popping glass fuses

[lt704]

Hi Folks. Last night i poped my brake light/tail lights fuse. I put in another one and it poped (broke). I still have directionals and hazards. One other thing i noticed after this happened when i had my hazards on my lisence plate light flashed as well, not sure if this is related or not. What and how should i progress?. Tomoorow i will start looking at the wires. I replaced the turn signal switch and headlight switch last year. My truck is a 68 C-10. Thanks for the help....Russ

[71swb4x4]

Sounds like you are grounding one of the power wires to a ground, maybe the frame?? I would check the harness from start to finish. Check both sides of the firewall plug and then follow it back to the crossmember plug and from there to the taillights and license plate light. If nothing there check it up through the brake light switch and into the column.

I would start by simply unplugging the plug at the firewall. If it doesn't blow, you can almost be certain that the problem is after that plug. Then unplug at the crossmember by the bumper, and so on and so forth.

[VetteVet]

I suspect your problem is in the tailight circuit or one of the front parking lights. The turn signal wires and the brake light wires use the same wires and since they still work it indicates the taillight wires are shorted to ground somewhere. You can verify this by replacing the fuse and turning on the key and activating the right and left turn signals and then the hazards. Leave the headlight switch off while doing this. If the fuse doesn't blow then turn everything off and step on the brakes. If the fuse doesn't blow then let off the brakes and turn on the headlight switch. I'm pretty sure it will blow. If it does then your ground is in the taillight wiring.

[Longhorn Man]

does the fuse blow when you;
step on the brakes
turn on the parking lights
turn on the dome light

If memory serves, that's everything on that circuit. (haven't had to mess with it in a few years)

If its when you hit the brakes, then you can turrn on the 4 ways, look at the the rear lights blink, and the dim one will be the side you'll be wanting to inspect.
If it is when you hit the lights, then the parking light wires will have the problem

As always, I say the first place to inspect is under the rear bumper where it has probably been hacked for a trailer plug a few times.

[Legolas894]

Originally Posted by 71swb4x4

Sounds like you are grounding one of the power wires to a ground, maybe the frame?? I would check the harness from start to finish. Check both sides of the firewall plug and then follow it back to the crossmember plug and from there to the taillights and license plate light. If nothing there check it up through the brake light switch and into the column.

I would start by simply unplugging the plug at the firewall. If it doesn't blow, you can almost be certain that the problem is after that plug. Then unplug at the crossmember by the bumper, and so on and so forth.

x2 for this-unplug the plug, replace fuse and see if it blows. If it doesn't, it's the harness going to the rear. Mine was messed up with an old trailer light harness grounded to frame.

Longhornman helped me find that.

[lt704]

Thanks for all the comments and suggestions guys. I will work on the problem tonight and get back to you all. Thank you....Russ

[lt704]

Okay guys I have an update.
The fuse only pops when i have the tail lights on and turn on the dome switch( p.o. took out dome light and my dash lights never worked).

Fuse does not pop when just the dome switch is turned on

fuse does not pop when the tail lights just on.

fuse does not pop when headlights are turned on.

fuse does not pop when brake lights and dome switch are turned on.

it was was suggested that i unplug the harness at the firewall?. In doing that do i take the bolt that holds the plastic harness with the wires, if so what would be my next step. I really do appreciate the support you all have given me.....

[Legolas894]

Originally Posted by lt704

Okay guys I have an update.
The fuse only pops when i have the tail lights on and turn on the dome switch( p.o. took out dome light and my dash lights never worked).

Fuse does not pop when just the dome switch is turned on

fuse does not pop when the tail lights just on.

fuse does not pop when headlights are turned on.

fuse does not pop when brake lights and dome switch are turned on.

it was was suggested that i unplug the harness at the firewall?. In doing that do i take the bolt that holds the plastic harness with the wires, if so what would be my next step. I really do appreciate the support you all have given me.....

If you disconnect at the firewall, then you've isolated the circuits to those in the cab and forward. If you can't get the fuse to blow with the harness unplugged, you know the problem is between the cab and taillights. If it blows the same, the problem is right where you are.

Check the wires that run along the driver's side door sill for a short to ground. That's where the fuel sender and dome light wires run.

[Longhorn Man]

the problem is not likely to be there. The dome light has power to it all the time, the switch is on the ground side of the circuit. If it was shorted there, then the fuse would pop as soon as it would get plugged into the fuse block.
Its a good idea to inspect it none the less, but todays problem won't be fixed there.

[fixit-p]

Originally Posted by lt704

Okay guys I have an update.
The fuse only pops when i have the tail lights on and turn on the dome switch( p.o. took out dome light and my dash lights never worked).

Theres your problem right there, the the orange and white wires for the dome light are shorted together like LHM stated the dome light is switched on the ground side so when you turn them on the white wire is connected to ground and if the orange wire is shorted to the white it too shorts to ground, check the plug first.

