‘87 V20 Alternator / Battery Wiring - Will not charge!

[FoxCustom]

Hey guys,
While cruising home on Friday night, I notice the voltage gauge drop to around 10 volts and all the lights go dim. Made it home fine. Swap in a new (remanufactured) NAPA alternator on Saturday with no luck - no charge, nothing. The truck is just running off the battery - just 12 volts.

Does this wiring arrangement look correct?? I’m not quite sure what the blue aluminum heat-sinked block is up on the firewall?? Any ideas what else I could check?? Fuseable links?? If it were a fuseable link, would I still be running off the battery or would the entire truck be powerless??

[DieselSJ]

Blue box looks sort of like a battery isolator for when you run multiple batteries.

[FoxCustom]

Ahhhhh! Thank you for the clue! I am not running an auxiliary battery in the truck so I will bypass that. I'm thinking the isolator itself failed or a bad / mis-connection there.

I found a factory engine bay wiring diagram that I will use to retrace and rewire this portion of the setup (fingers-crossed this works!)

[SCOTI]

Originally Posted by FoxCustom

Hey guys,
While cruising home on Friday night, I notice the voltage gauge drop to around 10 volts and all the lights go dim. Made it home fine. Swap in a new (remanufactured) NAPA alternator on Saturday with no luck - no charge, nothing. The truck is just running off the battery - just 12 volts.

Does this wiring arrangement look correct?? I’m not quite sure what the blue aluminum heat-sinked block is up on the firewall?? Any ideas what else I could check?? Fuseable links?? If it were a fuseable link, would I still be running off the battery or would the entire truck be powerless??

What's the voltage coming off the alternator (where the wire connects @ the stud on the rear of the alt)?
Also, check that wire from end-to-end. See what the voltage reads.

[Dead Parrot]

Another reason for an isolator might be to power something only when the alternator is providing charge. Whatever that blue(gray?) wire is going to across the top of the relay/fuse combo might drain the battery if hooked direct to power.

Chasing mystery circuits installed by a PO can be so much fun!

Also possible the PO was having a stealth leaky circuit and used the isolator to prevent having a dead battery every morning.

[FoxCustom]

Originally Posted by Dead Parrot

Another reason for an isolator might be to power something only when the alternator is providing charge. Whatever that blue(gray?) wire is going to across the top of the relay/fuse combo might drain the battery if hooked direct to power.

Chasing mystery circuits installed by a PO can be so much fun!

Also possible the PO was having a stealth leaky circuit and used the isolator to prevent having a dead battery every morning.

WE HAVE SUCCESS!!!! Starting strong and charging great at 13+ volts!!!!

I removed the isolator, repaired a couple frayed (nasty) wires down at the starter, and rerouted all the alt / batt wires to a factory arrangement.

Dead Parrot, I had thought that at first too but that blueish wire was running all the way to the rear bumper area of the truck... I believe the truck was set up as an in bed camper hauler with some fifth wheel duty back in the day.

Relieved to have my truck back in action! Thank you for the clues, guys!