Factory radio power wire not 12v

[kjackbrown]

 Okay, I just bought a 1978 K10 and was starting to add some comfort features to it. The latest being a new modern stereo and an auto dimm/compass/temperature mirror. I removed the functional factory radio and ran a new wire for constant 12v, for the new stereo's memory, from a add-on fuse box under the hood. My switched 12v power for the mirror is coming from the factory radio power source which is a yellow wire I believe. My issue is, when I plug in the mirror the display does not come on. When I put direct power from the battery the mirror functions correctly.

 I probed the power source (that yellow wire I mentioned earlier) and it seems something is not right. With the mirror disconnected and key on, my meter reads 8v and creeps up to 12v over the period of about 30 seconds. When I plug in the mirror, it immediately drops to .1v (again remember when i hook the mirror straight to the battery it works fine).  I do not have the new stereo in yet as that will be the next project after I complete the mirror install.

 What do you guys think could be the problem. I have tested this both with the truck running and not running. Sure, I could just find another switched power source but that would not fix the problem with this one. I haven't found the wiring diagram for this circuit yet...I wanted to ask here and get some responses while I google wiring diagrams.

Thnx!
Kev.

 

[bd]

Welcome to the Forum!

First, the 1978 Wiring Manual.  The radio feed (Ckt 43 - yellow) is powered via the accessory circuit from the ignition switch.

Second, you described symptoms of excessive resistance in the radio feed circuit.  Try replacing the 15-amp radio fuse and clean the fuse terminals.  Even if the fuse is good, the cartridge type fuse box terminals have a tendency to oxidize and degrade circuit continuity.

[kjackbrown]

Funny you say that....after reviewing the wiring diagram on this site and identifying the fuse, it was crusty as heck. At the time all I did was check continuity to make sure the fuse was okay. I did not pull the fuse and clean the contacts. Tomorrow, my first duty will be gething under the dash to pull all of the fuses and clean every contact.

Thank you for your reply!

[VileZambonie]

Also check your cab grounds.

[kjackbrown]

 Dudes....TOTALLY nailed it! I pulled the fuse and gave it a once over with a Brillo pad and used 600-ish grit sand paper on the fuse box contacts. I tracked down a couple cab grounds and cleaned them up and now it behaves as it should Thank you both soooo much for your advise.

 Last week I pulled the moist carpet out and Dynomat'ed the entire cab. While doing the pre-cleaning for this I kept finding clues the the truck might have spent some time crossing rivers. There was mud and stuff all over up to about the brake peddle. The guy I bought it from said it was his families (exclusive) hunting truck, and other than that no one drove it. Seeing the condition of the fuse box contacts and everything else I found so far...it appears it had a rough life but at least the cab was completely rust free. Now that I'm the caretaker of it, I'm bringing it back to its former glory and then some.

[bd]

I once rewired a truck that had a mummified fish wedged in silt way up under the dash.  I guess the driver went fishin' & thought she would float....  I thought it was a rather expensive way to get out into the center of the stream.

[PushrodV8]

I once rewired a truck that had a mummified fish wedged in silt way up under the dash.  I guess the driver went fishin' & thought she would float....  I thought it was a rather expensive way to get out into the center of the stream.

  that is an interesting story I must say!

[VileZambonie]

Don't be afraid to add a few auxiliary ground straps as well.