Originally Posted by lt704

Fuse does not pop when just the dome switch is turned on

EDIT>>> Oops, I missed your second symptom that rules out the orange and white wires being shorted, but still points to the white wire I would check to see if the PO did some after-market wiring trying and fix the dash lights and created a short in the process.

[raycow]

Originally Posted by lt704

Okay guys I have an update.
The fuse only pops when i have the tail lights on and turn on the dome switch( p.o. took out dome light and my dash lights never worked).

Fuse does not pop when just the dome switch is turned on

fuse does not pop when the tail lights just on.

fuse does not pop when headlights are turned on.

fuse does not pop when brake lights and dome switch are turned on.

it was was suggested that i unplug the harness at the firewall?. In doing that do i take the bolt that holds the plastic harness with the wires, if so what would be my next step. I really do appreciate the support you all have given me.....

Does the dome light work normally when the headlight switch is turned off? I know you said the fuse doesn't blow then, but does the light actually work?

Ray

[lt704]

Thanks for all the ideas folks. After work i am going to do some more work on it. Right now it does not pop the fuse as long as i don't turn on the dome light. I am going to disconnect the fire wall plug and go from there. Ray the fuse does not blow when I turn on the dome light only, It blows the fuse when I turn on the tail/stop lights and the dome light. I do not have a dome light itself in the truck, the previous owner has fabric in the headliner area but no actual light. Again thanks to everyone for helping out....Russ

[raycow]

Originally Posted by lt704

I do not have a dome light itself in the truck, the previous owner has fabric in the headliner area but no actual light.

Cheap and dirty fix: If you don't have a dome light now, and you don't have any immediate plans to install one, just disconnect the white wire from the headlight switch and tape the end so it can't short against anything. This won't fix your wiring problem, and I am convinced you do have one, but I think it will stop the fuse from blowing.

Ray

[lt704]

Hello folks, Update, I unlugged the firewall plug(unbolted the bolt?.) I then turned on different devices. Note. When I unbolted the plug I had no headlights, no tail/park lights no brake lights. I did turn on the dome light with the fire wall plug not connected. I then connected the fire wall plug to see if it popped and it did not. So do I start looking from there back to the tail lighrts? Ray yo have a good idea to stop the fuse from popping( disconnect the white wire fom the headlight switch for now, but I still want to fix my problem.. I thank you all for any more imput you can give....Thanks Russell

[fixit-p]

Originally Posted by lt704

Hello folks, Update, I unlugged the firewall plug(unbolted the bolt?.) I then turned on different devices. Note. When I unbolted the plug I had no headlights, no tail/park lights no brake lights. I did turn on the dome light with the fire wall plug not connected. I then connected the fire wall plug to see if it popped and it did not. So do I start looking from there back to the tail lighrts? Ray yo have a good idea to stop the fuse from popping( disconnect the white wire fom the headlight switch for now, but I still want to fix my problem.. I thank you all for any more imput you can give....Thanks Russell

When you turned on the dome light and you say the fuse didn't blow was the light switch also on? Because that is the condition you said earlier blew the fuse and it doesn't make sense that unplugging the connectors would have an effect because all indicators point to the white wire which is in the cab.

[mbgmike]

Probably not it but their was a TSB written last year or longer about the bad fuse batches that were coming out of China. All mfg had them or at least some of them. It advised to use only Bussman fuses until the problem was fixed. They blew way below what they were rated for. Maybe had a bad box.

[lt704]

FIXIT-P I hope i did not confuse you. When the fire wall plug was connected all I had to do is turn on tail/park lights, then turn on dome light , thats when the fuse would blow...Next I disconnected the fire wall plug, when I did this I checked and I had no headlights, no tail/park lights, no brake lights. I assumed I had none of these was because I disconnected the fire wall plug. Am I correct?. While the fire wall plug was disconnected and I had none of the above lights I turned on the tail/park lights then I turned on the dome light, I checked the fuse at the fuse box and it did not pop. Now at this point I reconnected the fire wall plug and I had all my lights, But the fuse will still pop if I turn the tail/park lights on and then turn on the dome light, sorry if i. confused anyone. MBGMIKE, I went out and bought some bussman fuses and it still poped the fuse. I do appreciate the great help I am getting here, you guys are great...Russ

[Longhorn Man]

i think you unplugged the wrong harness. It sounds like you unbolted the main harness on the backside of the fuse box. I believe they wanted you to unplug the tail light harness which is towards the right of the throttle pedal and up. It has 4 wires, brown, light green dark green and yellow. This will disconnect all the lights in the back of the truck.
However, after hearing that the dome light assembly is missing, I think that is your problem. Look at the fuse box. You'll see a plug with an orange wire and a white wire. unplug it. This goes to your missing dome light.

