I have been working on the restoration of a '78 and have encountered an electrical issue with the cluster. I get no indications when the blinkers are on, when high-beams are on, or when the parking brake is set. So basically all 4 lights in the middle are inop. I have verified that I have power coming out of the turn signal switch for the blinkers, and I have verified that I have power coming out of the connector which plugs into the back of the cluster itself. I thought maybe it was the card on the back so I replaced it. Installed and still have same issue...power goes all the way to the plug, but nothing out of cluster. I did notice that one of the lights are missing from the back of the cluster, but didn't think that would matter, or does it? I checked to see if any bulbs were burned out and they are not. I am at a loss here. Any guidance is appreciated tremendously!


Matt

 

The steel speedometer and tachometer housings have black ground wires going to them and if those wires aren't connected none of the bulbs will work.

 

Where do these ground wires connect that you are referring to? I don't recall seeing any grounding points on the plastic instrument cluster. All of the lights for the backlighting come on when the headlights switch is placed on. I just can't for the life of me figure out why the blinkers, parking brake, and high beam indication won't come on. I have power to the main cluster plug.

 

there is a ground connection stud on the back of the speedo and the tach, almost have to pull the left side of the dash to get at them. do the speedo and tach lights work? the instrument cluster, center console lights, the power for those come through the center console plug. the turn signal, brake and bright are on a harness with the speedo and tach lights.

 

this may sound obvious but you never know, do you have the correct bulbs in the correct sockets?

 

Also, those chinzy plastic bulb sockets with the little metal grounding ring are of very poor design (but cheap!) and don't survive disassembly/reassembly very well. I had to epoxy sevral of those rings to the socket so they could be reused. Other GM cars used metal housings instead of the Chevy plastic junk.

You can easily test for ground problems. Just get a length of #12 wire with bare ends. Connect/clip/touch one end to a known good grounding point. Pull your light switch out one notch, then touch the other end of the wire to the metal speedo or tach housing. If bulbs light up, grounding is your problem. Do the same with the gauge cluster, but touch the wire to some bare metal area of the cluster.

 

If your C3 is like my 72, the high beams, turn signals, and emergency brake lights are not grounded to the back of the speedo and tach like the instrument lights are. The sockets for the high beams, turn signals, and emergency brake lights should have 2 wires running to them, one of them a ground back to the harness. If you try grounding the sockets using the method described by 7T1vette above, you may find you have a faulty ground somewhere in your harness.

This gets messy. I had a nightmare tracing an instrument light short that ended up being inside of the original am-fm radio. Once I disconnected the orange wire leading to the radio, instrument lights worked great. I had to take apart my entire dash to get access to all the wiring. On the up side, you will ready get to know the guts of your vette! Good luck!

 

You said you replaced the "card" at the back of the cluster.

Do you mean the https://willcoxcorvette.com/corvette-speedo-and-tach-printed-circuit-78-82?

If so, it seems that you have a really strange problem. If not, it's a very fair bet that the printed circuit is the problem.

Something that makes your symptoms strange is that the background lighting, turn signal and high beam indicators all share the same ground (via the printed circuit) with +12V supplied to them via various circuits.

The brake warning light is different. It has constant +12V from the GAUGES fuse (I presume your gauges are working) but has three different sources for ground to make it light: 1) the parking brake switch attached to the parking brake lever mechanism 2) the ignition switch (so it comes on as a test when cranking) and 3) the brake pressure switch in the balancing valve down below the vacuum booster.

I think it's time to remove the cluster and verify that you have continuity from the connector area to the bulbs via the printed circuit using a meter. It's semi-transparent so it's easy to follow the traces. Those printed circuits are prone to delamination which allows the copper traces to move, short out against each other and even break/melt. Also the ends of the traces at the connection point are prone to coming loose and shorting out against one another.

The printed circuits can be re-laminated with an appropriate cement. Copper foil (used in stained glass) and solder can be used to repair breaks/burn outs in the traces. Thick super glue works to reattach the ends at the connection points but you must be very careful not to get any of the glue on the top of the contact points. I made all of the above repairs to both of the printed circuits in my car but got some glue on a couple of the contact points in the circuit for the gauge pack. I tried to abrade it off with an emery board but cut through two of them and after soldering on new ends and still having problems I gave up and bought a new one. The repairs I made to the cluster printed circuit are still OK after about three months.