Weird brake light problem?

mustangtommy · Jul 18, 2010

Hi,
I have a 1990 gt 5.0 hatchback.
I have a strange brake light issue. Both left and right brake lights do NOT light up when I step on the brake pedal. The strange thing is... the third high mounted brake light works fine. All other lights work fine. Fuse is good, both bulbs are good. When I turn on the lights, the tail light bulbs work (running lights) just don't brighten when the brake pedal is pressed. I have single element bulbs. I notice carquest and mustangs unlimited both think my car takes a different double element bulb, but they are definitely single element. The double elements wouldn't even fit because of completely different plug design.
My first thought was the brake light switch, but could this be the problem when the third high mounted brake light works fine? I disconnected the brake light switch terminals and of course the high mounted brake light indeed stopped working. I traced and wiggled wiring and harnesses to the bulbs, but got nothing. Should I go ahead and replace the brake light switch anyways? Any other suggestions? PLEASE HELP!
Thanks.

MustangLX-5.0 · Jul 18, 2010

It's not working because you have the wrong bulbs installed. Single element bulbs will only light up at one brightness level. There is no resistor or the like that changes the bulb brightness for park lamps. This is why you don't see anything when you step on the brake. They are already lit up. And second, when the park lamps are off, they won't ight up because... again they are single element. Single element bulbs only have one contact on the bottom and then the side acts as the ground. The socket for your brake lamps have one side terminal and two contacts at the bottom, meant for a dual element bulb.

Heres where you are probably getting it wrong... or the books have it wrong. In earlier year Mustangs, the rear lamps used push & twist bayonette type bulbs such as 1157NA (NA = Non-amber clear). On the later Fox Mustangs, they converted over to snap-in type bulbs. These bulbs have a blackor white plastic bottom connector that is narrow and just pushes to lock in. An example of these types are 3157 - 3156 - 4057 etc.

Which bulb type do you have? The push and twist round base, or the black/white snap-in to lock base? I'm not 100% sure. The books get some bulb types wrong and don't always update the listings for newer types.

mustangtommy · Jul 18, 2010

Thanks so much for the quick help.
You are right they are the double element 3157, I'm an idiot and was pulling the blinker single element bulb. BUT my problem remains, because both elements look fine on both bulbs. Where should I go from here? Thanks again.

HISSIN50 · Jul 18, 2010

IIRC, power for your body-mount lights go through the MF (Turn signal) switch but the power for the third brake light doesn't.

You might wanna check in/out voltage at the MF switch.

MustangLX-5.0 · Jul 18, 2010

Did you replace them with new bulbs? The brake lamp 3157 bulbs that is. Do the sockets have any corrosion in them? Check for corrosion then if that passes, try another bulb. If that don't work, get a voltmeter and test for 12V at the pins while the brake pedal is being depressed.

mustangtommy · Jul 18, 2010

You are awesome help MustangLX. I am in a small town so my two parts shops both closed at 3pm today. I suspect that I might not be able to visually inspect these elements like you can with a regular incandescent bulb. I tested with a voltmeter and when brakes are pressed current is getting to that pin in the socket. I was sure it couldn't be the bulbs because I thought they both went out at the same time, but I think I am most likely wrong about that too. I bet they went out one at a time and I never noticed. So in the end I am 99% sure I just need two new 3157 bulbs DOH! Will follow up after replacing proper bulbs.

90mustangGT · Jul 19, 2010

Time to grab a test light and see if you are getting power there. Are you getting ground?

mustangtommy · Jul 19, 2010

Ok MY lessons learned.

1. don't be an idiot and pull the blinker bulb. LOL

2. don't assume it can't be the brake bulbs because both left and right are out at the same time.

3. apparently bulb elements can look fine, but aren't.

4th just replace the RIGHT bulbs first! ROFLMAO

Thanks again, LX-5.0 and all, you guys are GREAT Fast help, even giving time to FNG's like me.

dannyp8105 · Jul 19, 2010

CHECK FOR A LOOSE GROUND/WIRES I HAD THAT PROBLEM THAT THE DRIVER SIDE BRAKE LIGHTS OR BLINKER WOULD'NT COME ON. I CHECKED THE WIRE ON THE TURN SIGNAL SWITCH ON THE STEERING COLUM AND SURE ENOUGH THERE WAS A WIRE NOT CONNECTED IN ITS HARNESS TRY THAT...

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