66 Dart 270 Brake light fuse heats up??

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Gublakhan

Gublakhan

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Hello Everyone,
Can anyone tell me why my brake light / stop light 20amp fuse heats up when I put my headlights on (66 Dart 270 convertible Slant 6)? During the day without headlights and house lights on it functions find. It's heating up so much that it's beginning to melt the fuse holder but not blowing the fuse strangely enough.

I've already replaced the wiring harness with a refurbished original, put a new brake light switch, ignition switch, new fuses and did the ammeter / bulkhead bypass as suggested per MadElectrical.com - Electrical Tech.

Now, I'm having this issue with the brake light fuse and I'm feeling at a loss here.
Any suggestions would be hugely appreciated.

Thank you!!
 
Ok when there is heat in a electrical circuit, there is High resistance. It can come from a conductor being too small for the current draw, or a poor connection. A short is an example of excess current draw. A corroded connection is an example of a poor connection causing heat. The higher the resistance, the higher the heat/current. . OHM's law. A small gauge conductor will get hot, and as it gets hot, will have more resistance, and get hotter till it fails. So if that fuse is getting hot, there is excess current, like a short circuit, or there is a loose connection there.
 
Refurbished scares me.

I stripped some of my original harness’s back including the body and rear light harness. The green in the wire went on to much. That is why it is was all replaced with new. You can take the time to OHM it all out but Remember most of these cars are 50 years old or more! Sooner or later you have to accept that.
 
Hey Mopar Tim,

Thanks so much for the fast response and the info!
I've kinda assumed it's a short or a loose connection, but I feel like I've gone through the entire system more than once and still coming up short -- no pun intended. I was hoping to hear that it might be something else that I've overlooked, but it looks like I'm gonna go on the hunt again. Any suggestions on how to track it down? That particular wires that are heating up the fuse terminal are (pink) the rear brake stop switch and door dome light switch (light pink). The other side of the fuse terminal surprisingly is not hot, which leads to the headlight switch (Red w/ with stripe) that also connects to the ignition, ammeter via factory splice. Because, it's only heating up on the one side, do you think that is where the source is coming from? Perhaps the brake switch or the door switch are grounding out somehow? Thanks again for your help!
 
1969383S said:
Refurbished scares me.

I stripped some of my original harness’s back including the body and rear light harness. The green in the wire went on to much. That is why it is was all replaced with new. You can take the time to OHM it all out but Remember most of these cars are 50 years old or more! Sooner or later you have to accept that.
Click to expand...

Thanks for your comment.
The wiring harness came out of another 66 dart that I have had for approximately 15 years which sat in storage and before that was driven very few miles. I know that the wiring is old, but the car was stored in a fairly dry climate and the harness had very few cuts in, so that's what motivated me to clean it up and use it. In any case, you make a very strong point!
 
If the door switches are grounding then you would have dome, courtesy, lamps lighting, the door switches “usually yellow” provide ground to the pinks +12v “ generally” on mopars to light the lamps. Start by isolating circuits. Unplug the rear body harness and see if the issue is there. Usual suspects are the wiring itself or sockets / resistance in the rear harness.
 
1969383S said:
If the door switches are grounding then you would have dome, courtesy, lamps lighting, the door switches “usually yellow” provide ground to the pinks +12v “ generally” on mopars to light the lamps. Start by isolating circuits. Unplug the rear body harness and see if the issue is there. Usual suspects are the wiring itself or sockets / resistance in the rear harness.
Click to expand...

Thank you much appreciated!
That narrows the playing field.
 
Knowing the car helps as we do not!

Mine had been hacked by folks before me in 1984. The other years just took a toll with the weather and time and all the insulation was cracked somewhere. Be true and get a good FSM if you do not already have one.

Best of luck going forward.
 
1969383S said:
If the door switches are grounding then you would have dome, courtesy, lamps lighting, the door switches “usually yellow” provide ground to the pinks +12v “ generally” on mopars to light the lamps. Start by isolating circuits. Unplug the rear body harness and see if the issue is there. Usual suspects are the wiring itself or sockets / resistance in the rear harness.
Click to expand...
The connector for the rear harness is under the left front kick panel.
 
The problem could be as simple as bad connections right at the fuse, IE bad fuse, bad fuse clips, poor connection between the fuse clips and the terminals in the back of the fuse box, and of course bad / corroded/ loose connections right at the wires

In addition to other posts above, you need to find out what is REALLY (actually) supplied from that fuse. Measure the current draw with a multimeter. This is easy to do. Get an inline fuse, same rating as whats in the box and remove the fuse from the box. get some good alligator clips and clip onto the fuse holder

Wire it From fuse clip.............to temporary inline fuse.........to multimeter........through the meter...........to remaining fuse clip

Carefully set your meter up for amperage and see what it draws.

Your factory service manual has a chart somewhere near the wiring diagram section showing what loads are on that fuse.
 
