Blowing a Fuse

[sdcbowler]

Hey Guys,

Trying to track down why I keep blowing the tail light/cigar lighter fuse. With the exception of the reverse light, all exterior lights work fine until I plug in either one of the front side marker lights.
I was comparing the aftermarket harness (M&H) to my original and it appears to be the same. However, when compared it to the wiring diagram I noticed a small difference. I would appreciate an opinion from the forum.

In the diagram below, the blue lines show the configuration of the new harness, which matches the original. The red ‘scribble’ of course is the wire runs in the original wiring diagram.
My question is: Do you think the harness is built correctly and the wiring diagram is not totally accurate in terms of the wire positions as they enter the plug, or is the harness built incorrectly?

For example: The diagram for the front left marker shows (L6-18BK/Y*) going into the female socket side of the connector and (L9D-18BK & L6A-18BY/Y*) both going into the male plug side of the connector.
Yet, the picture of the new harness shows (L6-18BK/Y* & L6A-18BK/Y*) going into the female socket side of the connector, and the single (L9D-18BK) going into the male plug side of the connector.

The left park/turn signal and the right marker also show a difference between the harness and the wiring diagram.
Thanks in advance

 

[pschlosser]

I will give this a go. Some thoughts as I try to think this through.

I agree there may be errors in the original wiring schematic.

In your image wiring diagram, I am confused about three wires going in to your two-connector side marker lamps. The blue line you've added seems to go right down the middle, making it ambiguous how your harness is actually wired. If you could show the blue line going to one side or the other, it will be easier to follow.

Next, if I read the schematic correctly, the left marker lamp seems to show one wire scribbled out in red, but then, reconnected again using blue. Again, this makes hit harder to for me to understand how your harness is wired.

I see the BK (black) wire is ground. It should always test as going to ground.

I conclude the BK/Y (black with yellow tracer) is power to the lights. With everything unplugged, this wire should NEVER go to ground. But it may test (using a meter) as going to ground, when its hooked to a lamp, because tiny current will go through the bulb filament.

If I understand you correctly, everything works fine UNTIL you plug in either side marker lamp and when you turn on the lights, BAM a fuse blows. Is that correct? The rest of this posts presumes so.

Keep your side marker lamps unplugged for now.

Its a long shot, but test your side marker lamps without bulbs, and confirm the BK/Y-power wire DOES not short to BK-ground wire. With bulbs installed, again, test and confirm there is a connection between these two with some resistance. The Ohms value should be pretty close to the value measured from the bulb directly.

With the lights turned off, test your wiring harness and confirm the BK/Y wire at the side marker lamp connectors does not go to battery/chassis ground.

With the lights turned on, test your wiring harness and confirm the BK/Y now shows power going to battery/chassis ground.

So far so good?

Now we can start connecting the side marker lamp assemblies to the wiring harness.

I would try testing for a blown fuse by connecting your side marker lamp assemblies to the wiring harness WITHOUT bulbs, and confirm nothing blows.

Finally, one side at a time, add the bulb and test again for a blown fuse.

I believe as you work through these tests, you are going to see the BK/Y-power wire showing continuity and connection to ground with little or no resistance. I write this believing that is the very thing blowing the fuse.

If you ARE seeing a short to ground, it is either inside the wiring harness, (and/or a connecting wiring harness) -or- one of your lamp assemblies connected on this fused circuit is shorting to ground. But after writing this, you said it only happened AFTER you connect one or more side marker lamps, so its either the wiring harness or the side marker lamps.

To prove/disprove the short is buried inside the wiring harness, you have have to disconnect every piece of hardware to said harness (or anything on the BK/Y wire), then test the pins and confirm the BK/Y wires are not ever going to ground.

I believe as you work through my lengthy list of tests, you will be able to identify the cause.

As always, should you or any of your IM Force be caught or killed, the Secretary will disavow any knowledge of your actions. Good luck, Jim.

 

[Challenger RTA]

70 cuda. Check that the wiring colors are correct.Dual and quad headlight very little difference. Then check the light sockets for rust or broken insulator. They do short out.

 

[Challenger RTA]

I don't see the diagram you posted. With harness plugged in at firewall. unplug all front side marker lights and parking lights. plug one in at a time until fuse blows. may all be bad. If fuse blows with everything unplugged look at harness.

 

[sdcbowler]

Hey Guys,
I have an update.

