911 SC brake light issues

[rpo420]

I have a 1978 911SC, and apparently a ghost has moved in to my machine!

When I apply the brakes during the day, the drivers side rear brake light is fine, but the passenger side rear brake light is much dimmer, but it does light.

Then when you turn on the headlights for night time driving, we have a new phenomenon...the drivers side light works as normal, but the passenger side light goes full bright when you turn on the headlight switch and it just stays on. Then when you apply the brakes it goes out completely.

I am pretty handy with most stuff, but electrical is not my forte so to speak so I am reading up on electrical stuff and I do have a test light and a multimeter.

I have been reading on this and some say it could be a bad ground? If so why would it light at all? Or could a ground do this? Could it be the actual rear brake light switch? Has anyone seen this before? I dont want to drive the car at night now cause of this!!! and I love to driver her at night! I am no electrical wizard so any help would be greatly appreciated.

[SC Eddie]

I'd start by pulling the right side cover and change the bulbs. you may have a burnt out filament. are your turn signals working correctly and how about backup lights?

[rpo420]

SC Eddie, yea the bulb is brand new, and I put new ones in both sides. The turn signals are working fine as well. Back up lights too...

[Bob Kontak]

One check I can think of is if the brake light bulb is put in backwards (some of the light bulb fastening ports are so tired, I believe this can happen - I think there are two filaments in the brake bulb, right?)

Also, check the springy tangs that feed the bulb. Do they look "OK"

So you want to get busy?

Here is what I am thinking - although it's mostly guessing with a little experience mixed in.

I think some connections may be talking to other connections when they should not be.

I can't come up with a scenario that explains all your symptoms but I do think that when your brake light is very bright (regardless of voltage source) that is the way it is supposed to look.

On the passenger side - rearward of the oil filter - way back in the corner is the circular plug to the tail light assembly. There is one on each side.

The plug only goes in one way. There is a notch.

Each male/female port feeds voltage to something. The body side is female. There are no ground connections that go through this connector. All grounds are to the car.

Take your multi meter and place it on the resistance/ohms mode. If you have an audible selection which beeps when there is no resistance - select this. Touch the probes together to confirm beep.

Now place one probe in a body side hole - at a slant to touch metal - and move the other probe around the circle. Slow enough to make contact and then wiggle both a bit to increase chances of making a connection. Should be easy with these holes as they are over 1/16th of an inch.

Go to the next hole with the fixed probe and repeat the circle run with the other probe. You get the idea. If you have two power wires touching you will get a reading on the ohm meter.

Next take one probe and put it in a hole. Touch the other to bare engine metal or other solid ground. There should be continuity between the port and ground. I don't think there is much resistance in the bulbs. If you have a ground issue, it should show up as high resistance or no reading on the MM.

Here is a pin-out of the driver's side I made a few years back by testing voltage on the male pins. To make sense of this, pull the connector on the driver's side and face the female port. I believe the notch is at 12:00. As you go clockwise around the circle, the connection to which light is referenced as to time on a clock (sorry so windy)

12:00 Tail lights
2:00 Not used on driver's side
4:00 Reverse
6:00 Tail lights
8:00 Brakes
10:00 Blinker

The two o'clock position on the passenger side feeds the oil level sending unit in the oil tank. I am also not sure if the clock is backwards on the passenger side.

[rpo420]

Bob thanks so much. I will check all of that and get back.

[gtrp]

Hello rpo420

Did you ever figure this out? I'm in the exact same dilemma on my 911sc.

At first the golden metal prongs in the housing weren't fully touching the bulbs but I gave them a little pull with a tiny hex key. That fixed the bulbs but now when my headlights are on and I press the brake, the passenger light goes out.

[rpo420]

GTRP I should have posted after I fixed this, but for me it was just the tangs behind the bulb. Once i moved them out toward where they touch the bulb my light issue was fixed. If it comes back I will let you know. I would check what Bob Kontak said. Good luck!