Last edited:
Re : 67dart273 's post, he is correct. The heat should be produced at the point of resistance.
Yote
 
67Dart273 said:
The problem could be as simple as bad connections right at the fuse, IE bad fuse, bad fuse clips, poor connection between the fuse clips and the terminals in the back of the fuse box, and of course bad / corroded/ loose connections right at the wires

In addition to other posts above, you need to find out what is REALLY (actually) supplied from that fuse. Measure the current draw with a multimeter. This is easy to do. Get an inline fuse, same rating as whats in the box and remove the fuse from the box. get some good alligator clips and clip onto the fuse holder

Wire it From fuse clip.............to temporary inline fuse.........to multimeter........through the meter...........to remaining fuse clip

Carefully set your meter up for amperage and see what it draws.

Your factory service manual has a chart somewhere near the wiring diagram section showing what loads are on that fuse.
Click to expand...

THIS.

Replace the fuse? If so, NO CHINA. There have been recalls and fires.

Try tightening the clips or look at the clip that holds the fuse. ANY signs of arcing?

Try measuring the voltage drop along the circuit. Voltage drop is FAR more effective for determining wire condition than ohm tests. But the heat will be where the problem be.

The side that's physically hot is either the source side or the circuit side. Which was it?
 
What gets my attention is the fact that there is no fault before headlights are pulled on. If yours is like most, you have 1 fuse that isn't hot either end before park lamps are pulled on. The headlight switch energizes this fuse and the park lamps. So lets remove this fuse, pull the headlights on and see what happens. The headlights would still work but park lamps, illumination ( orange wires ) would not. If the brake lamp fuse doesn't heat up we know there's a fault in the park lamp circuit. How/where it effects the brake lamp circuit/fuse is still a mystery ( rear lamp bulbs and sockets would be suspect ).
I am assuming everything else works as it should. If the dome lamp, some indicator in the inst' panel, license plate lamp, anything doesn't work as it should... we need that detail/clue.
 
^^Also bear in mind that the headlights ARE POWERED SEPARATELY. I suppose there is ome chance of a harness cross (two wires melted) etc which effectively hooks two circuits together
 
You are guys are amazing!
Thanks so much for all your help and guidance with trying to figure out this extremely annoying challenge -- but again I think I can say all electrical problems are annoying.

So to answer Jos51700, yes there has been arching at the fuse itself on the side that connects to the (Pink and light pink wires) rear brake switch and door light which would be the opposite side (Red with white stripe wire) which is the battery feed (Ignition switch, headlight switch)

To Redfish, yes everything else works find no other electrical issues other than when driving at night with headlights on there is a slight dim in lights at idle vs when I accelerate.

To add to the mystery, I've noticed that the wiring diagram only shows one pink wire connecting to the one side of the fuse that leads to the rear brake switch. However, when I look at the physical fuse box there is an additional light pink wire that connects to the left door switch (Triggering Map light) which is not shown in the 1966 Dart Manual. The refurbished wire harness that I'm using came out of a 66 Dart hardtop however my car is a 66 Dart convertible.

The diagram does not make a distinction between hard top or convertible so I'm assuming it's for a hardtop model, but does have a square bubble illustration in to the diagram that indicates the accessory circuit for the convertible, but no difference with the door light vs dome light. (no dome light on a convertible). Is it possible that this additional light pink wire is creating the havoc?

I've attached the wire diagram for reference.

Thanks again for all your help!!

Screen Shot 2017-11-01 at 10.17.50 AM.png


Screen Shot 2017-11-01 at 10.18.31 AM.png
 
Once you figure out the immediate issue, you might want to go back and deal with the ammeter properly rather than leaving MADelectrical's halfaѕѕed "fix" in place. See here and here and here and here (their site's still skeletal; best to call them) and here.
 
Gublakhan said:
thanks for the info you two!
Click to expand...
Like 67Dart273 said, Headlamps & stop lamps power distribution is from completely separate sources, however they do have something in common, park/stop/turn bulbs
in the rear. Check ALL of the bulbs, sockets, grounds, and wires at the terminals and socket terminations. A defective bulb can cause a short/draw, so...................
 
UPDATE:

I cleaned the fusebox, the contacts as well as the fuse holder. However, it wasn't until I removed the rear harness that the fuse stopped generating heat. I metered the fuse without the harness and I got a .01 reading, then with the harness attached, I got a reading of .07-.09. With the harness plugged there IS heat, and with it unplugged there is NO heat. I then unplugged the trunk lid rear lights and Voila! The heat seems to be caused by the trunk lid lights.

I inspected and tried to improve the connections by bending back the clips on the lightbulb socket to strengthen the contact, but there doesn't seem to be a lot more to do there. And, the rear lights illuminate well, so the contact is okay. Any suggestions on troubleshooting how to find a short or loose connection of the rear trunk lid lights? Perhaps I just pull out the wires and inspect and clean them since it's a self-contained wire assembly.

Thanks to everyone for helping track down the problem -- you all saved me a lot of time and heartache and endless searching. I don't know what more to say other than I love you guys!!
 
.01 "what?" If that is amps that is not much. Post a photo or at least the brand/ model of your meter and we can help you set it up.

Some meters have an internal fuse for current. The one in a Fluke, for instance, is fairly expensive. "It's easy to forget."
 
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