Just changed the 6.2 in my 84 Sivlerado to a 6.5 (daughter locked it up). Transferred everything but the heads over. Everythnig on the 6.2 worked fine before the swap and bolted perfectly to the 6.5. Replaced the GPC and relay, plugs were about 2 mo old. I cannot get the glow plug system to work at all. Have ran every diagnostic test in the GM manual and tested the resistance of all the wiring. Looking for thoer possible problems. All of the other electronics on the truck work fine. Am getting a light in the relay main feed and both wires on the side. as well as through the temp sensors in the right bank and fuel infection pump. Did a resitance check on all plugs and the block with them installed. Everything checked out. Am really lost on this on.

Just a thought but have you tried using a manual switch on the GP relay and disconnecting the GP controller? The controllers are finicky and not terribly reliable. If the GP relay pulls in with a manual switch and all the wiring to the GP’s is correct it should light the plugs.

Haven't tried the manual switch yet. Not really comfortable with how to put one in. Wouldn't mind a manul switch if I need to. Just need soem good gouge on how to install.