I’m starting to look at how to wire a “house” battery system into the alternator for charging while driving, and I was hoping to add a fuse to the high-current fuse block, but like some others in the thread with 2017 vans, all my connections are full.

Fuse Block:

fuse block

I went and traced cables, and I’m almost certain that the 250A MEGAVAL fuse is connected to the additional fuse box in the passenger B-pillar. There are two additional fuses where other vans have empty slots:

  • 70A fuse wired to the Upfitter Power Connector and documented in the body builder guide
  • 250A fuse wired to a second 50A fuse, with a narrower-gauge wire than the one for the Upfitter Power Connector

See photos below - I didn’t pull my seats off or run a continuity test to be certain, but the wiring is pretty conclusive that the 250A fuse is used for both the Auxiliary Switches and the Auxiliary Battery Prep. The aux switches provide 2 20A fused connections in the passenger B panel, with relays controlled by the dash switches. See Promaster Accessory Connector for details.

I have the switches but not the battery prep. This would explain why the 250A fuse is so large, and has the secondary 50A fuse for the wiring I do have.

Since I do have the Auxiliary Switches option (LHL), as does beewill, and the switches provide 20A as above, the 50A fuse would make sense. From my wire tracing it looks like the narrower-gauge wire feeds into the fuse and relay block adjacent to the upfitter connector, with several 20A fuses. Someone else reported they had a 2017 without the 250A fuse, and they did not have Aux switches or battery prep.

Based on this, I’m thinking I’ll tap off the 250A fuse with a second lug and wire (sized for 250A, or add a secondary fuse) for house battery charging.

50A fuse attached to block cover:

50A fuse

Upfitter connector wires - driver’s side

wires to under seat

Upfitter connector wires - passenger side

passenger B pillar