnect passenger battery, then disconnect driver side battery, then connect a charger (to this battery only) for at least 4 hours.
Then Reconnect driver side battery.
Check voltage and record this as voltage 6.

Start truck once a day, see how long it takes to drain to the point where it won't start again.
I'll try to respond faster each time you post...hope this will speed everything up.

 

If voltage 1 and voltage 2 were measured with the batteries disconnected, then the voltages you posted indicate your batteries are at least seriously drained. While disconnected, they should read at least 12.5 volts....fully charged, and measured before they've been connected, they should read close to or above 13 volts...new batteries, fully charged read 13 to 13.1 volts (mine read 13.05).
If, after charging (but still disconnected), your batteries are not reading above 12.9, you should take them both into a parts store and get them load tested.
And even if you batteries are bad, you may still have a short to find.

 

Ok Bustedknuckles so charged both batteries at four hours at a time. The truck still had trouble starting but finally after a couple of cranks it started. Ran the truck all day yesterday with no problems. Started this morning no problems got home from work, truck is dead. Jump started the truck drove in it for a couple of hours went home waited an hour went to start truck dead again. Both batteries were both load tested an tested good. Voltage 1 and 2 were done exactly how you asked me both were at 11. At no point did they ever read 12 or above.

 

Jaime,

Did you measure the voltage on both batteries, while disconnected, right after they were on the charger for 4 hours?

If they do not both read significantly above 12.5 volts (while disconnected) after charging, they are bad, no matter what the person that load tested them says.
Where did you get them load tested?
The way they should have load tested them is by placing them on a carbon pile for 15 seconds. If, after they have been on the carbon pile load for 15 seconds, they read less than 12 volts, they are bad. What happens with a marginal battery, is the voltage holds up for some amount of time, but eventually (before 15 seconds transpires) it starts to drop below 11 volts and this means the battery capacity is failing to meet spec.

So, even though I would like you to tell me who load tested your batteries, I suspect they don't know what they are doing and do not want to replace your batteries under warranty...just a guess.

 

Ok I took the batteries to autozone to get charged an load tested not really sure if they tested them on a carbon pile, they told me they both held charge at 12.5 after getting them back i tested both batteries myself an both read 12.5 volts. So I went ahead an connected them up to truck, left it running for about 20 minutes. Checked voltage on my batteries while running both batteries tested at 11.5 volts what in the world could be draining my batteries. I turned off the truck waited a bit to no surprise truck won't start batteries dead.

 

12.5 volts after a full charge is not enough; the AutoZone that I took mine to measured my voltage after charging them and they showed 12.5 volts (after they charged them). They then put them on the battery tester and they failed the load test; they did not hold voltage for 15 seconds. They guy told me mine tested at 85% cranking capacity, which he said fails the test.
The real test is to put the batteries on the tester after they charged them. The voltage after they charge them is not a complete test.
I suspect they will not hold voltage for 15 seconds.
By the way, I think I told you this, but my new batteries showed 13.05 volts after they were fully charged (but before connecting them to the vehicle). The voltage will drop almost immediately after connecting them...but this is fine.

Another thing to realize; just because you have bad batteries does not mean you don't have a short.

At one point, I had both.