No Brake lights!

[tarafied1]

AGGGH!
I go from blinking/flickering dash lights/tail lights to now I have no brake lights! The brake light switch must work cause when I put on the 4 way flasher and hit the brakes they stop blinking. The turn signals work but the rear 4 way flashers don't. The front and dash indicator lights blink with the 4 ways. So the turn signal switch, 4 way switch and brake light switch all seam to work and the bulbs are okay. I'm so confused!

[Midlife]

Yes, it is confusing, but examination of the circuit will reveal the culprit: be patient!

The rear turn signals are shared with the brake lights.  The running lights are on a separate filament, and that line is shared with both sides.  Either the turn signals blink, or the brakes are lit, depending upon the appropriate condition. 

 The 4 way flasher bypasses the stop lamp circuitry entirely: all 4 corners should blink.  First, disconnect the 4 way flasher, since that simply complicates the system, and it may be bad.  Now test the system: headlights out, brake on.  Then headlights out, brakes off, turn signals left, then right. 

If your brake lights work, but the turn signals don't, then the turn signal switch is bad.  If neither brake lights or turn signals don't work, it is likely a connector not fully plugged in, or a bad bulb.  Report what your results are and I'll fix you up with the proper diagnosis.  Using the 4 way flashers for diagnosis is not recommended.

[tarafied1]

okay, I disconnected the  4 way flasher. Headlights off, still no brake lights. Head lights off, both left and right turn signals work. (thanks for helping, this is frustrating)

[Midlife]

Good!  This indicates that the problem is definitely in the brake line system.  12V is applied to one side of the brake switch itself, and when you press the brakes, a circuit is closed, allowing 12V to pass through the switch and up to the turn signal switch, where it combines with the turn signals and sent on it's merry way.  Since we know that the signals travel from the turn signal switch fine, the problem lies in the generation or initial propagation of the brake signals. 

Start at the brake switch and measure the voltage of the two leads.  One should be hot.  If you can, jumper the switch and check your tailights: they should now light up as though you hit the brakes.  Try the turn signal switches and see if they are still there.  If not, then the problem is in the turn signal switch itself, and may have to be replaced: they are notorious for these kinds of problems when they begin to go belly-up.

[tarafied1]

I got 9 volts at one of the leads. I jumped the switch and still no brake lights. The turn signals still work...

[Midlife]

what lead did you get 9V?  You should be seeing 12V or more or 0V.  If you see 9V going into the brake switch, then something along that input path is bad, and explains everything.

[tarafied1]

it was a green wire with a red stripe. If I jump a 12V source to the 0V lead will the brake lights come on?

[Sluggo]

it was a green wire with a red stripe. If I jump a 12V source to the 0V lead will the brake lights come on?

If you have 9 volts at the switch and jump the switch the brake lights should light up albeit dimmer than normal.

See if the 9 volts is carried back to the brake lights with the switch jumped. If so you need to look at the ground side
of the circuit.

If not you have an open circuit on the hot side between the switch and brake lights.

Check for proper bulb usage (ie. dual filament bulbs in single filament sockets and vise versa) front and back. May not
be an issue but would cause some confusing feedback.

[Midlife]

The green/red wire is the output of the brake light switch.  If the other wire going to the switch is 12V, then the problem is the brake light switch itself.  It's a $10 part and replace it.

[Sluggo]

If he has voltage on the output side of the switch wouldn't the brake lights be on?

[Midlife]

If he has voltage on the output side of the switch wouldn't the brake lights be on?

9V may not be enough to light the bulbs.

[tarafied1]

I'm in New York City (Times Square) right now, I'll double check the wire when I get home to make sure I told you the right one. The bulbs are fine, brake lights had been working okay. In fact turn signals still work fine even with headlights on. As I mensioned earlier, when I have the 4 ways on and step on the brake the flashers stop flashing (front bulbs only) so I know you said not to use the flasher for trouble shooting but if the application of the brake makes the flasher stop wouldn't that mean the brake light switch is working? Anyway I did jump the switch and had no brake lights, what I was asking was if I bring a 12V supply from the Battery and hook it to the 0V side will the brake lights come on? If they do not what could that mean?
Sorry for all the confusion...

[Midlife]

Yes, if you hook up the 12V battery supply to the outgoing side of the brake light switch (green w/black stripe), and if everything else is A-OK, you should get brake lights. 

Something is dragging your voltage down, and that may be why your front lights don't blink when you use the flasher.  First things first when trouble-shooting electrics: start simple and isolate one problem at a time.  If you did not disconnect the e-flasher, then it is entirely possible that switch is bad and is dragging down your brake light signal voltage and possibly your front lights as well. 

[Sluggo]

what I was asking was if I bring a 12V supply from the Battery and hook it to the 0V side will the brake lights come on? If they do not what could that mean?

See my previous post.

As far as lights not working or feeding back through the harness when flashers, blinkers, or brake lights are switched on, that is a classic symptom of circuits being shorted together. Most common cause: wrong bulbs in sockets (ie 1156 in a 1157 socket or vice versa) You need to check all blinker and brake light bulbs before you do any more diag.

A bad flasher can cause the front park lights to come on when applying brakes as well. I have seen this one personally.

