I recently learned a little bit about how the tire pressure monitoring system works on my 3500 Hi EXT van. With only 120 miles on the clock I drove to Asheville to pick up Polyiso insulation board when I noticed the TPMS light stayed on. Walked around the van and all tires looked good so I continued the 100 mile round trip. That afternoon it was about 75F and checking all the tires I found ~ 66 psi in all of them. My door sticker says 65F, 80R and when I brought the fronts up to 67 and rears/spare up to 80 the warning light stayed on.
I had read about problems with programming of the TPMS on early PMs so I called the dealer (Egolf in Brevard, NC) to have them check out the system. The next morning about 8:30a (about 60F) the technician (Mark) checked air in all the tires and added a few psi to what he said was 67 in the front and 82 in the rear. The TPMS warning light immediately went off (engine idling).
I then checked with my digital gage and got within one psi of Mark's reading. Mark told me that each wheel housing has a receiver and that the rears checked for 80psi and the fronts for 65psi. According to Mark the pressure in the spare is not monitored. The only difference we could determine from the previous afternoon was the cooler morning temperature. I mentioned to Mark that my Nexen 225/75 CT8 HL E tires specified a max pressure of 80psi and he agreed that was more than needed in my lightly loaded 3500 van. The max tire load specified at 80psi is 2,680, which doubled comes to slightly over the rear axle rating of 5,291 lbs. I believe they switched to the E tires on the 2015s versus the 2014's European C type which had a max load rating around 3,200lbs. (Egolf did not charge me although I suppose they could have).
The next morning (about 60F) I set all tires to 70psi and have since driven over a hundred miles without the warning light coming on.
My tentative conclusions from this exercise:
1. The TPMS system is set to warn if tire pressures are just slightly lower than 65psi (good feature in my opinion).
2. The system appears to check for 65psi in all tires, not 80 in the rears.
3. The system may take temperature into account and require a higher pressure at higher temperatures (not sure about this).
4. The van rides well at 70psi and I will only increase the rear pressures if my total weight goes over about 8,000 lbs.
Bill