I tested the new harness only against my original harness. Nothing plugged in or attached
Observation: Refer to insert I probed between the BK (Ground) wire of the Left Side Marker and followed the path thru to the Right Side Marker. The signal (continuity) followed the path……Red Line

On a whim, I decided to check for continuity with the BK wire at the other connectors. The results are that I found continuity at all the locations……Blue Squares. Value Range .001-.000. There was no continuity between the BK wire and any Parking/Side Marker Light

Possible conclusion: There is no BK wire coming from the bulkhead connector that is associated with the lighting. Even with the harness plugged in at the firewall, or the Body Harness disconnected, the results are the same.

I am thinking the next step is to unravel the harness and see if anything looks out of place…….Unless you all think I should go another way.

Thanks much

 

[pschlosser]

Possible conclusion: There is no BK wire coming from the bulkhead connector that is associated with the lighting.

That is spot on. Well done. In most automotive vehicles, the metal chassis, sheet metal, engine parts, and so forth, are all connected forming ground, or chassis ground. In this way, to feed power to an item, there need be only one wire feeding power, because the chassis ground acts as the other wire.

I would not expect the bulk head connector(s) to waste space providing a ground when the whole vehicle provides, more or less, the ground connection.

It's good to note here, the grounding strap going from the chassis over to the engine block is often needed, because the engine may be sitting on rubber mounts, and its possible for it to be isolated from the chassis ground without the strap. When this strap is missing, I've pulled my hair out in the past working through problems only to see them resolved once its back in place. But this is not your issue, I'm pretty sure.

 

[pschlosser]

When I said in a prior post I thought the original wiring diagram may be in error, I was referring to the L & R side marker lamp connections. My intuition is the BK-ground wire goes into the exposed half of the bullet connection, while the BK/Y connections go to the shielded side.

The wiring diagram, particularly the left marker lamp, seems to suggest the BK-ground and the BK/Y from the left park and turn signal are connected, I am concluding that would be a short. If so, that must be an error in the diagram.

I'm going to concede I may be in error, since the turn indicator lights need a ground, so they can be powered on a turn indication with the parking lamps off.

I am thinking the next step is to unravel the harness and see if anything looks out of place…….Unless you all think I should go another way.

I would NOT do this. I have done this before when I could not identify a short. Only to later realize if I were patient and thought things through, I could identify a short without unravelling.

Let me clarify there is a difference in identifying there IS a short, and finding WHERE the short actually takes place.

When and IF you identify a short in your NEW wiring harness, it may be a warranty item you can have replaced. Once you untape it, that may no longer be an option.

I think you're getting closer. On the unplugged wiring harness, you're looking for ANY of the BK/Y powered wires to show continuity to the BK-ground wire.

If you CANNOT find that, then the wiring harness may NOT have a short within. If you DO find it, then there is a short within that would be corrected by untaping it and inspecting the wiring.

So, the only two problem areas I see in the wiring diagram are:
1. Right Marker Lamp connector - Do the two BK/Y wires combine on one terminal, and the BK-ground on the other?
2. Left Marker Lamp connector - Likewise, do the two BK/Y wires go to one terminal, and the BK-ground to the other?

 

[sdcbowler]

Hey,

Before chasing down this issue, I thought it was a 'simple' grounding problem. As you say, the body, engine, and Chassis all act as grounding points. I Think I have at least 5 grounding straps in various places. I also removed all the lights and housing to be sure there was metal-to-metal contact.....reassembled with dielectric grease.

You were right in thinking the wiring diagram is not very accurate. That's what I was trying to show in my first confusing diagram with the scribble lines. The connectors, I believe are accurate in that they indicate which wires go into the connector, but the way it is drawn makes one think that the BK (Gnd) and the BK/Y* go into the same side of the connector. I thought that was my problem until I researched online. The two BK/Y* go on the female socket side and the BK on the male 'bullet' side. I don't know what to say about the Right Side Marker. Whoever drew that one must have been very tired at the end of the day...lol.......looks like all three wires are twisted together in the center of the connector... Aside from the two you mentioned, I believe the Left Parking Lamp is also not drawn correctly. The two BK/Y* wires should both be on the female socket side of the connector, and the single BK/LGN* should be on the male bullet side.

I bought the harness almost 10 years ago so the warranty doesn't apply. My reasoning for thinking the problem exists within the harness is that a correctly built stand alone harness would not have a grounding point that would have continuity with several other locations where is should not. Based on the wiring diagram alone, there should be no continuity to those locations. I also verified by testing my original harness..

I was all prepared to start looking at other locations for the short, and I would have had I not found continuity at those other location. Will post an update whether I find something within the harness or not.

