I have a 79 vette and have never had electrical problems in the past 15 years. Saturday I was coming back from a carshow and the dash lights went out. I pulled over and found the tail lights were also out. I made it home with flashers on. A black car at night with no tail lights isn't good. I found the tail light fuse was blown, but after replacing it and turning on the lights, it blew again. The dash light fuse is ok. I checked wiring along door kick plate and it is all encased in molded shroud. Can it possibly be the light switch itself, or any other ideas? You know how hard it is to get under to even chnage a fuse. I can't even see the back of the light switch. Please help!

 

I think you have diagnosed your own problem.... I suspect it's the light switch based on the way you describe your dashlights going, then the tail lights. But before you start dismantling things, make sure you don't have a grounding problem. Directly above the emergency brake pedal at about the same height as the headlight switch you should find a grounding point, a bolt that seems to stick out from the side panel up behind dash. It'll have a number of black wires running to it. Be sure that everything is good and tight (and clean) on that bolt. Look for sloppy connections or rust. If everything is good there... well let's just say that "the fun begins."

The only way I know to toubleshoot the headlight switch is to remove it...which as you might guess is a bit of a PIA. You might be able to get your hand up far enough to unclip the harness that plugs into the top of the headlight switch... If you can, great...then skip down to the paragraph that begins "Next step...."

I couldn't, so here's what I did:

First, dissconnect the battery.

Then what I do it on my '81 (your '79 should be close) is to remove the plastic cover over the speeedo/tach. Then continue to remove screws (an Assembly Manual will show you where they are) until you are able to extract the speedo and tach. (By the way, I baggie and label the screws as they come out so I know where they go when putting things back together. Most of these screws are obvious, but I don't trust my memory).

The cable that goes to the back of the speedo can be tricky to unclip. There is a "pressure band" that makes up the outside part of the speedo cable plug. Squeeze it as you pull back from the speedo and the cable should release from the speedo.

The last obstacle is to get the guage housing out of the hole far enough to unclip the harness from the top of the light switch. If you have a tilt/tele steering column tilt it down as far as possible. I can push the gauge housing to the right far enough to get mine unclipped.

From here you'll need to either pull the light switch out (as though you are turning on the lights) or remove the entire ****/stem from the switch. From the front of the dash you'll see that where the switch shaft enters the switch there is a collar.... that collar is actually a nut, so you'll want to turn that counter clock-wise to back off the nut. As you do you should notice that the switch (the part behind the dash) will get loose. Once the nut comes off the front the switch will fall away from behind the dash.

Next step is to trouble shoot the switch. My guess is that you'll find it carries 12V to the headlight output on the switch, but not to the dash or tailight output. New switches vary from $40 to $60 and all of the Corvette parts suppliers have them.

I'd have an Assembly Manual and a schematic before starting in unless you all ready have some experience taking your dash apart and tracing electrical harnesses.

One other thing... when I'm working under the dash I remove the seat. It's just 4 bolts and unclipping a plug-in wire harness... takes maybe 5 minutes and it gives you a lot of room. Others might suggest you remove the steering wheel to gain extra room to get to the guage cluster easier. I was able to work around it.

Sorry for the long reply and I hope you find the quick solution,

Good Luck,

kdlp

 

Thanks KDLP, I will attempt to go after this problem next week. It sure would be nice if they put things in a easier place to work on.

 

I really believe the fuse box location is the poorest design item on the whole car. Haven't a clue what they were thinking.... or if they WERE thinking for that matter. How much more (or less) would it have cost to locate that fuse box somewhere where you wouldn't have to stand on your head (and read their labelling upside down) to get to it?!?

kdlp

 

For the fuse to be 'blowing', there has to be a power short to ground somewhere. I wouldn't suspect the headlight switch as a likely candidate. It would be much more likely for one of the tail lamp sockets to have failed to a 'short' condition somehow...or one of the lamp feed power wires to have worn through the insulation and be touching 'ground'. It would be best to disconnect the battery from the system and trace the wiring system with your ohmeter to find a short in the lighting system wiring.

 

I would check all the grounds first and look the the headlight switch as the last candidate. when this happened to me it was the HL swith, but easy solutions first rigth....



tim

 

My short was in the switch, in fact the plug into the switch was burned brown on the outboard side "hot" (orange wire) connection. It shorted the dashlights and tail light circuits but not the headlights (maybe they pick up another ground from the front harness?)

Checking the continuity of the tail light harness is easy to do and would rule out another possibility. And I would suggest doing that before getting into the dash.

My .02...

kdlp

 

One thing I don't understand is why the dash lights go out when the tail light fuse blows. The dash light fuse does not blow. Does the switch have some effect on this so that it cuts power to the dash and tail lights but not the headlights?

 

whaler97 i'm under the same situation, tail and marker lamps out, and fuse burns out if replaced. I'm in for a short as it comes some smoke when the fuse burns out. Head lamps works fine (high and low possition). Dash lights also out( i think.). I didn't solve the problem yet as it started this last weekend and haven't use the car since then. So if i find something i let you know what came out of it!!!

 

I just found another description of how to replace your headlight switch and it seems a whole lot easier that removing the dash cluster.
http://www.corvettemagazine.com/content/view/93/

 

Since I'm not the best electrical guy, how is the best way to check the tail light circuit for a short? Can I use a ohm meter at the light sockets or do I need to get into the wiring up front also?

 

One way is to follow the wiring from the sill up to the fuse block (behind the kick panel) and disconnect the rear light harness and check them each to ground with a meter.

Another way is to pull all the tail lenses and bulbs and look at the sockets. Bulbs have also been known to short to ground internally too.

The colors of the wires:

Dark green and yellow are turn
Brown is tail and side marker
Tan is fuel gauge
Light green is backup
Black is ground
Orange/Black stripe is courtesy light

 

That's pretty standard practice on cars of that era and perhaps into today. Thedash lights and the taillights share the same fuse because you may never notice that your tail lights are out, but you almost always notice the dash lights when they don't work - the dash lights act as a telltale for the taillights. Think of it as a safety feature.

Cheers,
Pete

 

If your not that comfortable with ohm meters and test lights one thing you can try is to remove all your turn and parking light bulbs and side marker bulbs in the front of the car and all the parking and brake light bulbs in the rear along with the license plate light bulb and side marker bulbs.As your removing the bulbs look for a problem in the sockets.Now replace the tailight fuse.Turn on your parking lights.If the fuse didnt blow then start installing the bulbs 1 at a time (leaving the front marker lights until last)until the fuse blows and note which bulb causes the fuse to blow.(Once you get a bulb or 2 in, just watch one of the lit bulbs to see when the fuse blows.)

 

Thanks for all your help. I'm off to the garage.

 

I did just what you said, removed all the bulbs, found a couple that were burned out and seemed to short the system. Replaced bad bulbs and everything works without blowing the fuse. Thanks a lot.