[fixit-p]

lt704, you didn't confuse me I was just trying to get on the same page, IMO the hardest part of threadshooting is fully understanding what the OP and posters are trying to say and making assumptions can lead you in the wrong direction. When I said

it doesn't make sense that unplugging the connectors would have an effect because all indicators point to the white wire which is in the cab.

I had assumed that you disconnected the taillight harness but if LHM is right that you disconnected the wrong plug (and I think he is) it now makes sense because if you removed the bulkhead plug then the cab has no power.

You say that the PO removed the dome light assy but since turning on the dome light causes the fuse to blow when the lights are on indicates that there is a circuit present (could be a short circuit or too large of a load) just to verify, are you are using a 20 amp fuse?

[lt704]

Okay folks I think Iam getting to the route of the problem, Thanks so much for being patient with me. LONGHORNMAN and FIXIT-P I Did find the right plug to pull on the rear light harness ( thanks LHM ). The fused still popped after disconnecting that harness when I did the tail lights and dome switch. I then unplugged the orange/white wire connector near the fuse box. I then turned on the tail lights and dome switch with the orange/white dome light connector unplugged and the fuse still popped. So today I am going to try to disconnect the white wire leading to the headlight switch to see if that helps?. If that doesn't help I think my next adivice is to follow the orange wires, according to my diagram one orange wire goes to the fuse and the other goes to my brake light switch?. A question on my brake switch?. The wire going to my master cylander is that my brake switch or is that just my brake warninig indicator and if not wher is my brake light switch located?. I am sorry for making the these post go on but I can say I have learned learned a lot from everyone.... Thanks Russ

[cdowns]

brake light switch is down by the brake pedal under dash//wire from proportion valve is warning light

[lt704]

Thank You CDCLOWNS I found the brake light switch.

[lt704]

Okay guys an update. I cut the white wire about 1 inch from the headlight connector that goes to the headlight switch ( i tapped of both ends of the white wire ). I then turned on the tail/park lights on and then the dome switch, the fused popped. I know the orange wire that goes from the dome light as well as the orange wire frome the brake light switch then goes to the the fuse box, so I cannot disconnect that wire to the fuse box because i will not have qa brake light . Any suggestions? I know If i don't turn turn my dome light on I will not blow a fuse , but i really would like to fix the problem, Thanks for all the help....Russ

[fixit-p]

If you eliminated the white wire and if the switch is wired correctly then the switch must be bad, do you have an ohm meter so you can test the switch?

[VetteVet]

Originally Posted by lt704

Okay folks I think Iam getting to the route of the problem, Thanks so much for being patient with me. LONGHORNMAN and FIXIT-P I Did find the right plug to pull on the rear light harness ( thanks LHM ). The fused still popped after disconnecting that harness when I did the tail lights and dome switch. I then unplugged the orange/white wire connector near the fuse box. I then turned on the tail lights and dome switch with the orange/white dome light connector unplugged and the fuse still popped. So today I am going to try to disconnect the white wire leading to the headlight switch to see if that helps?. If that doesn't help I think my next adivice is to follow the orange wires, according to my diagram one orange wire goes to the fuse and the other goes to my brake light switch?. A question on my brake switch?. The wire going to my master cylander is that my brake switch or is that just my brake warninig indicator and if not wher is my brake light switch located?. I am sorry for making the these post go on but I can say I have learned learned a lot from everyone.... Thanks Russ

I have been following this thread for a few days and it is very interesting to say the least. I have a theory about what might be the problem given what has been detailed so far. I will have to re-examine what I think I know so if any detail is incorrect then please correct me.

1. Tail brake lights fuse pops when dome is turned on with tail lights on,
2. it does not pop when the brakes are activated or when the tail lights are on by themselves.
3. The dome light is not installed and does not pop the fuse when the dome switch is activated by itself.
4 The tail lights to the rear were disconnected yet the problem remains. This means the short is in the cab. I know that has been stated already.
5. The thing that I have been dwelling on is you said that the dash lights did not work. So----
I have come to the conclusion that the problem is in one of two places, The dash light circuit or the headlight switch rheostat.

The dash lights come on with the tail lights through the headlight switch and the fuse panel. If there is a ground in the gray wire that feeds the dash lights, then when the tail lights are turned on they won't blow the fuse until the dash light switch rheostat on the headlight switch is turned on far enough to overcome the resistance to ground. This may not be until the switch is turned all the way to the dome light position.

The other cause might be the headlight switch itself as was stated by Fixit -P.
If you can isolate the dash lights by disconnecting them then you can use an ohm meter to find the ground.
This may also be caused by drawing more than 20 amps through the three circuits if you have something else wired in.

The brown wire to the master cylinder can be ignored because it only activates the brake warning if it's grounded.
Here are some diagrams of the orange wires which you may have figured out by now.