I have 75 Vette that the instrument lights don't work.  I have the speedometer and tach out of the Vette.  I bought a new light switch hoping that would do the trick.  It still doesn't work.  Does the dash have to be back on the vehicle to make the bulbs work?  I noticed that only one side of the fuse appears to have power (originally there were no fuse).  I have never seen anything like this?  Also there were two bulbs to the left side by gauges (and they had two wires in them) wires not the gray ones to the dash lights.  They didn't have bulbs in them so I put some in.  One lighted up.  Also I noticed that when I checked the gray wire with light tester I did have power to sockets but the bulbs wouldn't light up (with a new light switch can you light them up without putting the dash back into the car?)   I am thinking I have a ground problem.  Do you know where I should look for the ground wire for the instrument lights.  Any help you can give me will be great.  I do have a fluke meter as well.  Also the turn signal lights use to blink on the instrument panel however they don't blink now that the dash is out.  The turn signals work in the front/back of car.  Need your help.

Thanks Dean

|UPDATED|2/7/2014 1:55:34 PM (AZT)|/UPDATED|

The bulbs get ground from the tach and speedo housings.  There are heavier black wires in the harness with a spade connector on them that plug into the tach and speedo. 

So if the lights are not plugged into the gauges, they won't light up because they'll have no ground (note that there is only a single wire going to them).  If I remember right, the parking brake warning light has two wires.

Take a piece of wire, ground it to the car somewhere, and touch it to the metal barbs on each instrument light wire (with the lights on).  They should light up!

The bulbs should look like this:



Except the wire is grey.  If you have power to the center of that socket (+12v) and a good bulb, there is no reason it shouldn't work.  You have to touch the ground wire to the little metal prongs surrounding the bulb though, not the plastic of the socket.

Are you sure your ground wire is on a good ground?

Again, there is no way it can't work if the bulb is good, 12v to the center of the socket, and a good ground.

How are you testing for power at the socket? Test light, or multimeter?
Even checking with a multimeter, and showing full battery voltage(12+v), a bad connection can cause the lights to not shine. The wiring, including all connections(including the socket itself) must be able to carry the amperage/load once the light is "on". If you have a solid 12v at the center terminal of the socket(measured from the bulb side), and a good ground, the lamp has to light. If you lose your 12v power while under a load(bulb in, power and ground supplied), then you have a bad connection somewhere.
Try this. Connect the positive lead of your multimeter to your makeshift ground wire, and connect one end of the ground wire to the metal tangs of the socket, but leave the other end of your ground wire loose. Put your ground lead for the multimeter to a good ground(the battery is best). Turn the lights on. You should have 12v showing. Then put the loose end of your ground wire to a good ground. Your voltage should go to zero....and the bulb should burn.
What we're attempting to do here is verify that you have 12v THRU the socket.

Doyel,

Houston we have lights.  You know I purchased some new Super Green 3SMD 2835 LED Instrument Panel BA9S 2835 Light Bulb 1895 off Ebay.  I was going to replace the original bulbs with them.  Yesterday when I put one in and I didn't ground it.  Tonight when I got home I went back out there and put 2 new LED bulbs in grounded them and they worked.  Do you think I can use the LED's or should I just put original bulbs back in?  I guess the bulbs were all bad.  My 75 is painted GM 42, Bright Green Metallic and I thought it would look cool with a green dash.  Let me know what your thoughts are on replacing them with the LED or leave it original?   Will they get to hot?  I am just glad that I got it working.  Dean

Glad they are now working! 

You can use the LED bulbs with no problem, except you will not be able to dim them.....they will be either on, or off, without some sort electronic converter/dimmer. The headlight switch will not do it.

Bad bulbs... Whoda thunk?  Glad they're working! 

The LEDs will actually run a lot cooler than incandescent bulbs and draw a lot less power. 

Glad to see there is another fighting with dash bulbs! If you go LED please post your thoughts. I'm on the fence. Whoever posted the solid black ground wire with spade connector comment....thanks! I buttoned up my gauge panel and I found one unconnected black wire with spade and for the life of me couldn't figure out where it went. Hope I can find the brass terminal without pulling the gauge panel. Its sooooo darn tight up there!