My 135 diesel frankentractor isn't charging. I replaced the alternator (external VR) and the VR with a solid-state type. Traced all the wires, checking for continuity. I'm getting about 13V across the battery terminals when it's running. I'm getting the same when I put the leads on the ground at the block and on the F terminal on the alternator. Looking at the diagram, I expected to get some sort of voltage across the oil pressure switch - I get none. Should I have voltage there? What else can I check? The ammeter is reading zero. I have no lights, so nothing to draw current once the engine is running.