No power to blower fuse

[rustysquare]

I have a 78 c10 Ls swap with 700r. I had been trying to figure out why my blower motor is not kicking on I checked the switch and the blower its self and they are both good. When I got to the spliced brown wire next to the vent I noticed it did not have power. I followed the wire to the fuse and the fuse has no power going to it. The fuse is not blown. I checked both ends with a test light and nothing. Where does that fuse get its power from. Don't really know too much about automotive electrical. Thanks

[Rapid Roy]

Welcome to the site.

[JohnnyPopper]

Welcome From SoCal!

Ask BD for a wiring diagram. OR, use the search engine on the subject of wiring, very effective!

[rustysquare]

I've done several searches trying to find something without any luck. I'll keep trying tho. Thanks

[bd]

Assuming that your LS converted truck continues to use the 1978 OEM cab wiring harness, the 12-volt power that feeds the A/C - HTR fuse (as well as the fuse block power tap in the cavity located directly below the fuse) is provided by the ignition switch when in the "run" position, via a 12-gauge orange wire (circuit 300). 

When probing a glass fuse type fuse block for power, be sure to probe the fuse block terminals, not just the end caps of the fuses.  Corrosion between a fuse and its terminals is a common problem that will interrupt circuit continuity and prevent current flow.  Study the 1978 Wiring Manual for OE circuit details. 

If there is no power on the orange wire (ckt 300) at the fuse block with the ignition switched on, check for power on the orange wire directly at the ignition switch.  If orange is dead at the ignition switch, check for constant B+ on both red wires that connect to the switch.  If both red wires are 12v all of the time and the orange wire never energizes, replace the ignition switch.  Otherwise, trace the wiring as necessary, using the factory diagram for reference, and verify that the fusible links and their connections are intact.

[rustysquare]

Thanks BD I cleaned and probed the fuse again and nothing. I probed the cavity also and nothing. Should there be something plugged into that cavity if I'm looking at it right, that wiring manual you sent me looks like there should be an orange wire plugged in. Mine has nothing plugged in there.

[bd]

Generally speaking, anything that plugs into that cavity would be running out to an appliance, not supplying power to the fuse.  The 12-ga orange wire that connects to the fuse from behind the fuse box supplies power to the fuse from the ignition switch.  Check for power on the orange wire that connects to the ignition switch with the switch in the "run" position.

[rustysquare]

Ok. Oddly enough I found an orange wire that was going to anything it was hot without the key on. I connected it to the cavity and the heat and AC fuse had power. I did this before I read your last response. I'll be able to check the wires behind the block tomorrow morning

[bd]

With respect to standard wire color-coding in GM vehicles of this era, red is "unfused" B+ (actually, protected by a primary fusible link more often than not), orange is "fused" B+, pink is "fused" I+, brown is "fused" ACC, and black is ground.  But, as with most things in life, there are exceptions.  With regard to Ckt 300, which powers the A/C - HTR fuse in your truck, the 12-ga orange wire in question is restricted I+, being 'hot' only when the ignition switch is in the "run" position.  This prevents the blower from running when the ignition is switched off and also while cranking the engine to start in order to limit unnecessary load on the battery.

[rustysquare]

Fixed!!!  I checked the orange wire behind the fuse box like you said and it had no power I looked at the wires around the ignition I found a four prong plug that was not plugged in it had the brown wire coming from the switch the orange wire that you were talking about and a red power wire. Plugged it back in and probed the AC heat fuse again and it lit up like a Christmas tree thank you so much BD

[bd]

You're welcome.