Brake Light problem, need HELP
 

55 Merc Situation,
Brake lights, tail lights and signals all work fine without lights being turned on. As soon as you trun on the lights, the right side works fine (tail lights, brake and signal), but on the drivers side, the brake light goes out when applied, but the tail light and signal light still work. Any ideas????:confused:

sorry, could you clarify that sentence..

when the lights are on, left side, you should have the parking lamp.

when you step ON the brake, what happens?
when you lift OFF the brake what happens.

the brake/signal use the same part of the bulb. there is a relay that handles the triggering..

Sam

I'd bet money that it's an issue with the ground in the tail light. When the lights are off, the brake and turn signal are grounding through the running light circuit, when you turn the lights on, it loses the ability to ground through the other bulbs in the running light circuit.
I used to be a mechanic for a fleet of concrete trucks and have seen my share of faulty ground issues that caused bizarre things like yours.

I agree, faulty grounds in automobile wiring have made me scratch my head many times. I had a Uhaul company hook up a trailer once to my rig and the lights reacted very bizarre on the trailer, similar to what you mention, and it turned out to be the hitch on the trailer was so rusty it wasn't grounding correctly to my vehicle. It took them nearly and hour to figure that out.

Whoa guys wait a sec.... The lights are grounded from the socket through the light housing to the frame......On lights that have two elements. (on lights with one element like the (front)parking lights on a 51, the lights are not grounded throught the socket but rather by the second post on the bulb to a grounding wire).

Now I need to clarify what youare saying. You said "the lights work fine (tail signal and brake) until the lights are turned on." (huuuh?) So lets get more semantically specific. Do you mean your tail lights work fine when the light switch is in the "parking light" (first detent position) position. But when you have the headlights on, and you step in the brake, the drivers side running light goes out? Because the brake light shouldn't be on in the first place, right? Or are yousayin gthat if you step on the brake, th ebrake light works on th edrivers side until you turn on the headlight then it goes out? Same difference I think......Anyway.......

There are two wires going to each light, one for running lights filiment (low) and one for the brake lights filiment (high). The running lights are wired directly from the headlight switch. The running lights are completely independent of the signals and brake lights. The brake light is routed throught he signal switch. So, when the lights are on, the low filliment is illuminated. Push the brake pedal and the second, bright filiment comes on. Turn on the lights and the low filiment is running light and the hi filiment light comes on when brake pedal depressed and flashed when brake is applied and signal is switched. Right so far?

So, if both filiments are working without the head lights on, the grounds for both must be ok. Rrrrriiight????

With the running lights off or on, the signal and brake still uses the brake light filiment.

So, you're wires are crossed from the running light and signal light on the drivers side. When you turn on the headlights, the rear light thinks it is the signal light, and because there is no flasher to make it blink, it stays off. I'd be willing to bet that your drivers side tail light is brighter than the one on the passengers side when both are on.

Where that wire is switched is going to be hard to find. Hope you have a good wiring diagrame. If not send me a PM and I'll send you the one I made - lots of pretty colors - easy to read.

I have a headache now!

Good luck!

J!

Julie,
Where the problem usually develops is when a little corrosion develops between the tail light housing and the body of the truck, or even the bed and frame if they don't have metal to metal contact. I recommend placing a star washer in place of a lock washer on the tail lamp mount. They tend not to loose ground and corrode like the alternatives.

The theory behind my bad ground idea is that when a tail light housing fails to ground, the two filaments of the tail light bulb are still connected together via the brass part of the bulb where it usually completes the circuit to ground through the housing. If ground is lost in the tail light housing, the current, instead of flowing directly from the brass part of the bulb to the frame, flows from the brake switch/turn signal, through the brake light filament, through the brass part of the bulb, can't find ground so goes back through the running light filament and essentially puts all of the lights on the running light circuit in series with the bulb that isn't properly grounded.
Clear as mud right?
I've read and reread this before hitting the reply button trying to clarify it and at this point it only makes sense to the penguins in my head that tell me what to do...
Just kidding about the penguin part, they're actually llamas... Best of luck with the wiring issues 55 Merc, they can be a handful.

