My issue started over about a days time. Instead of firing up quick the engine is turning over like the batteries are low. The volt meter in the dash is reading almost down to the left line on the "normal" range when I turn the key to the ON position. Then when I try to start it, the engine turns over slow for a couple seconds then speeds up a second then starts. The voltage gauge in the dash almost immediatly goes up to the 14-16 volt range. The truck will start the same way every time regardless if it sits for 10 seconds or 10 hours.
I had the batteries tested at Napa. One was at the proper amperage (maybe 850) and the other was around 680 which is low but the tester said the low battery was "Good." The batteries were disconnected and tested seperately for accuracy. I have heard people just running one battery so I did not know if this would be the culprit or not. The terminals are all tight on the battery posts. I think that covers everything.

 

Check all the cables/connections, replace if needed. I'd advise replacing both batteries, not just one. If you R&R just one at a time, you'll still have two unequal batteries in the truck, and the lesser one will continually try to suck the life out of the better one.
And yes, it is possible to use just one battery, and in fact, Cummins says one 850 will start this engine. But that doesn't take into account hot/cold outside temperature extremes, or lots of start-up/shut-off cycles during city driving, which is why our trucks have two. HTH

 

I think that you may have a bad connection in the starter...that is, the plunger may not be making a good contact with the washer that ensures the connection. I can't think of the correct terms right mow, but there is a member on here who makes a heavy duty part to remedy the situation (LarryB?).
OTOH, if one of your batteries is low, they both are. It is the weakest link theory. Try disconnecting the weaker of the batteries and things may work well
Good luck

 

Most places cannot put enough load on a big battery for a diesel to rule out the battery. Just because the voltage is there does not mean that you have a good battery. Take it to a battery shop & have them check it out.

 

After checking all the above and all is ok, then it's probably the starter contacts. I had the exact same issue, fully charged good batts, and a good working alt, I swapped out the starter and no probs since. I think Geno's has a heavy duty starter contact replacement. A new starter is around $200.

 

Has to be the starter b/c I went to start the truck Fri morning and NOTHING. All electronics worked but not even a click when I turned the key to Start. Tapped on the starter with a hammer and the truck fired right up! Is that the contacts or something else in the starter?

 

If the main (+) connection is tight, I would think you need to replace the contacts and/or starter. Pull it off and have it tested.....although just testing it on a bench tester may not show that it's bad because there is no "load" being placed on it. I would say the starter contacts are your problem.

 

ohh, yea make sure to disconnect the batts prior to removing the starter as the main (+) wire always tends to fly around and mate with (-).....unless you like fireworks shows.

 

The starter bolts can be tricky to remove. You need a special socket, 7/16, I think, that has 12 points...you better check, since I am having another sr. moment.
george

 

I'll check into getting into the starter to check the condition of the contacts. Would Larry B's be my best option?

 

Thanks for the reminder. Not to mention that joker is heavy.

 

Didn't have the time to do a rebuild so I just replaced the whole starter.

Starts up in less than a second or two.

Problem solved!