I'm having a problem getting 12 volts to my Heater system input on my 78. I have temporarily rigged up 12v to the heater system and it works fine when I can get power to it.

I have checked and replaced all the fuses that were blown or that just looked dodgy. When I put a volt meter on the Heater A/C fuse I get varied readings (+ 0.5 volts, the occassional -0.00 volt), this is the case with the engine running too. I have checked the entire fuse block and I get 12 volts on every fuse except the top right 3 fuses (looking directly at the fuse block). The other two may be a result of the same thing that is causing my heater voltage supply problem.

I have tried to remove the fuse block, I could find only 2 bolts. Looking at a repair manual there are only 2. I cant get the damn thing to budge.

I noticed that in the engine bay there are a whole pile of wires going into/coming out of what looks to be at the same position as the fuse block on the inside of the firewall, are these connected??? The manual I have here shows that it could be separate. It only shows a brand new wiring loom.

Anyone had a similar problem? Any general advice on common problems? Will that fuse block just come out easy??

I think from memory, the top right fuse is the heater/AC, then Dir Sig / Backup and then Gauges 3rd from the top, I think. These dont show 12v at all.

Cheers

 

 

 

 

 

 

I would check your fuse links located where the wires go into the fusebox from the engine side. There are at least 2 of them.

If you look closely at the center of the wire harness, engine side, where it goes to the fuse box(...a whole pile of wires going into/coming out of ...), you'll see a small bolt in the middle. Take this bolt loose(it will NOT come all the way out), and you can separate the fuse box from the main harness.

I agree with Big Fish on checking the fuse links first, tho. They should have 12v on both sides of the "links". There are several on the car, in various places. A wiring diagram should show them, and their locations.

Are you checking for voltage with the key on, or off? It will make a difference, on some circuits.

Thanks for the tips, particularly for the tip about the bolt on the engine side, that sounds like it will make all the difference. On the engine side of the fuse box it's a bit dirty so it was hard to see anything.

I have been checking voltages on both ends of the fuse, with the key on and off just to make sure. Do the wires on the engine side individually unplug (or in groups) or ar ethey just one big connection?

I almost had the interior fully put back together on Saturday and ready to drive her again but this problem had me going in circles, maybe next weekend now.

The con. block(bulkhead connector) on the eng. side is normally split into 2 sections, but I'm not sure on the '78. All of the terminals can be removed individualy, after separating the connector, but it is basically one big connector.
I solved a problem similar to this on my '85, by unbolting this "bulkhead" connector, and cleaning all of the terminals. I suspect you will have the same luck!
Adams' Apple38696.9109027778

Thanks again, that'll be my first job next weekend.  I am hanging out to drive her again, all this nice weather down here is going to waste.

Cheers

Well, I pulled it al apart, again and again, after cleaning the terminals and testing etc but I still cant get 12 volt on the top right two fuses

Behind the two fuses there is a big orange wire going to them, common 12v I'm guessing. Ignition on or off it wont power up.

I jumpered 12v from another source to the same connection as the orange wire and the heater fan etc powers up which is what I need it to do. The other item requiring power from that same orange wire on the fuse box is the turn signal, that doesn seem to work even if I jumper 12v to it, so that's another problem, yippee.

The orange wire that is meant to deliver 12v to the top right two fuse disappears up the wiring loom. I've cut back a little insulation tape to see if it comes back down but it seems to keep going up to somwhere else.

I have 4 books here about 68-82 Vettes but none go deep enough into detail about the wiring.

Does anyone have any more detailed wiring diagrams for a 78 or does anyone know where the orange wire goes? I'll try to fix the direction indicator proble later once I can get power to it, the yway it is meant to.

I dont really want to have to pull my dashboard back out if I can help it.

 

Doug, it sounds like you have a blown fuse-link.
If you can get under the car, check around the starter for 2-3 red wires, comming off of the center terminal of the solenoid. (The really big one is the batt. cable)

About 4"-6" from the solenoid terminal, you will see what looks like small "tootsie-rolls"(I guess you guys have them down there!). These are fuse-links. One or more of yours is burnt, simmilar to a blown fuse, but you can't just look at it and tell it's blown.

With the batt. connected, test for 12v on both sides of the fuse-link(s). If you look hard enough, you'll see that at least one of these red wires turns orange after the fuse-link. When you find the blown link(s), jumper around them, or cut them out, and splice the ends of the red/orange wires back together, just as a TEST. If you now have power at the fuse box, then you have found the problem. You do NOT want to leave these wires spliced like this- you must have some form of circuit protection.

There are auto parts stores that stock these links, but I don't know about your location. If you can't locate fuse links locally, I know some of the dealers used to have them, and some of the 'Vette vendors carry them.

Hope this gets you on the right track!

Adam's has it right about the MAIN fuse link located at the starter, but I don't believe that's the one you need because you have partial power. At the bulkhead connector on the engine side there are 2 (orange 20 gauge) fuse links. Most likely it's one of those that opened. Check for power on BOTH ends of the links. Any auto parts store should carry replacement links with crimp connectors. There is also a 16 gauge red link, but that supplys the entire fuse block. Try these people for a laminated wiring diagram for your car: http://www.lectriclimited.com/

Note: on the factory diagrams fuse links are referred to as "thermo wire"

Big Fish38696.7780092593

Do these fuse links tend to be covered up? ie under the plastic wiringing loom covers. When you say small tootsie rolls, bigger in diameter than 2mm.  I'm having trouble seeing any Tootsie rolls under my hood.

It appears there are no fuse links from my solarnoid, just 2 red wires heading up to a connector a the top rear of the engine. Not good I take it.

Also on the other side of the engine bay, I've had a scout around for 2 Orange fuse links but nothing jumps out.

Although I'm new to all this Vette tinkering , I'm sure I'd be able to spot fuse link if there stand out and are actually there.

Maybe I'll get the digital camera out and see what I can photograph for you.