Ah the "phantom ground syndrome"........ very common....lonestaraggie explained it very well. Electricity is sneaky stuff and will find a ground when the normal one is missing....... this sure sounds like your problem.
Now if it was the front lights that were missing the ground the usual indicator is the turn signal light on the dash lit up very dim when the parking/running lights are on.......the rear is usually indicated by dimmer than normal brake/turn signal lights.
And I thought I was the only one to hear the penguins talking in llamas voices.......it feels so good to not be alone :)

Hmmm, perhaps I'm confused (which would be nothing new) But I'm intregued by your theory. And I do agree that proper grounding is the source of many electrical problems. Electrical trouble shooting is hard enough withthe truck and a meter there but on the internet it's even harder.

Ok so with regard to your theory: Seems like you switched from grounding (outbound flow) about half way through, to power application through the brakes and signals.

Lets talk basic electrical signal flow for a second: the running lights are powered off the TWO positions on the headlight switch (one for parking lights/running lights - first detent and one for headlights/running lights-second detent). Running lights (in the back as opposed to parking lights in the front) work off both detent positions. The current flows from the switch throught the wires, into the point on the socket base that contacts the tit on the bulb, and up into the low light filiment. After passing through the filiment, the current flows to the bulbs brass base casing, into the socket housing holding the bulb and out to ground via the metal light body/bracket/frame.

The brakes are powered off the hot bus (or directly from a hot source depending on how you have your power distribution set up). The current flows fromthe supply down to the brake pedal/MC switch then back up to the signal light switch. With the signal light switch off, the current flows back to the tail light through both left and right wires, into the point on the base of the socket and through the tit on the bulb and up to the bright element of the bulb. It flows through the bulb, to the brass bulb base (say that fast 3 times) to the socket housing holding the bulb, out the light fixture to the bracket and frame. Difference is if you turn the signal switch on, then that power is diverted to the flasher and to the appropriate wire supplying the left or right light.

I think what you are trying to tell me is that if the light fixture is not adequately grounded, then the flow of electricity will rereoute throught the other element of the bulb (because they share a commom base through which to ground) and thus complete the ground VIA THE POWER WIRE OF THE OPPOSITE LIGHT SYSTEM, back in the brake or signal circuits probably through the ground of another light in the same circuit (where it would have to becasue there are no other grounded points in the running lights circuit).

IF you did have the running light grounding itself through the brake light/signal wiring, then both filiments would be on every time power is applied because power would be flowing through both to get to ground.

Also, it seems to me that if you were running power through the running light circuit and then had it back grounding through the brake/signal circuit (or vice versa), you would be blowing a few fuses when you step on the brakes or turn on the signal. Also, the brake lights switch would have to be stepped on or signal lights turned on to complete this ground, which means none of the lights (in that fixture) would work until the brakes are stepped on or signals switched on, and in this case the reverse is true.

Either the fixture/socket/bulb is grounded or it isn't. All of the lights work correctly in this scenario until the HEADLIGHTS are turned on, so there is an adequate ground to the fixture/socket/bulb-one way or the other (perhaps your way).

I know electricity is voodoo. I have many mirrors and cans of smoke in my garage just for electrical troubleshooting. But, theoritically, I don't understand how what you are saying could be possible this scenario (so it must be true). Maybe you could expand for me.

I think it's time for Dave Boley to jump in as well.

One thing is definately certain: it would take a heck of a lot less time to walk out to the garage and test for continuety between the light bulb holder on that light and truck frame than, it did for me to write this!

J!

PS I just edited this in: I've kept reading your original post and is it possible that you have been naughty and haven't told us everything. Question: do you have a plug or wiring for towing hooked up? Any extra lights like cab lights, etc?

If you can't see them, only hear them, and they sound like llamas, how did you know they were penguins to start with?

My headackhes back! :-banghead

J!

