Bad batteries?

Alternator

 

2nd vote for alternator. Or loose belt. Most likely Alternator not charging.
Also check the fluid level in your batteries.
I just tested my batteries and 3 of 4 tested weak. Check acid level and it was low. Charging was OK and now all the batteries are holding charge as well.

 

Regulator? Yank off the alternator and take it in to a shop, let them spin it and see how many amps it's pushing out.

 

Answer to question # 1 is yes.

 

Worked for a company several years ago and my truck was having issues after being serviced by said company. Alternator got replaced. Same issue. Batteries got replaced. Same issue. Alternator cable replaced. Same issue. Then a greenhorn mechanic with less than six months experience noticed that the batteries were hooked up wrong. Problem solved. Point is...... Check everything. Again.

 

May trouble shoot better if you can test output while another buddy turns on lights/ heater fan/ups RPM - I'd guess slipping belt under load.

If you do buy batteries get the 740 CCA, not 1000 CCA - less plates, less chance of corrosion , last longer. Buy from Freightliner - brand that is warrantied by any TA/Petro.

Load test batteries.

 

To check if your alternator is working, after you get truck started, disconnect the negative posts. Truck should keep running. If you disconnect positive posts the motor will die if equipped with those small power leads to ECM. Well, that's what happened to mine.
I had the 750 CCA batteries. They lasted me 1.5 years. I bought those last time because that's what was available.
I had 1100 and now I have 950.
The battery store owner explained the batteries all have the same number of plates. The difference is the thickness to the plates and the purity of the lead. He explained batteries from many moons ago. Were the same size but only rated for 550CCA. This allowed for more acid which also acts as a coolant during the charge/discharge cycle.
When you solve your problem, let us know. Thanks. Good luck.

 

get a cheap load tester. one with heating coils. batteries could all read 12v but you may have one that drops off under load. if not I would check all the cables. check the ground connected at the frame. most of the time that's where it would be. disconnect it and clean the connector and the frame. check the cables going to the starter and wires going to the alternator. look for cable swelling. that's a sign the cable got to hot. if all looks good take the alternator and have if tested. make sure all your battery's are all the same CCA. one battery higher or lower can damage the rest.