Charging issue?

Time to bench test each battery. If either battery fails then replace both batteries.

Then pull the alternator off and have it bench tested. Normal output is 13.5 to 14.5 Volts DC and Less than 0.1 Volts AC. If either are out of spec replace the alternator.

Or it could just be the grid heaters doing their thing! For assurance pedal down to local parts store and have them check both batteries, being sure to disconnect one so you test each separatelly.... they can also check alternator for free. I like pulling alternator and having it bench tested.
So far it sounds normal. Oh! clean all heavy battery terminals.

Thanks for the help! This is my first diesel/cummins so I don't know what to look for. Just for me to stop worrying I'm gonna get the battery's checked out and the alternator checked at my local autozone and I either are bad then I'll get those replaced. And as for the grid heater, it is normal for it to drop the voltage gauge like that? It has been cold lately but this was when the truck was warm and it's been a lot colder than this evening and it's never done that before so that's why I'm assuming it's a charging issue. At first I thought the slow start was the starter going bad but now that this happened I'm leaning towards the alternator. Again thanks for the help I appreciate it!

grids suck 95amps, normal for lights to dim then bright, and fan to slow/speed up,

nickg said:

grids suck 95amps, normal for lights to dim then bright, and fan to slow/speed up,

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Double that... 190 Amps is what they draw. It's 95 amps per element so when both are used then its 190 Amps.

Thanks for that! I had a 1993 Ford Bronco before my cummins and it was a nightmare, everything and anything went wrong with it. I guess I'm just scared about it. But if that sounds normal then it must be ok. But why would it have a slow start? That's what's leading me to think it's an alternator issue

Bnbbwhite said:

Thanks for that! I had a 1993 Ford Bronco before my cummins and it was a nightmare, everything and anything went wrong with it. I guess I'm just scared about it. But if that sounds normal then it must be ok. But why would it have a slow start? That's what's leading me to think it's an alternator issue

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most likely weak batteries

Don't forget starter brushes wear with use. The more wear they get the less oooomph and starting (tend to turn at slower rate). You will need to remove the starter to check the brushes.

Thanks for the info! I looked at multiple forums and I learned that the grid heater is to cause the voltage drop. I'm 19 so this is my first diesel, would have never known. At first I knew I had to probably check the starter, I assumed the slow start up and voltage drop was from a bad alternator, lesson learned on that one. But starter is gonna be addressed. Thanks again for the help!

be sure to have each battery load tested, and just because the starter turns the engine over it needs to turn it quickly

Alternator brushes are in known to go out too. You can check the alt if you tear it apart with just a couple tests. If your a diy kinda guy then taking apart a alternator isn't that bad but I don't know how available individual parts are.

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dozer12216 said:

Or it could just be the grid heaters doing their thing! For assurance pedal down to local parts store and have them check both batteries, being sure to disconnect one so you test each separatelly.... they can also check alternator for free. I like pulling alternator and having it bench tested.
So far it sounds normal. Oh! clean all heavy battery terminals.

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yep ................

Everybody is right above. Grid heaters are the cause of voltage from right after starting. Normal. Poor battery(s) is the likely cause of slow crank and or not wanting to start. Bad contacts in the solenoid is usu\ally the only starter problem and it will click only if that is the case. Cheap easy fix for contacts only. Alternator probably OK. Have the batteries load tested first. Also check and clean all the cable ends, significant dimming can be bad ground or hot connection.

Bnbbwhite said:

Thanks for the info! I looked at multiple forums and I learned that the grid heater is to cause the voltage drop. I'm 19 so this is my first diesel, would have never known. At first I knew I had to probably check the starter, I assumed the slow start up and voltage drop was from a bad alternator, lesson learned on that one. But starter is gonna be addressed. Thanks again for the help!

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alternator would not be a direct cause of a slow start.

have batterys load tested first, if ok
inspect/clean all connections, any green on wires, replace cables
if still slow cranking, have starter tested(check starter bolts have seen a few trucks where bolts are loose and back out)

these starters are pretty robust, usually just the contacts fail, they cost 30$ or so for parts and 10mins to replace once the starter is out, typical symptom is a "click" noise and after turning the key a few times the truck will start.