Follow the path...
 

Hi Sharon,
Jules Cool alerted me to your post. I have read the entire thread to this point. First let me say that at this point, I am siding with the Grounding Camp on this thread. We could spend days discussing the therory behind the reasoning... Whoops , sorry, we already have I think...

I don't know if you are using the old original lights or later produced types. This alone can be a determining factor as the more recent fixtures are made of plastic and require a wired ground as opposed to a grounded metal case. In general, most if not all vehicles use the body and/or frame as a conductor for the ground path, which in most cases is the negative side of the 12 VDC supply from the vehicle's electrical system. Since this path is dependent on getting an electrical current path through a maze of mechanical connections and welds, it is most often the least dependable path, as opposed to the positive source path which is electrical wire and electrical switches all the way from the source to the device. Before you do anything, check ALL the grounding. Even that whitish film corrosing you sometimes see in the sockets can be the problem. Laugh if you like, but I have seen these problems cured by simply using some Scotch-Brite on the bulbs and sockets or even as little as re-seating the bulbs. All that has been written here about securing the grounding should be tried. Get some new bulbs and try them too. Make sure the body, frame and bed are all securely grounded with electrical conductors in the form of electrical wire and/or woven grounding straps. Use the anti-corrosion compound on all connections that is available at RV and trailer places for putting on trailer electrical plugs and sockets. I even squirt a bit inside the sockets. I strongly believe that based on what I have read here so far, that you have a missing grond path and the path is being found through an inactive filement to it's ground in another circuit. Typically, this path is negated when that other filement is energized.

Unfortunately, in most cases of restored or customized vehicles, the electrical systems seem to end up being the ******* step-child of the project. This is mainly due to it being ignored or at least considered to be the least important part of the project. Another problem is that most people who are really great at restoring and mechanical work, are often very weak in the electrical areas. I do not say this as a put down or insult to anybody, it is just merely an established fact. It is also unfortunate that even those who claim to be profesionals in this area, are seriously lacking in their ability to do safe and correct installations. I blame this on a serious lack of training in theory and practices. I do not believe that these mistakes are intentional on the part of those who do the work.

Later...

55Merc M-100

Try and run an independant ground wire from a good ground source, to the Tail Lamp then try your lights again.
If this solves the problem, then it is a ground issue.
If it doesen't check or replace the Bulb, the ones that they are making now, some are complete junk.
I had the same problem as you and it was the Bulb. Also check your connections, they can also play tricks, and are some times hard to find, good luck and keep us posted.:-X22

if all else fails and you get pulled over------ just tell the man that it's a factory recall and the dealer never contacted you! oh yeah, listen to BOLEY!!!

Just try swapping the wires at the connection from the left taillight assembly to the harness from the master cylinder switch (travels along the left hand frame rail). Then report back what happens.

Lots of great input supported by a tremendous amount of verbage - mostly on my part.

I think the long story short here is: If it's wired right and properly grounded it will work right.

So, based on all of this, check to see that the light is properly grounded. It's simple, just get out the multi meter and touch it to the bulb socket and a piece of the frame in the "0 Ohms" setting. If the needle moves, it's grounded, if not its not, and you have to provide the grounding. You can do that by cleaning all the connections and installing star washers instead of lock washers as suggested, or run a dedicated ground wire as suggested. Recheck the ground after the fix.

If it isn't the ground, then disconnect the signal/brake light wire and check that the running light works ok. That's the simple circuit. If that's ok then you are cross wired in your brake/signal wires. (including the possibility that one of those wires has a bare spot touching the frame and is grounding when activated - which I would think would be more likely in the phantom grounding scenario)

J!

Did we ever get an answer if there was a trailor plug or towing/cab lights installed that might have a bare wire or be crossed?

Problem solved
 

Thanks for everyone's input. It was a grounding problem. Hubby hooked up the new tailights the same as they were before when we replaced them. That was where the problem was. Bad grounding repeated. He sorted that out today after reviewing the schematics. Thanks for everyone's help.