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I have a 1978 corvette I installed led’s and the tach works the blinker lights work the bright light works and the brake light works only thing that doesn’t work is the illumination on the speedo/ tach cluster the lights on the center console illumination work I’m believe it getting power up there because I shorted the whole illumination out when I was checking out the plug in and the center console illumination shut off.. just curious what my next steps are? Could the ribbon be bad and how to check that out thanks
I originally had similar issues because I had replaced my dash harness and installed LED bulbs. For me, everything worked except the speedo/tach lights. I tracked my issue down to the light switch. If I remember correctly, the speed/tach gets its 12v from the light switch on the left hand side. Check to see there if you getting 12v. If you look at a wiring diagram, I believe it traces back to that switch.
If you need, I can take a pic of the colored wiring diagram tonight and send it to you,
Richard
P.S. after re-reading your verbage above, since you believe you are getting power there, maybe the issue is a ground or the circuit overlay itself. I ended also replacing my circuit overlay. The speedo/tach unit is quite easily removed and you can check the condition of the overlay.
Last edited by lakerider57; Apr 17, 2019 at 05:23 PM.
Yea I have power coming there with a test light and I used the ground on the plug in and used the power for the dash lights and it lit up so I’m thinking it’s the overlay I’m going to test the bulb holders themselves Willcox says that’s a common problem although I think it’s hard to believe I have 12 bad bulb sockets thanks for your quick response!!!
Last edited by 78c319900991; Apr 18, 2019 at 10:52 AM.
Remove your speedo/tach and visually inspect for any broke runs in the overlay.
Next do a continuity test on the overlay circuit, it won't take you long to realize which runs are the grounds and which runs are the 12v (follow them back to the main connector).
I agree with you that doubtful that all your bulbs are bad. One thing I did was insert some test wires into the connector for the various pins (i.e. #4 ground, #10 dash lamps) and connected the connector to the overlay and turned the key to ACC. That's when I realized that the voltage coming from the light switch was the wrong voltage (too low). Maybe you can use a similar approach....
Gauge cluster issues