Thanks much for all your help

 

[Challenger RTA]

The FRONT L-R Turn signals signals and the front parking lights get a ground from the mounting screws. and fender turn signal ground on the socket that it pushes into.If you have them. Don't think of some of the wires as a ground. The red wire in you diagram Yes they are a ground. Hope this helps more than confusing. What year do you have

 

[Challenger RTA]

In short pun intended : If you understand using a meter. to check if the is a short in the harness.With harness unplugged from everything. put a test lead on one terminal and touch to all the other terminals and you should get no reading. check all terminals to each other.

 

[sdcbowler]

In short pun intended : If you understand using a meter. to check if the is a short in the harness.With harness unplugged from everything. put a test lead on one terminal and touch to all the other terminals and you should get no reading. check all terminals to each other.

That's correct. However I got the opposite result. I put the test lead on one terminal and got 8-10 readings across entire harness. Thats how I knew something was not right.

 

[Challenger RTA]

Open it up you might find it.

 

[sdcbowler]

Open it up you might find it.

Well,

It appears I am back at square one. Before I took the harness apart I tested it again. I looked at what other possibility could exist that would cause me to find all those points of continuity. I then disconnected the sealed beams frome the harness, and then probed the blades. There was continuity between all the blades. I don't know whether or not that should be the case but that is what it was. I suppose thats was why it showed up when the harness was connected.

Steps in testing:

1. I plugged in the harness at the firewall
2. Unplugged the body harness
3. Unplugged all front end lighting
4. Unplugged cigarette lighter
5. Moved light switch up one position
:Fuse did not blow
6. Moved switch all the way up
:Fuse did not blow
7. Plugged cigarette lighter back in
:Fuse did not blow
8. Reconnected body harness
:Fuse did not blow
9. Added all rear bulbs
:Fuse did not blow. All rear lights working except reverse
10. Added all front lighting except side markers
:Fuse did not blow. All front lights working
11. Added Left side marker only
12. moved switch to first position
:bye-bye fuse

How would I test the dimmer switch? I am open to suggestions.

Thanks

 

[pschlosser]

There is no right or wrong or definitive answer, here. But again, your side marker lamps seem involved in this crime against unblown fuses.

I'm not sure the dimmer switch is to blame, if it goes untouched during your most recent above-listed test.

Does the BLOW occur when the left unplugged, but the right plugged in?

Testing the side marker lamps, at their connectors (not the wiring harness), and unplugged from the harness, do either of the two leads show continuity to chassis ground?

Is there continuity between the two leads? I'm guessing it may show some resistance (in Ohms) that matches the resistance of the bulb.

 

[sdcbowler]

Yeah,

I believe somehow the side markers are involved as well. Tomorrow I will test the side markers at their connectors. Will test both with and without the bulb. It does not matter which side marker I connect, the fuse still blows.

 

[pschlosser]

When you're testing the side marker lamps, go gently so you don't inadvertently clear the potential short as you're testing. You want to take care to find the problem. So leave them installed in the vehicle, at first, and rule out some of the issues I mentioned in my prior post.

The bulbs, in all your lights, they are regular traditional filament bulbs, am I right? None of these are Made in China LED bulbs?

 

[sdcbowler]

The bulbs are tradition with a filament. I will perform all the test you mentioned.

Thanks much for your assistance.

Have a good night.

 

[Challenger RTA]

You are going about this right. It's a processes of elimination. pschlosser is giving good advice. Try not disturb the harness.It might be a phantom short. Check to see if bulb and socket are correctly made and installed and the right bulb.The springs in the socket can short out.
Continuity between all the blades: there should be continuity between the grounds,low's and the highs only. I'm sure you understand that just want to say it. I would have to say that the short probably melted melted the headlight wiring or vise a versa.
As far as the high low switch: They do melt from high current check it. After you find the problem do yourself a favor. Do a relay headlight circuit.

 

[sdcbowler]

Thanks to both of you for all the great advice. I am learning a lot from what you are posting. I will consider installing a relay light circuit.
Wanted to clarify one thing. When I said I found continuity between the blades of the sealed beam, I was referring to a single sealed beam only. For instance, the passenger side sealed beam that contains three prongs on the back. A prong each wire (V/W*, Red, Blk) in the connector. I disconnected it from the harness and removed it from the car. I bench tested it while it was not connected to anything else. No matter which two of the three prongs I probed, there was continuity. This lead me to believe that all three are somehow connected within the sealed beam. I didn't know if that was normal or not.

 

[Challenger RTA]

When not connected to the firewall connector and no headlights plugged in. You should get no reading between any color of wires at all. only should get a reading on same colors. With headlights plugged in you will be reading across the filament from one color to the other. All 3 blades or prongs on the same color wire are independent of each other. Ground Hi and low.