Brake lights and emergency flashers wont work

rmontoya746 · Saturday at 2:01 PM

Good morning,

I am looking for some guidance trying to figure out this issue. About a year ago i replaced the turn signal switch on my 71 mustang (base model) which fixed the issue before. I think It could be the switch again.

I bought a new stop lamp swtich and installed that.

Things I have checked:

Both Turn signals work.
The horn works.
The head lamps all work, which is also a new.
Some times one of the emergency flashers work(LH)

I am getting 12v to the stop lamp switch on one wire and when switch is pressed, 12v to the other end. The 12v continues to the turn signal switch when the stop lamp switch is pressed. BUT when i check the wires on the turn signal switch that control the lights in the rear, I see no voltage to those. The emergency flasher is getting 12v to both sides of the plug. I think the turn signal switch is bad again... Would there be anything else to check in this circuit? Thank you for the help!

midlife · Saturday at 8:15 PM

I've talked with you on the phone, and you isolated the problem to a bad flasher can for the turn signal switch.

rmontoya746 · Saturday at 10:10 PM

midlife said:
I've talked with you on the phone, and you isolated the problem to a bad flasher can for the turn signal switch.

Thank you for the reply, I don't think we have ever spoke on the phone. What is a "bad flasher can"?

midlife · Saturday at 10:45 PM

Oh sorry...I had a call just today for exactly the same problem. The turn signal flasher can is the relay that takes power from the fuse box and when the turn signal switch is activated, it sends pulsating power to the turn signal switch that activates the appropriate lights.
Re-reading your post, it is entirely possible that your turn signal switch is bad.

Spike Morelli · Sunday at 12:10 PM

I had very similar issues not long ago. I did some preliminary checking of curcuits, but everything seemed to check out. It was the turn signal switch! I know people tend to hesitate to just change it out, but it's not all that big a deal. I would think most enthusiasts/mechanics will own a steering wheel puller.

fast67cat · Sunday at 3:15 PM

rmontoya746 said:
Good morning,

I am looking for some guidance trying to figure out this issue. About a year ago i replaced the turn signal switch on my 71 mustang (base model) which fixed the issue before. I think It could be the switch again.

I bought a new stop lamp swtich and installed that.

Things I have checked:

Both Turn signals work.
The horn works.
The head lamps all work, which is also a new.
Some times one of the emergency flashers work(LH)

I am getting 12v to the stop lamp switch on one wire and when switch is pressed, 12v to the other end. The 12v continues to the turn signal switch when the stop lamp switch is pressed. BUT when i check the wires on the turn signal switch that control the lights in the rear, I see no voltage to those. The emergency flasher is getting 12v to both sides of the plug. I think the turn signal switch is bad again... Would there be anything else to check in this circuit? Thank you for the help!

Brake light failure is a very common problem with these turn signal switches.
The problem is that they are manufactured with too much resistance at the rivet contact points.
Based on SAE standards, this resistance should be below .005 ohms. I measured a range of .5 to 10 ohms.
This resistance creates enough heat to melt the plastic surrounding the rivet.
This allows the rivet to move and the connection is lost.

This picture is from a switch failure analysis that I did for a well known supplier a few years ago.
It shows the underside of a 1970-72 standard column switch cam brake light failure.
I made several material recommendations: most were ignored for cost reasons, so the problem continues to plague Ford owners.

One good solution, after replacing the switch, is to convert to LED bulbs.
This reduces the bulb current by 85% and therefore the heat generated.

rmontoya746 · Sunday at 3:40 PM

Good afternoon, I appreciate the responses with my problem.

So what I found, see pictures. The turn signal part that activated the turn signals, there is a circuit in there where the stop light switch applies power to. The brass rod, was not making contact with the two brass connectors to complete the circuit. I may have to get another turn signal switch as I soldered them back together and now trying to get the thing back together with no gap.

fast67cat · Sunday at 3:46 PM

rmontoya746 said:
Good afternoon, I appreciate the responses with my problem.

So what I found, see pictures. The turn signal part that activated the turn signals, there is a circuit in there where the stop light switch applies power to. The brass rod, was not making contact with the two brass connectors to complete the circuit. I may have to get another turn signal switch as I soldered them back together and now trying to get the thing back together with no gap.

What brand is your switch?

rmontoya746 · Monday at 2:44 PM

fast67cat said:
What brand is your switch?

No idea, my best guess is that I got it from CJ pony or NPD.

Update: The break lights work now the emergency flashers aren't.

rmontoya746 · 2024-12-03T10:50:30-0500

fast67cat said:
Brake light failure is a very common problem with these turn signal switches.
The problem is that they are manufactured with too much resistance at the rivet contact points.
Based on SAE standards, this resistance should be below .005 ohms. I measured a range of .5 to 10 ohms.
This resistance creates enough heat to melt the plastic surrounding the rivet.
This allows the rivet to move and the connection is lost.

This picture is from a switch failure analysis that I did for a well known supplier a few years ago.
It shows the underside of a 1970-72 standard column switch cam brake light failure.
I made several material recommendations: most were ignored for cost reasons, so the problem continues to plague Ford owners.

View attachment 95673

One good solution, after replacing the switch, is to convert to LED bulbs.
This reduces the bulb current by 85% and therefore the heat generated.

Is it simple to convert to LED bulbs?

fast67cat · 2024-12-03T12:54:52-0500

Replacing the rear bulbs is a matter of unplugging the sockets and changing the 4 bulbs to 1157 LED equivalent bulbs.
An electronic 2-pin LED flasher is required when using most low current (~100ma) bulbs.
On my website, I have published a list of tested 1157 LED bulbs: L ED Bulb List
While specifically tested for sequential applications, they will also work in non-sequential applications.

Due to the unique way the side marker lights are grounded on a 1971 Mustang (through the element of the front turn bulb),
an LED side marker light adapter kit is required when installing front turn signal LEDs.
This kit redirects the side marker light ground circuit so that it doesn't pass through the front turn signal bulb.
It also maintains the feature where the side marker lights flash the opposite of the turn signals while the headlights are on.
(A feature of 1970-71 Ford vehicles)
If the front bulbs are replaced with LEDs without the side marker adapters, you will get very strange results when the headlights are turned on.
Side Marker Adapter

Brake lights and emergency flashers wont work

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