2015 VNL780 Starting issues

2015 D13 I shift 133k

 

Suggestion,just above the battery you have the master switch with two big studs on the back (where the big electrical cable are connected),for a quick test put your fingers on those studs to feel there temperature, start the engine two times between 5 second, check the studs temperature again with your finger.
If you feel a small heat it's normal but if you can't keep your finger on those studs because they are to hot then it showing you your master switch is to much restrictive and need to be replace.
On Volvo with a I-Shift transmission the starting circuit is complex a little bit. I already spend a lot of time to full understand the complete circuit. (see explain below)
You have a little relay bolted on the starter motor, the coil of that relay is (Ground controlled) by the Engine EECU and the positive side (feed) for the starter relay is come from key switch but in serie of that there is a micro relay (position R-13 in tec tray) who is (coil) ground conttroled by the VECU to confirm the neutral position.
The positive side of the R-13 relay is supply by a fuse.
The VECU and Transmission ECU (TECU) are talking together over Data Link.
The clue that i have seen before is: If your battery master switch begin to be defective when you turn the ignition switch at start position it generate a quick voltage drop (peek) but the ECM modules (EECU,VECU,TECU)will feel it and cancel the cranking operation.
Let me know what you will find.
Hope this can help.
Ben

 

If you run in the rust belt, The battery and inverter cut off switch lever on the right side of the battery box is starting to short out. like benrab said, throw a heat gun on each of the red cables going to and from the switch in question, if the temp. is high then the out side temp more then 15+ degrees on the cables going to the switch is bad. You check with a volte meter, turn the lever in the off mode. check if you have any voltage at the red stud on the back, if you do, the switch is bad.

 

if starter/solenoid replaced big chance could be starter relay

 

A quick test that i can suggest to you,remove the big cables on the back of the master switch clean terminals and install them under the same stud to eliminate the master switch for a short period of time then you will have the answer shortly for the master switch condition .
Low cost suggestion.
Ben

 

Should have no volts at the off position none. Switch is bad.

 

I believe you are looking at the wrong place. Those autoshift/ishift transmissions have a pretest sequence that will disable starter if not satisfied. Even if you start the truck shorting the relay on the firewall, the transmission will not engage in gear. Usually problem is with green/yellow ECM wires. Since they run for tens of feet it is not easy to pinpoint exact location. Good luck