Cummins qsm11 problem - solved!

I'm assuming your Cummins tech is spending a lot of time on his phone.
Skippy J has valid points that should of been looked at when the tech came on board.
These newer computers, pumps and surrounding relays need juice as far back as DDEC.

Carbon pile those batteries at the connects to the starters and lightly at the ECM B+ taps to ensure their getting good clean +V juice.
There is also a 24 to 12Vdc converter in use if your running smart crap (no relation) displays and controls that may lead the can bus a-stray in errors if there is a problem there.

Keep us up on what is found, were always learning.

Sadly, that does not mean anything.
Check them out and all connects from them to the can-bus (engine harnesses).

Every station has a box where the engines bus cables are tied together and the displays/clutch7throttle control are tied in.
Here is where one connect can cause problems for both engines.

The 21 v sounds low .
My 24 v MAN,s bat gauges are usually 28 v +
For me I would be looking @ the batteries and the connects + earth straps on the engines .
Do you have a x over switch sometimes called a parallel button .
Pressing this during starting connects the domestic bank to the dedicated engine bank .If you have this try it to add more juice when attempting to start .
As well as fuel solenoids requiring a lot of juice the ECM does require a surprising amount too .
Once the starter motor is running there’s not enough juice to power up the ancillary 21 st century’s stuff that diesels need these days .
It’s also my understanding that the fuel pump will not squirt below a certain cranking rpm too- again it may be turning over ,but not quite fast enough for a signal to be sent to the fuel pump ,nethermind if it gets a signal ,there’s not enough juice anyhow for it to work properly.

If you might be losing voltage somewhere, test the voltage across the battery posts (not clamps), and then across the battery clamps, and then as close to the starter motor connection as possible, all while cranking engine. You should be getting the same voltage under cranking load at all those places, and if not you have found the problem.

If one of your new batteries has a bad seal around a battery post, that could be slowly creating corrosion under a clamp and not obvious.

None of this makes obvious sense for both engines to fail, but there are a lot of automatic connections between batteries you might have.

I remember ordering an electric lift pump for one of two QSCs. After the replacement, when I went to light up the second engine, it's pump was dead.
Batteries and all were separate.

It's a reminder that Murphy has a warped sense of humor. I hate that guy...

Happy & Safe 2018.

Those solenoids were likely manufactured on the same assembly line one after the other. Maybe they had a manufacturing problem that day and both had the same weakness at the same level, and you got lucky.