Belsom

All,
I was trying to diagnose why the defogger was not working on my 1978 Corvette with a test light. At the defogger switch, I had power to the Pink/blk wire when the ignition was turned on but no power to the Lt Blu wire. While poking the test light I actually crossed a couple tabs at the defogger switch and there was a tiny spark. After that spark I could get no power reading at the Pink/blk wire. I gave up on my diagnosis and put the driver's side console access panel back on. When I went to move the car I discovered the power windows don't work, the tach no longer goes below 3500 and the electric charge gauge does not work. I can do without the window fogger, but not everything else.

I tested the gauges fuse and it is good. Hot on both sides. The Pink/black wire at the PW relay was also dead, which is not surprising because both get power from the same source. I jumped a hot wire to the Pink/blk wire connection at the defogger switch and was able to put the windows back up. So I know the problem is upstream of both the defogger switch and PW relay. At the fuse box it is apparent that not every wire is virgin, but not too bad. On the ignition connection, next to the gauges fuse on the fuse box is a plug that looks unique to fit into that connection and a pink wire runs from that to the brake light switch. That wire is hot when the ignition is on. At the brake light switch is a red wire that connects at the same location. It was hot when the brakes are not on and it was dangling down, unprotected or covered....something I don't like to see.

Near the fuse panel, I did discover a Pink/blk wire that was not connected to anything with a stripped area all aged. Maybe at one time someone had stuck that into one of the fuse tabs, as the length was about right. The Pink/blk dangling wire had was not hot, with or without the ignition switch. Using an ohm meter, I tried to see if the dangling wire end had continuity to the Pink/black wire at the defogger switch, but could not get a reading. Verified the ohm meter was working by demonstrating connectivity between the pink/blk wire at the defogger switch and the pink/blk wire at the window relay.

Feeling desperate, I connected power to the dangling pink/blk wire, but got no power to the pink/blk wire at the defogger switch.

Short of tearing out the dash to trace the pink/blk wire from the console, does anyone have any ideas?

Appreciate any help.

Keith

Gazehound

Here is an electrical schematic for the defogger in your 78. See if this helps you out.

Belsom

Thanks Gazehound, I had found that diagram when I searched forums before posting. Appreciate peoples help. But my mystery is where the pink wire travels and where it terminates at the fuse box so that I can try to trace it to understand the problem. Maybe I should be posting this in the tech forum.

Keith

bradleyb66

I can't help alot, but it sounds like your original problem was the switch. I just finished debugging the rear defogger on my 79, and you should have power to the blue wire when the switch is held in the 'on' position (but it's a momentary switch, so as soon as you release it, the blue wire won't have power anymore - but relay should stay energized for a period of time). The light on the defogger switch is fed from power thru the relay, so it will only light up if the relay received the energize signal.

My problem ended up being bad/dirty connections on both ends of the 10 gauge orange wire that runs from the fuse box to the defogger relay (under the arm rest in the center console).

For what it's worth, have you tried removing the gauges fuse, and re-inserting it? It could have power on both sides of the fuse, but have a bad connection between the blade of the fuse and the fuse block terminal?

And I hate to say it, but make double sure you're checking the right fuse. I spent a couple of hours trying to figure out why I wasn't getting power to my blower switch, when the fuse had power - turned out that I kept checking the wrong fuse when I was upside down sticking my head under the dash....

As for wire routing, there is a big bundle that runs from the fuse box to the center console (fairly direct path) - I would think the pink/black wire should be a part of that bundle. If you need a wiring diagram, check out this link: https://www.corvetteforum.com/forums...or-anyone.html