Brake/Tail Light Issues

[71Mach1429]

Hello all! New member here and i was curious if anyone would have an answer to a problem I'm experiencing. 

I've got a 71 Mach 1 429 that is in the process of being restored, by in the process of being restored I mean that it was originally restored by a local classic auto shop and I could not accurately describe how terrible of a job they did. that being said I've taken it upon myself to go back and fix all of their cut corners and shoddy work.

All has been going well until I realized the other day that when i turn on my headlights i no longer have running tail lights. This is a new problem I currently haven't run into yet. I've checked all the grounds from the switch on the dash panel, checked all connections running to the taillights and i have a constant 12v until i check the bulb sockets themselves. The sockets read 0v unless you pull the light switch on the dash and then they read 0.5v or half a volt. 

Since this problem started I've also had intermittent problems with my brake lights coming on. Most recently my driver side would come on but not passenger, then i wiggled some wires behind the dash and the passenger side came on and not the driver side.... HOWEVER.. If I pull the hazard light switch on the steering column and then press the brake pedal, both sides light up clear as day.. 

Does anyone know if these two problems are most likely related? Where should I begin looking for the issue? Do you think the brake light problem could be a result of a short in the hazard light switch?

 

[Don C]

Yes, your brake light problem may be associated with the hazard flasher. They are both powered by the same circuit breaker, and, the brake light circuit passes through the hazard light switch and they both pass through the turn signal switch.

What were you using for a ground reference when you checked the voltage at the taillight bulb socket? I would try it with a jumper ran directly from the ground terminal on the battery. That will eliminate the chance that the ground reference point you were using is not a good ground. If you still do not show any voltage at the bulb socket go to the next connector towards the front of the car and try it there. It will just be a process of eliminating potential problem locations until you isolate the connector, or section of wire, where the problem is. The problem can also be in the bulb socket, with the feed wire broken off the bulb contact.

You can go to the Mustang Data tab at the top of the page and download wiring diagrams. However, the '71 diagrams are not complete, missing the second page of the exterior lights diagrams, plus a couple of others. The '72 diagrams are close enough to use, though.

Also, please go to the Introductions section of the forums and introduce yourself and car. Be forewarned, we like pictures.

 

[71Mach1429]

Yes, your brake light problem may be associated with the hazard flasher. They are both powered by the same circuit breaker, and, the brake light circuit passes through the hazard light switch and they both pass through the turn signal switch.

What were you using for a ground reference when you checked the voltage at the taillight bulb socket? I would try it with a jumper ran directly from the ground terminal on the battery. That will eliminate the chance that the ground reference point you were using is not a good ground. If you still do not show any voltage at the bulb socket go to the next connector towards the front of the car and try it there. It will just be a process of eliminating potential problem locations until you isolate the connector, or section of wire, where the problem is. The problem can also be in the bulb socket, with the feed wire broken off the bulb contact.

Hey Don! Thanks for your response!

The ground I used for testing the bulb socket is the ground in the trunk that I'm currently using for the stereo amplifier so I assume its a good ground. Although we all know what happens when we assume. I did some detective work just now on the relay box behind the light switch, using a multi-meter i checked each port on the relay individually. There's a port on the relay with a wire coming from it marked "Tail light switch", that port is only producing an intermittent 0.30V when the switch is pulled out. I also noticed several other ports on the relay that are completely dead, the only ports getting any power are the headlamps and another unmarked port. 

So if i pull the plug for the tail lights out and plug it into one of the ports that is getting a constant 12V and they turn on i have basically narrowed the tail light problem down to a bad relay?

 

[Don C]

There's no relays used in these cars, originally. There's the headlight switch and the fuse panel. If you have relays they were added, which isn't a bad thing, reduces the voltage through the ignition switch and other circuits, if the relays are powered directly from the battery through some form of circuit protection. The downside is adding a failure point. Yes, if you can connect a constant 12 volts to the taillight wire under the dash and then get taillights you'll have ruled out the wiring and connections from the dash to the rear of the car.

 

[71Mach1429]

There's no relays used in these cars, originally. There's the headlight switch and the fuse panel. If you have relays they were added, which isn't a bad thing, reduces the voltage through the ignition switch and other circuits, if the relays are powered directly from the battery through some form of circuit protection. The downside is adding a failure point. Yes, if you can connect a constant 12 volts to the taillight wire under the dash and then get taillights you'll have ruled out the wiring and connections from the dash to the rear of the car.

I could be wrong I just assume they're relays. What I'm referring to is a black box that sits behind the headlight switch and has several ports on the top of it, not sure if that's the fuse panel or if you're referring to the main fuse panel on the firewall. Either way I will try connecting the tail light wires to another port tomorrow and see if that is the issue. Thank you for your help and I'll post an update once I get to mess with it some more!

 

[Don C]

You're likely looking at the headlight switch housing. It extends out of the back of the dash and has a connector on it that has several wires coming out of the connector.