[tarafied1]

The front flashers work fine. They are the only bulbs that blink with the 4 ways. The rears do not. While they are flashing, if I step on the brake they stop flashing like they are supposed to do. The rear lights don't flash at all with the 4 way and don't come on with the brakes but the turn signals all work fine front and rear. All the bulbs are correct and the system has been working fine until just recently. I did disconnect the flasher and still had no brake lights. The 9V should at least make the lights dim wouldn't you think rather than nothing. Again sorry for the confusion, I don't think you guys understood what I was saying. The front turn signals, flashers, parking lights work properly, not the rear except for the turn signals with or without the headlights on. Tail lights and turnsignals work, no brake lights or rear 4 way flashers.

[Midlife]

I still think the brake switch is defective.  The  way flasher feeds into the circuitry and should over-ride the brake switch signal.  However, the 4-way flasher signals do travel back down to the brake switch itself, and if there is a short there, then that can wreck havoc with those rear lights. 

To tell you the truth, I really don't know what happens when the 4-way flasher is on and brakes are on, and I'll check Saturday when Midlife comes out to play.

I'd still replace the damn switch and see if that improves things. 

[tarafied1]

okay, thanks

[Sluggo]

I spent some time looking at the wiring diagram and there are only two things the brake lights and flashers share.

1. The ground at the rear body.
2. The turn signal switch.

My money is on the turn signal switch.

The headlight switch supplies the power to the brake light switch on the green w/red wire.
The brake light switch outputs power to the turn signal switch on the green wire which is
where all the magic happens.

You can also read this post I hunted down to add validity my diagnosis. (like i need validation) :naughty

http://forums.mustangmonthly.com/70/6715630/vintage-mustangs/brake-lights-and-turn-signal-not-working/index.html

[tarafied1]

I'll probably replace both the brake light switch and the turn signal switch since they are both original 40 year old parts but it seams odd that when I apply the brakes the flashers stop blinking which means the switch did something and the turn signals still work which means the turn signal switch did something too. Weird.

[Sluggo]

You can't have flashers and brake lights at the same time. They share the same filament in the bulb.

If you are driving with your flashers on and have to slam on the brakes what do you want the guy behind you to see, brake lights or the flashers he's been staring at for five miles?

Turn signals take precedence over brake lights only because one brake light remains lit.

All this logic is going on inside the turn signal switch.

And don't buy one of the 10 dollar economy switches the parts house sells. get the good one.

Do the turn signal switch first so you can prove me right :show

[Midlife]

Sluggo: I don't disagree with you, but....

The Original Poster implied that when he examined the voltage coming out of the brake light switch (I'm assuming, of course, that the output line was not connected through to the turn signal switch) he was getting 9V.  If that was true, then the turn signal switch is not part of the equation.

The brake switch is an order of magnitude cheaper, both in cost and time to replace, than the turn signal switch.  I thought it would be worthwhile to at least explore the chance that it was the brake switch first.

[daveSanborn]

If it's any consolation Mid, I'd change out the brakelight switch also.  It couldn't take more than 5 minutes to have a new switch installed.

[Sluggo]

I guess you missed the two posts where he stated that stepping on the brake pedal caused the flashers to stop flashing.

Does this little trinket of information not indicate that the brake light switch is in fact closing the circuit? :show

If the brake light switch is inoperative, how pray tell would it have any effect on the flashers? It's one wire in and one out right?

Two other relative statements he made were:

9 volts on green with red wire (he thought)
0 volts on the other side of the connector.

I'm assuming this check was made with the connector disconnected.

We need a pot stirring smilie.....I'm gonna get on that.

[Midlife]

I did not miss that at all.  In electrical troubleshooting, the best thing to do is to first isolate as much as possible the various contributors to the problem, and test each in turn.  I like to test the easiest first, then progressing to the most difficult to diagnose.  I find that this method is the quickest and cheapest to resolving the problem. 

I first recommended removing the 4 way flasher from the circuit, and trying to see if he had brake lights and all other functions.  This isolates the 4-way flasher.  He said that did not help.  Next, isolate the brake switch, and see what the input/output voltages are of the switch.  At that point, he said 9V out.  Wouldn't you then assume that the brake switch was worth replacing?  If he did not isolate the output line, then yes, I would next move to the turn signal switch.

[Midlife]

Having re-read all of Tarafied's posts, here's the low-down:
No brake lights under normal conditions.
When using 4-way flasher, rear lights stop blinking when brakes are pressed (this is expected, as the fronts should stop blinking as well).
With 4-way flasher disconnected, still no brake lights.
9 VDC at one lead into the brake switch, 0 at the other.  Jumpering the brake switch still yields no brake lights at rear.
Rear brake lights don't flash with 4-way blinker, but front ones do.  (This is contrary to description above).
Rear running lights (separate filament) and turn signal indicators work.

With 4-way flashers enabled, you should see all four corners blink. 
With 4-way flashers enabled and brake depressed, all four corners should be on and no blinking.

With all of the above, I would agree that the problem is at the turn signal switch or the connector to the turn signal switch.  First, I would ensure that the green/black wire going to the turn signal connector at the base of the steering column is making positive contact.  If that is true, then the problem is in the turn signal switch itself.  The switch is a mechanical/electrical connector, and is often gummed up with grease and dirt, which can affect the electrical continuity.  Remove the positive battery terminal, remove the steering wheel, and clean the turn signal switch (use electrical spray cleaner with small straw into the guts of the part just under the steering wheel.  You should try and lightly pull it out of the base to access the guts).  Try everything again.  If still no positive results, then replace the turn signal switch.