Join Date: Feb 2010
Location: Grass Lake, MI
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P0882 is TCM Input Power Low, which means that the main 12V power feed to the TCM (from the trans relay) has been cut. By the way, you have an ECM and a TCM (separate engine and trans controllers). A PCM is the combination of these two, and was used in 2010 and later 6.7L trucks.
The trans relay is in the Power Distribution Center (PDC) or Totally Integrated Power Module (TIPM), both of which names refer to the black box near the battery that contains fuses and relays. I don't know why there are two different names, or which model years use which name, but they essentially mean the same thing. In some cases, the trans relay is removable (and therefore, replaceable). In others, the trans relay is built into the main circuit board in the PDC/TIPM (so if it's bad you have to replace the whole thing). Again, I don't know which years have which style.
The trans relay receives direct battery power. When the relay is turned on, it feeds 12V power to the TCM (and to the trans). Loss of the 12V power to the trans will give you full limp-in mode (4th gear only, in Drive), and in fact the TCM commands limp-in mode by turning off the trans relay (the TCM controls the relay). The TCM is also fed with other 12V power, but the trans relay output is fed back to the TCM (at several different terminals on the TCM) so the TCM can check whether the power to the trans is present or not.
When you roll the ignition key out of the LOCK position, the TCM goes through an initialization routine. As part of this routine, it checks the trans relay output power. Initially, the trans relay is OFF. The TCM first checks to see that there is NOT 12V power on the trans relay output (if there is, it sets a P0883 [TCM Input Power High] fault). If that checks OK, it then commands the trans relay to close, and checks that there now IS 12V power coming from the relay. If not, you get the P0882 fault. Note that this is only checked at key-on. As far as I know, loss of power from the trans relay while driving will not set a P0882 fault (although I think the PDC/TIPM itself may set a fault if this happens). But it sounds like you're losing the 12V power feed from the relay to the TCM/ trans while driving (which gives you 4th gear only).
Diagnosing a P0882 can be difficult, especially if it's intermittent (as yours is). That's because an intermittent wiring problem (for example) will usually check OK when you test it with a meter. Also, if you jumper 12V into the trans relay output, you'll get a P0883 fault. I would check the wiring between the trans relay and the TCM (both the main 12V feed that goes to the TCM and trans, and the control wire that the TCM uses to turn the relay on and off). Note again that the trans relay output goes to several different terminals on the TCM (4, if I recall correctly). Check for open circuits, or for shorts to ground or into other circuits. But it sounds like you've already checked this, and again for intermittent issues it's unlikely you'll find the problem. So you could try replacing the trans relay if it's removable (if so, you can also swap it with one of the other relays as a test). If the relay is built into the main board, then maybe try another PDC/TIPM from a junkyard. Or simply assume the wiring is bad and run new wires in place of the existing ones.
FYI, you can find wiring diagrams and connector pinouts at rambodybuilder.com. You have to drill down through a series of PDFs to find these, and you have to use Internet Explorer (Chrome doesn't open the next PDF when you click the links). For a 2008 truck, I would instead look at the diagrams for 2009 MY (I think some of the 2008 diagrams still show the old 48RE trans wiring).