1978 GMC Sierra Classic

[sputtle]

First post!

 

This truck has some electrical issues. I made modifications when I drove it some 15 years ago, and so have others. Electrical doesn't scare me at all. However anything in the dash of this truck does. It's old, it's brittle, and I just can't start tearing it all apart without a plan.

 

With that said, if anyone can help out with wiring diagrams, pictures, advice, or any kind of knowledge, it would be very helpful.

 

So the problems. Aside from random wires hanging out of the bottom of the dash? Well, the running/parking lights aren't working. Looks like a blown fuse, but all the lettering has worn and I don't know what amp fuse belongs there. There was one present, but was rusted and I couldn't read the lettering. Headlights, brake lights and blinkers all work but the blinkers are dim in the rear. Looks like a bad ground, but again, I'm really hesitating taking anything apart until I have basic knowledge of how it's wired. Dash lights are also out, high beam indicator light doesn't work. This will eventually get all wiring replaced.

 

My post is getting lengthy, so to get to the point, really I'm just seeing if anyone can help me out with some wiring diagrams or your own knowledge. Then I'll go from there.

[grampadirt]

Get a repair manual from Haynes or Clymer,although not as good as a factory manual they still have a lot of good info.I have a Haynes I picked up when I had a '77 truck and it has many wiring diagrams in the back.

 

Here's a scan from my '77 Operator's manual for the fuse sizes:

They should be the same or similar to your '78

 

[sputtle]

Yeah I suppose I should. I do plan on having the truck for a long time and I can't rely on googling everything.

 

Thanks for the fuse info! Looks like I need a 20 amp

[grampadirt]

You're welcome and Good Luck with it.I sure miss my '77.

[sputtle]

Got quite a few things done on the truck in the last week or so and I'm pretty happy with how it's coming along.

 

I got all the exterior lights working great, turned out it was all because the box of the truck wasn't grounded properly.

 

The dash light bulbs seem to be working ok, but the numbers are very dim. I've already cleaned this truck top to friggin bottom (removed the seat to vacuum and scrub, door panels, blower motor, inside all the light assemblies and found a total of 6 dead mice) so I figured I'd keep with the trend and clean it out. But the assembly stayed pretty well sealed. Can't confirm if it is original though. But once I took it apart, I started playing around with a flash light. The gauges are opaque, even the numbers on the gauge. There's also blue paint on the back, I suppose for color, but what am I missing here? Do I really just need to tinker in a way of brighter dash lighting? Or go the other route and try to make the gauges more translucent? I'm trying to keep mods to a minimum on this truck so I'd like to see what some others think first.

[2018GMC]

Your dimmer switch is working properly? The dash lights in my 79 K10 are bright enough but I can always tell right away when one bulb goes out because it dims the entire cluster. Dash lights in these trucks are certainly not as bright as newer vechiles. If you like I can take a pic of my dash lights at full brightness.

 

6 dead mice? LOL glad to hear you got them out. I remember changing the rotors on my truck and found a dead petrified mouse wedged in between the cooling holes in the rotor. Couldn't believe it! Must have been a warm place form braking lol.

[sputtle]

The dimmer is working pretty well, feels flimsy though. I think I'm going to pull the dash out again and double check for bulbs. I thought they were all on but I was struggling to hold two wires on the contacts and check bulbs at the same time. Also, I searched around amazon and found LED replacement bulbs. That should run better and brighter on the old wiring, I would think.