'72 interior lights blown fuse
Thread Starter
Racer
Joined: Aug 2017
Posts: 352
Likes: 47
From: Bham, Alabama
'72 interior lights blown fuse
I recently pulled my engine to have some work done. Before pulling it, the instrument lights/tail lights worked fine. After install, I found the fuse was blown. I replaced the fuse and it blew again. Obviously I have an issue with a wire somewhere, but thought I would ask everyone here if anything obvious jumps out to them before I start the hunt.
The engine is original, although it has an HEI distributors, There is almost zero clearance between the distributor and the wiper motor. This distributor was on the motor before I pulled it, but I thought I would point out the tight space, in case it matters.
Aside from running a 12v wire from the ignition to the distributor, the harness hasn't changed, In fact, the dash harness and engine ignition harness have been replaced a couple years ago and are in great shape.
So, what could I have done in the engine bay to case myself issues? Again, I haven't started the hunt, although I would probably start with the parking lights and work my way back, as I believe don't think any of the light wires would be anywhere except along the driver side fender.
Thank you all,
Keith
The engine is original, although it has an HEI distributors, There is almost zero clearance between the distributor and the wiper motor. This distributor was on the motor before I pulled it, but I thought I would point out the tight space, in case it matters.
Aside from running a 12v wire from the ignition to the distributor, the harness hasn't changed, In fact, the dash harness and engine ignition harness have been replaced a couple years ago and are in great shape.
So, what could I have done in the engine bay to case myself issues? Again, I haven't started the hunt, although I would probably start with the parking lights and work my way back, as I believe don't think any of the light wires would be anywhere except along the driver side fender.
Thank you all,
Keith
Last edited by Hig116; Oct 5, 2021 at 04:27 PM.
Melting Slicks
Joined: Feb 2018
Posts: 2,328
Likes: 724
From: Marietta GA
You should probably check to see what lights /devices are on the same fuse as the one that keeps blowing. Then you can start looking for the wire that is skinned / shorted and causing the problem. You will need a wiring schematic to see what color wires may be the culprit. Since you were only working in the engine bay, it seems that the issue would be there. Can you easily disconnect the wire you added. In my trouble shooting experiences, when something goes wrong, first thing I do … is undo the last change I made.
Thread Starter
Racer
Joined: Aug 2017
Posts: 352
Likes: 47
From: Bham, Alabama
The fuse has the instrument lights and outside marker lights (tail light, front markers). The wire from the distributor has been there for a while and before I pulled the motor. Although, I do agree that this is a good place to start. Thank you
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
Unhook every wire off the motor. Especially alternator and ignition. Now put the fuse in and see if it pops. If it still pops, it’s going to be something with the harness got tweaked motor got set on it or something. But from the firewall forward is more likely where are you going to find it.
Thread Starter
Racer
Joined: Aug 2017
Posts: 352
Likes: 47
From: Bham, Alabama
Good idea. I'm baffled, as I think the front wiring harness stays along the drivers fender. I could see the ignition harness getting damaged, since it routes across the firewall. So frustrating. Engine is in and running great, but now this. At least I can drive during the day and the brake lights work. Ugh!
Thread Starter
Racer
Joined: Aug 2017
Posts: 352
Likes: 47
From: Bham, Alabama
I disconnected the alternator and temp sensor, which is the only thing from the headlamp harness that connects to the engine and it still blows the fuse. I've studied the AIM and wire diagram and all wires associated with the lights are tucked along the drivers side fender and shouldn't have been able to be pinched during engine install. I'm stumped and not sure the best method of finding this issue without just visual inspection. Any thoughts for my next steps? Any way to use a multi-meter to find the issue? I guess I could pull instrument fuse and all parking light bulbs to see if it blows, although I feel like that is a lot of effort when I just don't think is a bulb that coincidentally when bad during motor install. 
Thread Starter
Racer
Joined: Aug 2017
Posts: 352
Likes: 47
From: Bham, Alabama
After studying the wire diagram, I'm questioning whether my light switch coincidentally went bad (still hard to believe). Per the diagram, it looks like an orange wire goes from the fuse to the switch. Then, a green wire exits the switch and goes back to the fuse block to the instrument fuse. There is also a brown wire that exits the switch to power the parking/tail lights.
So, the possible problems, as I see it, are:
Thanks
Keith
So, the possible problems, as I see it, are:
- Bad switch
- Problem with orange wire supplying switch
- Problem with green wire going to interior light fuse
- Problem with interior lights that isn't blowing the interior light fuse
- Problem with brown wire going to lights
Thanks
Keith
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Melting Slicks
Joined: Feb 2018
Posts: 2,328
Likes: 724
From: Marietta GA
Are you referring to the strange looking all metal door jamb switches that turn the interior lights on? Use a open end wrench to remove them (with the battery disconnected). I won’t say how many new switches I bought before I assembled the wires (with the special brass connectors) so everything worked correctly. The switches on a 73 also are tied to the theft alarm. There are three wires to each switch. I am not sure if 72 cars have the same alarm as the 73 cars.
Thread Starter
Racer
Joined: Aug 2017
Posts: 352
Likes: 47
From: Bham, Alabama
Update: I replace the fuse and the lights worked, which lead me to think that I may have had a loose wire somewhere that I correct. Then the fuse blew again at some point.
