Voltage after fuse box

[2square]

Hey guys I've got a problem.... I have a new battery and let me say all issues are with the truck off ( not started I've yet to fire it up) but I noticed the aftermarket gagues are working, but when I turn anything on ie. Fan, turn light, head lights the volt meter goes to 0. So I grabbed a meter and when i have 12.8 v from starter to fuse box no matter what but I go into fuse box and everything is 12v until I turn anything in cab on and it drops the voltage after fuse box to everything in fuse box but again the wire that feeds the fuse box does not move from 12 v. I've taken every fuse out same problem. Volt meter in fuse box to ground I get 8 volts with anything on, turn it off 12 v. But the wire that feeds fuse box says 12 v with everything on... What gives

 

[MikeB]

If your 87 is like my 82, there are two large red wires that feed the fuse panel through the firewall connector. Both wires are connected to the fuse panel BAT bus, which in turn feeds some circuits like headlight switch & brake light switch. The BAT bus then feeds the ignition switch, which in turn feeds the fuse panel Ignition and Accessory buses, as well as the starter.

So are you saying everything on interior side of fuse panel drops to zero volts or 8 volts when you turn on anything? Is that true with the ignition switch in all positions? I think you may have a problem where voltage from the ignition switch is not getting to the IGN or ACC buses on the fuse panel.

Regarding headlights, are you saying they work until you turn on an accessory like the heater blower, or until you turn on the ignition switch?

 

[Poppy 87]

I would check for integrity of the fusible links. Test both sides of the fusible links for voltage drop and continuity

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