05 Freightliner runs then dies when VCU plugged in.

Check the wire at pin 1 on the 18 pin connector at the vcu for short to ground. Also check the pin on the VCU itself. It should only be grounded with the idle shutdown timer or engine protection shutdown.

 

You don't have to run the truck. Just disconnect the 18 pin connector, and check pin 1 on the vcu and see if its grounded. If it is, then it is the VCU telling the engine to shut down. You could verify by removing the pin 1 terminal from the connector, plugging everything else back in and see if it runs normally. But it may be shutting the engine down for a reason. Are there any fault codes?

 

Sorry. The schematic I have showed 18 pin connector/pin one in the list, but 18/2 in the drawing. Ill attach it. With the CAN connector disconnected, the VCU cant talk to the engine. It sounds like the VCU is getting an incorrect input to shut the engine down, and does it once the can lines are connected again. The gauges drop out and throttle doesn't work because those are inputs/outputs of the VCU that relays the info to/from the engine.

 

Most likely the VCU is still good if you don't have any faults from any other module. You would have to try a test VCU to verify. Without the truck wiring diagram its hard to say. I would still suspect something with the vehicle power shutdown circuit in the wiring diagram I attached previously. It says "IGN" so I don't know if the VCU is expecting to see 12v on that pin when the engine is running and is shutting down because there's an open circuit.