With the fuse blown, the courtesy lights still worked (only by the switch, as they haven't worked by the door pins since I've owned the car for 3 years). Today, I replaced the fuse and dimmed the interior lights completely, so I could test to see if the interior lights were the cause of the blown fuse. I turned the lights to their brightest setting and the fuse was fine. Then I turned on the courtesy lights and the fuse blew and the courtesy lights also went out but didn't blow the fuse for the courtesy lights.
So, I'm thinking my switch may be the issue. What do you all think and is there a way to test the switch?
Thank you,
Keith
With the fuse blown, the courtesy lights still worked (only by the switch, as they haven't worked by the door pins since I've owned the car for 3 years). Today, I replaced the fuse and dimmed the interior lights completely, so I could test to see if the interior lights were the cause of the blown fuse. I turned the lights to their brightest setting and the fuse was fine. Then I turned on the courtesy lights and the fuse blew and the courtesy lights also went out but didn't blow the fuse for the courtesy lights.
So, I'm thinking my switch may be the issue. What do you all think and is there a way to test the switch?
Thank you,
Keith
Melting Slicks
Joined: Feb 2018
Posts: 2,328
Likes: 724
From: Marietta GA
Keith
Do the easy tests first.
If it’s the Main Headlight Switch, you won’t like the pain and suffering required to replace it. You should first remove all the courtesy light bulbs and then see if the fuse blows. Also at the same time ensure the light sockets are not shorted, positive to ground (they have these really archaic brass “crimps” with springs that make contact with the bulb). I have experienced problems with the light sockets before. Let us know what you find out.
Good Luck
Eli
Do the easy tests first.
If it’s the Main Headlight Switch, you won’t like the pain and suffering required to replace it. You should first remove all the courtesy light bulbs and then see if the fuse blows. Also at the same time ensure the light sockets are not shorted, positive to ground (they have these really archaic brass “crimps” with springs that make contact with the bulb). I have experienced problems with the light sockets before. Let us know what you find out.
Good Luck
Eli
Melting Slicks
Joined: Jan 2006
Posts: 3,180
Likes: 297
From: Sarasota Florida
I can only commiserate with you. I had a strange issue with my turn signals. Seemed like every time I ran a wire down, a different gremlin popped up. These old cars are fragile and sometimes you can do more damage than good.
Ain't it fun though?
Ain't it fun though?
Thread Starter
Racer
Joined: Aug 2017
Posts: 352
Likes: 47
From: Bham, Alabama
It's been a few months since I posted and I still haven't gotten around to trying to figure the out. Instead, I quit driving at night. 
But, I need to get to it and move to the next issue.
Today, I took a drive and realized my blinkers weren't blinking. I check the fluid (joke)... The lights come on, but don't blink. So my first thought was the flasher is bad, but I can only assume that my other issue caused the flasher to go out. I know it could be a coincidence, but those don't happen often. Cause and effect.
So I imagine I have a short or ground that is causing issues and now I'm concerned I'm going to cause other issues if I don't figure it out. Before diving in, I thought I would update this post with this new information to see if anyone said "Oh, now I know this issue."
If this new information helps, please let me know you thoughts. Otherwise, I'll dig in soon and see what I can find.
Thanks everyone!
But, I need to get to it and move to the next issue.Today, I took a drive and realized my blinkers weren't blinking. I check the fluid (joke)... The lights come on, but don't blink. So my first thought was the flasher is bad, but I can only assume that my other issue caused the flasher to go out. I know it could be a coincidence, but those don't happen often. Cause and effect.
So I imagine I have a short or ground that is causing issues and now I'm concerned I'm going to cause other issues if I don't figure it out. Before diving in, I thought I would update this post with this new information to see if anyone said "Oh, now I know this issue."
If this new information helps, please let me know you thoughts. Otherwise, I'll dig in soon and see what I can find.
Thanks everyone!
Burning Brakes
Joined: Jul 2018
Posts: 979
Likes: 258
From: Maryland
After studying the wire diagram, I'm questioning whether my light switch coincidentally went bad (still hard to believe). Per the diagram, it looks like an orange wire goes from the fuse to the switch. Then, a green wire exits the switch and goes back to the fuse block to the instrument fuse. There is also a brown wire that exits the switch to power the parking/tail lights.
So, the possible problems, as I see it, are:
Thanks
Keith
So, the possible problems, as I see it, are:
- Bad switch
- Problem with orange wire supplying switch
- Problem with green wire going to interior light fuse
- Problem with interior lights that isn't blowing the interior light fuse
- Problem with brown wire going to lights
Thanks
Keith
1. Disconnect the battery.
2. You can access the light switch and the connectors from the side, that's what I had to do. Very tight, but I managed to get my fat fingers there. Disconnect the connector for the Green (goes to the Gauge Lights) and the Brown wire (goes to the exterior lights).
3. Get a Multi-meter, set it on ohms setting. Make sure you find a suitable ground to use and clip or connect one side to this. Set the Ohms setting on the lowest one - for my meter it is 200 ohms.
4. Connect the other side of the multi-meter (set on ohms) to either the green or brown wire connector. You can see the metal connector inside the plastic connector housing. For each wire, what is your reading?
5. Use Ohms law to see if the resistance is so low that it will blow a fuse - this indicates a damaged or expose wire. Ohms law states Current E = IR, so the minimum resistance you can have for say a 10A fuse (Gauge)
is E/I = 12V/10A = 1.2 ohms. I would say that any reading below 5 ohms is very suspect.
6. So check both green and brown wires wires. This will indicate which circuit (interior gauge or exterior lights) is causing the issue.