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Discussion Starter · #1 ·
Hoping I might get some help or suggestions. I have a '76 Trans Am, 455, 4 speed that is mostly original. My dad bought the car brand new.

I have searched and tried everything that I found. The short story is that the dash lights (gauge lights, heater control light) do not work. Everything else works....gauges, blinkers, cigarette lighter, radio, rear defrost, dome light, floor lights, glove box light etc.. I checked the small 4 amp fuse and put a new one in just in case. I tested the lights by using a 12 volt battery and running wires to the circuit board and they worked, so the bulbs dont seem to be the issue

The long story is that the speedometer cable was broken and I wanted to replace the 1980s Cobra radio/8t track unit that my dad had put in when the 8 track player stopped working. While I was at it, I decided to replace the dash bulbs. I also wanted to get the original 8 track working so I had it converted to work with a modern radio. I installed a USA-230 radio, connected to constant power via the orange lighter wire. I found a 12 volt wire that was running to the old plug for the 8 track player so I used this for the ignition power to the radio. I hooked the new 8 track power to the accessory tab on the fuse panel. Both the radio and 8 track work perfectly. Other issues were that for as long as I could remember, the oil/water temp gauge did not work. I installed new senders, made sure they were connected and replaced the dash printed circuit board. Both senders are working now. I also replaced the speedometer cable and changed the speedo gear successfully. Upon putting everything back together, I found that the dash bulbs were not lighting. I replaced the headlight switch, thinking the old one might be at fault, but they still dont work. I have a wiring diagram that shows all of the lights that are not working are the same colored (gray) wire. This wire runs to the direction signal and hazard switch connector as well. The gray wire is powered from the small 4 amp fuse marked instrument lamps on the fuse box.

I have been a backyard mechanic for 20 years or so but Im not the most experienced with automotive electrical. I am thinking that I have to check and ensure that the gray wire is getting power from the fuse box and making sure its carrying it to the circuit board and other bulbs. Is this correct? Do I just use a test light, probe the wire and attempt to find where its losing power? Is there a way to see if power is at the fuse?

Any suggestions or ideas of things to check would be really appreciated. Im just confused as to why they worked before I did the rest of the work, and they dont now. As far as I know, I did not mess with any of that wiring.
 

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Given the other things you checked already, power comes to the fuse via a dark green wire from the headlight switch. You could test for power there. From the fuse then is the grey wire that splits off to a few different areas like the lighter, heater controls, multi-pin connector at the back of the cluster, fuel and volt cluster and radio.

You didnt by chance replace the gauge cluster bulbs with LED did you? Thats been brought up recently. Some LEDs are polarity sensitive and some dont dim so if the stalk on the headlight switch is turned counter clockwise the LEDs may have shut off if they arent dimmable.
 

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Discussion Starter · #3 ·
Thanks for the reply. I did not go with led, but they are 3 watt bulbs instead of 1 watt. Good info on the green wire from the headlight switch. I will start there and then follow it up with the gray wire.
 

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Discussion Starter · #5 ·
Ok I really got in there today. The green wire has power when I turn the headlights on...as it should. I put a new fuse in. The gray wire is not getting power. I tested it above the steering column, where it is about 2 feet away from the fuse box. No power. So the gray wire is not getting power. Can someone help guide me on what to do? How do I check the gray wires connection at the fuse box? Or is there something else I can do? I'm at a loss on where to go next.
 

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Best I can suggest off the top of my head is try to find the grey wire where it exits the fuse box and probe it for power there. Ive honestly never removed one of these fuse boxes so Im not sure how the connection is made between the wire and fuse but its possible that connection is severed where it would connect to the non hot side of the fuse terminal.
 

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Discussion Starter · #8 ·
Ok so I couldn't really get to the back of the fuse box. I decided to skip the fuse box and jump the green to the gray with a fuseable link. When I turned on the headlights, it instantly blew the fuse. So there must be a short and I am going to have to trace all the gray wires to there destinations.
 

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I had a Similure issue years ago, I had to remove the wiring harness from the back of the fuse box, I found 1 of the tabs bent shorting out the tail lights and dash lights. Straightened the pin on the connector and reinstalled it. Problem solved but it was a pain to get the 1 center bolt out that secures the 2 wiring harness's to the back of the fuse box. I also cleaned and reapplied dielectric grease to the connectors.

I wish you both luck.
 

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I had a Similure issue years ago, I had to remove the wiring harness from the back of the fuse box, I found 1 of the tabs bent shorting out the tail lights and dash lights. Straightened the pin on the connector and reinstalled it. Problem solved but it was a pain to get the 1 center bolt out that secures the 2 wiring harness's to the back of the fuse box. I also cleaned and reapplied dielectric grease to the connectors.

I wish you both luck.
Thank so very much! I’m going to look into that tab right now.
 

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Good to go on the gray wire that powers or illuminates some of the dash lights. I’m currently trying to find out why my tachometer (aftermarket) doesn’t work at all. There’s power coming in from the ignition and coil, but maybe it’s a bad ground, which I just can’t solve
 
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