The code indicates instrument cluster to ECM communication problem. Motorcarman pointed out the potential issues are cluster failure, ECM failure, or CAN bus wiring open circuit/short circuit. I find the "Pinpoint Test" in the manual and all is good until I get to "check for short circuit between CAN + and CAN -"



According to this the CAN bus should have at least many thousands of ohms resistance. Mine measures 60 ohms. I was excited to have narrowed down the problem here, then more excited when I found if I unplug the cluster my measurement of the CAN + to - suddenly is within the specified, namely it is greater than 10k ohm. I measured the CAN pins on the cluster and get 120 ohm. Must be a short in the CAN bus wiring in the cluster...

Being lazy I googled "can bus fault 120 ohm" and I find out that CAN+ to CAN-, pins 6 and 14 of the OBD connector, should measure 60 ohm and this is because the cluster and the ECU each have 120 ohm resistors to terminate the bus. I see article after article and wiring diagrams and CAN bus diagnostic videos that all say OBD pin 6 to 14 should measure 60 ohms because of the two 120 ohm resistors in parallel. This is generic CAN bus, applies to almost all vehicles. So my suspected short in the cluster is in fact exactly what I should have expected, 120 ohm. One example I found is below but there are a lot that are the same basic info.





Now I'm having trust issues with the shop manual pinpoint tests.

 

I have decided the shop manual is wrong. Test D6 should NOT give greater than 10k ohms until after Test D7 which requires disconnecting the ECM. If you swap steps D6 and D7 in the manual it makes sense, namely with the ECM and instrument cluster disconnected you have a bunch of very high resistances in parallel resulting in a high resistance, >10k between CAN bus pins 6 and 14 at the OBD connector.

In my case I believe the P1638 was probably due to working on the TCM on a nice day with the windows rolled down. I reattached the battery briefly and turned the key on so I could roll up the windows. Unplugging the TCM stops communication between cluster, J Gate, etc and ECM. The next day I put the TCM back in and had P1638 from turning the car on with no TCM the day before, I think. Yesterday I cleared the code and went for a drive and it did not come back so fingers crossed it was just a result of powering up the ECM without the TCM connected.