Brake Light Issue

Help Support 7173Mustangs.com:

perryseth

Member
Joined
May 13, 2018
Messages
10
Reaction score
3
Location
Bakersfield, CA
My Car
1973 Mustang - 351 Cleveland
Hello -

I am troubleshooting my brake lights (not working) in my 1973 Fastback. Here is what I've done:
~Confirmed that the rear turn signals both work...but mystically, the left turn signal was intermittent
~Tested for voltage at the brake switch - None
~Inspected the ground at the brake lights - Looked fine
~"Jumpered" across the female connector going into the brake light switch, brakes didn't light up...and tested this switch for 12V power, which it didn't have
~Removed the turn signal switch...From reading a separate post somewhere on the web, I believe the brake switch gets its power from the turn signal switch and it seemed like a good long-term investment especially since the left signal was acting funny. It was tough breaking off that connector, HA

Now I'm waiting for the connector to arrive and trying to clean up my wiring. I ordered a blown up laminated wiring diagram I'm waiting on...I find it all fascinating, but also quite intimidating.

I posted this to see if anyone had any other troubleshooting ideas I can try if this new turn signal switch doesn't worth. ALSO, if you have a website or YouTube video to help someone who really doesn't know much of anything on the car electrical system, that would be great. I'm even up for taking an online course to learn more.

Thanks! Seth
 

Attachments

  • 66942856280__C77FBE9C-7BF6-4D0D-A557-D1854F6893E2.jpg
    66942856280__C77FBE9C-7BF6-4D0D-A557-D1854F6893E2.jpg
    1.9 MB · Views: 11
Joined
Dec 10, 2017
Messages
542
Reaction score
107
Location
Georgia
My Car
1973 Mach 1 "Q Code"
Midlife will probably be around to help out soon, he is a vendor here and knows these electrical gremlins like they are family. In the meantime, I would continue to check all the grounds and maybe even get a new brake switch since they are cheap. I had no brake lights on my fastback and it turned out to be the switch. Not a hard job to replace, just a pain to get to. Good luck.

Tom
 
Joined
Jan 1, 2012
Messages
8,173
Reaction score
931
Location
Springfield, OR
My Car
1971 Mustang Sportroof M code
The brake light switch receives its power from the same 15 amp circuit breaker that powers the emergency flashers. If your emergency flashers work the brake light switch should be receiving power, too.

The wiring after the brake light switch runs through the steering column to the turn signal switch. A large percentage of brake light issues are due to a bad turn signal switch.

Edit: You can download the wiring diagrams for your car in this forum's 7173 Wiki.
 
Last edited:

perryseth

Member
Joined
May 13, 2018
Messages
10
Reaction score
3
Location
Bakersfield, CA
My Car
1973 Mustang - 351 Cleveland
Thank you much- I’m going to test this hopefully over the weekend - I am thinking it’s the turn signal but we shall see!
 
Joined
Apr 24, 2020
Messages
265
Reaction score
170
Location
Pittsford, NY
My Car
My all time favorite vehicle is our 1969 Shelby GT500
Lynda and I are on the road, and I do not have access to the schematics and other electrical system files I normally have at hand. But, I do plan to be home by this coming Sunday evening. I have a lot of info I think will help you out. I just hope I am not leaving you hanging mercilessly as we make our way back home over the next few days. Some of the info I have includes the circuit breaker for the brake light circuit, photos of the Relay Bar above the glove box that may (or may not) be involved in this situation. We also have two 73 Mustangs, so if you need more photos than I already have of the circuitry (and schematics) I ought to be able to get the additional info to you quickly. Although I prefer to post this info within this thread, so others can gain also, I am also available at [email protected].

I have some schematic info on my laptop, so that is attached already in this post. If you are lucky it is all you will need.
 

Attachments

  • 1973Mustang)InstrumentGauges_WiringSchematic_Pages8-35_20200917.pdf
    17.6 MB · Views: 14
Joined
Jul 3, 2010
Messages
673
Reaction score
81
Location
The OC (California)
My Car
31 Model A Station Wagon “Woody”
69 Mach 1 - 351C
72 Mach 1 - 351C
96 Cobra Convertible
If you do replace the brake light switch I would hang on to the old one if it is still working. Some of the newer brake light switches are not of the same quality and do not activate like the old ones.
 
Joined
Apr 24, 2020
Messages
265
Reaction score
170
Location
Pittsford, NY
My Car
My all time favorite vehicle is our 1969 Shelby GT500
We are back home. Attached are some files I think you will find useful. Let me know if you are still in need of help.
 

Attachments

  • 73MustangConvertible_RayRinaldi_FuseBlockPhoto_OffendingFuseForConvertibleTopRemoved_Fuse#2_20...jpg
    73MustangConvertible_RayRinaldi_FuseBlockPhoto_OffendingFuseForConvertibleTopRemoved_Fuse#2_20...jpg
    630.2 KB · Views: 13
  • 1971-1973Mustang_FuseBlockImage_FromRayRinaldi_20220122.jpg
    1971-1973Mustang_FuseBlockImage_FromRayRinaldi_20220122.jpg
    152 KB · Views: 13
  • 1971Mustang_Fuse-4_Circuit-54_Schematic_20210818.pdf
    4.4 MB · Views: 6
  • 1973Mustang_FuseBlockCircuitry_20210801.pdf
    5 MB · Views: 6
  • 1973Mustang_FuseBlockCorrectedSchematicSnippet_FromKennethPattison_20220201.pdf
    33.5 KB · Views: 7
  • 1973Mustang_FuseBlockGraphicApproach_20201205.pdf
    645.4 KB · Views: 6

perryseth

Member
Joined
May 13, 2018
Messages
10
Reaction score
3
Location
Bakersfield, CA
My Car
1973 Mustang - 351 Cleveland
ITS FIXED!!!! I installed the new turn signal switch, but no luck...still no power to the brake switch and hazards wouldn't work. I should have read more closely above because it ended up being the circuit breaker on the relay bar. Apparently...when I was young (and more stupid), I removed this bar and all the components completely. My whole dash is gone and its a gaggle of wires everywhere. Thankfully, I had an old wiring harness off a 1972 and unplugged the right cb. I plugged it in to the wires used in the description above (and followed the wire from the flasher I believe) and the emergency flashers came on. I then used a jumper wire across the connector that goes to the brake light switch and the snap crackle of current flow made me giggle. I probably didn't need a new turn signal switch but it was a good investment. I learned a ton in this trouble shooting and am grateful for this forum. More to come!
 
Top