No brake lights

Hi and welcome back.

When you say you have 12 volts going into the plug in the back, I assume you mean the brake switch?

So if you have 12 volts going in what is the reading coming out? Is it 000 or is it showing some voltage with the brake applied?

Rich

Have you pulled the bulbs and checked the voltage in the socket using the socket ground?

I suspect there may be a bad connection somewhere which is why I asked what you were reading under load.

I would like you to plug everything in, apply the brake and test the voltage in the socket using the ground post in the socket. See what you get and then test it again, this time using the body as a ground. Let me know what you get.

Also you can test for a bad connection with a test light. Ground the light where the socket is grounded and test the socket. Does the test light light up normally or is it dim?

Here is a test light guide if you need it.

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

Rich

The turn signals interrupt the brake circuit so it is likely that if the bulbs are flashing that your ground is ok. It is just a very common issue that I wanted to rule out. Once the brake signal leaves the brake switch it should go through the turn signal switch which will likely be your issue unless there is a brake relay in the circuit. The car is too old for me to pull up wiring diagrams.

Do you have a Chiltons on it with diagrams?

Rich

Okay, so I can't read the diagram you posted. Too small and fuzzy so I went looking and found this link...
http://www.mymopar.com/72to76_wiring.htm

In there I found the 2 diagrams uploaded below.

Praise the lord things have changed since then. These things are a nightmare. I had to study them for 30 mins just to get a grip and I still don't get it all.

Anyway, near as I can see, as I said the brake circuit comes out of the turn signal switch.

So, you say you have 12v going into the brown wire in the plug which I assume is the one on the bottom right of diagram "B" that I uploaded.

But you say you have nothing coming out of that plug. When you tested it did you unplug it or was it still plugged in when you got the 12v?

Rich

Can you unplug it and use a test light on the brown wire and see if you get a full glow on the light and then plug it in and do the same testing from each side of the plug.

Also when it is plugged in how many ohms are you reading across the plug on the brown?

Rich

If you have 1.6 to 2.6 ohms across that plug then you have a bad connection.

This will be the tell. With the plug together, put one lead on the upstream side of the brown wire at the plug and the other lead on the positive lug in the brake light socket. How many ohms do you read?

Rich

You should have 000 ohms everywhere you check. That bulb should be a 1057 which is about a 1 ohm bulb. If you have a 1 ohm or more connection issue that would be enough to either light the bulb very dimly or not at all.

Rich

So you had 12 volts on the upstream side of the plug with it unplugged, correct?

And you have 0 volts at the socket with it plugged in?

And you are testing the brown wire for all of these as that is the brake light wire?

We are either losing info in the translation or something is missing because if the above info is correct the only possibility is a bad connection between that plug and the socket which is causing a voltage drop but all the info isn't adding up. It is in fact conflicting.

So here is what I need to help you.

Taking all these measurements can get hard to remember all the results, so I will itemize the tests I need you to do by number and description. Print this post out then take it to the car and write the results down for each number. Then come back and simply type just the number of the test and the result and I will compare that to this post.

1: Voltage reading on the upstream side of the plug.

2: Voltage reading on the down stream side of the plug.

3: Voltage reading from the upstream side of the plug to the socket.

4: Ohm reading from one side of the plug to the other side.

5: Ohm reading from the upstream side of the plug to the socket.

All of these tests should be done on the brown wire.

Rich

Also I should specify all socket tests should be to both sockets. Just give me an "L" and an "R" so I know which reading goes to which socket.

Thanx, Rich

Okay,

#3 was, I need you to measure the voltage from upstream side of the plug to the socket. So positive prob on the upstream side of the plug and the negative lead on the ground in the socket. If you don't get 12 volts you have a bad ground. Check in this manner on each socket.

My first thought was a ground issue and if your plate bulbs are flashing then that means you have feedback through those bulbs from another circuit seeking a ground. In other words you have a circuit going through the bulb to get a ground.

So next step is to Ohms check all your rear sockets to the body. Positive lead on the socket wall and negative lead to the body.

Write down what the reading is for each socket. Any of them that are not 000 ohms is a bad ground.

Trace the ground wire from any bad socket readings and then test the ground wire to the body like this.

You need a probe that can pierce a wire and then.

Set meter to ohms, pierce the ground wire 1 inch back from the connector with positive lead and put the negative lead on the connector. Any that don't read 000 is a bad connection from wire to connector. Need to cut wire, clean it up and put a new connector on it.

Also test connector its self to the body. Any that don't read 000 you need to pull the screw, clean connector, screw and metal where it connects. I use Permatex anit-sieze on my grounds cause it stops corrosion and it has a powdered copper base for conductivity.

Let me know what you find out.

Rich