brake pedal lights up marker lights?

[duane79mustang]

I got a 79 mustang. Never had a issue with wiring but now I got this issue that when I hit the brake pedal the front marker lights, front park light/turn signal and rear licence plate lights come on as well. Any ideas on where to start checking

[wraithracing]

Start checking your grounding wires first. You have the two grounding wires up on the radiator support just above and a bit to the outside of the headlights. Make sure you remove the green screws 7/32 or 1/4" IIRC and clean the metal underneath and reinstall.

The rear grounding point is on the metal for the driver side rear quarter and you have to remove the rear hatch trim to get to it, so I would try the front grounds first and go from there.

Have you chanced or swapped any parts recently into the car?

Trey

[duane79mustang]

Have you chanced or swapped any parts recently into the car?

I put after market headlights in and added a rear spoiler with the brake light on the trunk lid its a coupe by the way if that makes a difference. But it didn't have the issue when I the added parts that I remember. Just started I believe, so I dont think they are related but I could be wrong .

[wraithracing]

I would check your brake light wiring for the rear spoiler. The ground for the coupe should be in the same basic position as the hatchback, but easier to get to in a trunk.

I would bet that your power lead for the rear spoiler brake light is the issue, but checking the grounds is always a good idea anyway. You could disconnect the spoiler brake light and see if the problem stops.

Good Luck!

Trey

[duane79mustang]

Cool I'll try that.

[gt pony]

The very first thing I would do is check all of the fuses, then proceed with the other suggestions.

[duane79mustang]

Unhooked the spoiler 3rd brake wiring no change.the only other thing I see is the passanger side front blinker/park light is lit bit very dim everything else is bright. When I turn on the blinkers either direction both marker and the leds in the headlights flash because I got those wired into the marker lights as well. But when i turn on the right blinker everthing flashe's but the right front blinker bulb. I checked and cleaned front and rear grounds. And I cant get the high beams to come on with the turn signal switch think it's the turn signal switch causing all these issues

[wraithracing]

Are you sure your right front blinker bulb isn't bad?

Sounds like you have made a lot of changes recently. I would double check everything before you start throwing parts at it personally. The Multi function switch in the column could be part of the issue, but it sounds like you have other things going on too.

Good Luck!

Trey

[duane79mustang]

When I added everything it all worked like normal..this issue just came about.

[duane79mustang]

I changed everything back to factory wiring and still same issue.. any other thoughts as to where to check next

[gt pony]

I retired after nearly 40 years as a tech. One of the first things I learned when confronted with weird electrical feedback issues like you've described is to always check every fuse with either a test light or DVOM. You may have a blown fuse in a sub-circuit that is causing the affected circuit to search if you will for a feedback ground. That's the best way I can explain it.

Another way to look at it is if you don't know where the problem is, find out where it isn't. I always checked the fuses first when I had a problem like you described. You'd be surprised how many times that fixed the problem.

[JACook]

Brake lights back-feeding the marker lights is not a fuse problem, it's a grounding problem. The ground
for the brake lights, which is on the trunk (or hatch) striker support, has been compromised, and the
brake lights are finding their ground through other means. The fuses are only on the supply side of the
lights, not on the ground side.

When an electrical load, such as the brake lights, 'grounds' through another load, such as the marker
lights, the lights are effectively series wired. Because the brake lights draw a higher current than the
markers, they may not light, or may glow dimly, while the markers will light up at nearly full brightness.
If the lights are closer to the same load, then both will light up at something closer to half brightness.
This is THE classic symptom of a compromised ground.

The front harness grounds at the radiator support won't cause this, because those grounds are not used
by the brake lights (except right now they are, through the markers).

[duane79mustang]

Checked and cleaned front and rear grounds with no change. When I turn on the turn signals now all lights front and rear flash except the passage side if that makes any difference

[duane79mustang]

I changed out the turn signal switch with no change. Anywhere else that ties these things together?

[wraithracing]

If everything works except for the passenger side rear, then I would look at that bulb, socket, and wiring. It's quite possible you have a bad bulb. You could also have a socket that isn't making good contact with the bulb due to corrosion, gunk, etc. There is also the possibility that the wiring to the socket has been damaged at some point and the power isn't getting to the socket.

You can swap the bulb to another socket to check the bulb or just replace. I would visually check the socket for corrosion, gunk, damage. Last I would use a test light and verify that you have power to that socket with the turn signals on if neither of the first repairs fix the problem. This is a simple matter of deductions to determine why that bulb is not working. The odds are that the issue is in that bulb, socket, or the wiring going to it.

Good Luck!

Trey

[5.0 bird]

Not sure if this is your problem, but I had some crazy backfeed issues on my old Cougar because of a brand new tail light bulb that had a short in it.

[tonysilver82]

Make sure the bulb is a 1157 not a 1156

[duane79mustang]

Ended up being a bulb fixed it thanks guys your a life saver.