Interesting electrical gremlin

[Beladona_13]

Hello, new to the forums here, but owned 65-79 Chevy/GMC trucks for many years.  I recently purchased a 1978 Chevy K20 with a knocking 350.  I swapped the good 350 from my haggard old '76 C20 Camper special into it and have been really loving the 4 wheel drive.  Been driving it now for about a month and had this little electrical demon rear it's ugly head now that it's colder here in Oregon.  If I use the heater blower fan on high with the headlights on the fuse for my tail/park lights blows.  According to the FSM they're supposed to be on separate fuses.  I just placed an order for the factory wiring diagram supplement to go with my factory service manual, but I was hoping someone here might have a suggestion or three.  I still have a few kinks to work out, but I think I've just about cured the "Kmart Rash" it came with!   

Cheers!

[hatzie]

AC or not?

[Beladona_13]

Non A/C truck.  I think I figured it out.  I had the ground cable attached from the battery to the engine, but no ground from engine to chassis or body.  Body and chassis were already tied together, just not to the engine.  After rectifying the issue it ran in my driveway for about half an hour with both headlights and fan on high, no issues!  Now I can concentrate on other things... like why the oil pressure gauge is always maxed out.  I'm thinking it's gotta be the sending unit....

I'll report back if the issue comes back.

Cheers!

[hatzie]

Oil pressure and fuel gauge work the same way 0-90Ω = 0-60PSI. 
Maxed gauge usually means the sender is borked or the wire is unplugged or damaged.
Teflon tape in the threads has been rumored to electrically insulate the sender from the block.  I'm not sure If I believe that wives tale or not...  It's possible.

I don't trust the factory ground arrangements.  Not enough of them.

On my 76 & my 2000 T400 GMC I ran the battery ground(s) to the frame rails close to the batteries and proceeded to tie the whole chassis together electrically. 
I hid most of the braided straps.
--From the frame rails on both sides to... the radiator support, Inner Fenders, A bed crossrib, rear cab support, and  big ones to the block next to the motor mount clamshells.
--The inner fenders to the outer fenders near the battery tray locations and outer fenders to the hood through the center of the springs. 
--One strap from the outer fender to a heater fan screw on my 76 & the M1008.
--I also eliminated the sheet metal grounds behind the tails and added a ground wire from both tails to the frame rails on all three trucks.

My ground issues magically went away.

[Beladona_13]

I'm just about ready to tackle the gauges.  It's a driver, but not my daily, so I have plenty of time.  Today was working on the heater, or lack thereof.  I just discovered the engine I stuck in it has had no thermostat for the entire 10 years I've owned it!  Once I find the gauge issue I'll report back.  I did use Teflon tape on the sender...   Maybe I'll test this wives tail before I replace it.  If I had an extra Aftermarket gauge trio I'd add it.  Next Time I hit flaps Ill see what they have. 
Adding grounds here and there may fix a few woes I have too...

EDIT:  Now that I've added a thermostat, the temp gauge is working...