Half operating rear light

Hi, Ferraristi
I have a small question concerning the elektronics at rear lichts of my 328GTS (europe model)
During our last trip (=Italy trip with some great foto's/videos at Dutch/Belgium section) some of the guys pointed out to me that the lower part of my left rear light was not working. Normaly I scrub the connecting points and it is fine, however not this time

As you know the rear light is split into two sections which are iluminated by two bolbs. Each bolb has two glowing elements inside, one for night driving and one for breaking. In short; light switched on element A is working, puting brakes on element B is also switched on in each bolb.
The top bolb is working ok, the lower one only operates when braking.?????? Also each bolb has only 2 wires and I don't understand how is is posible (when breaking) the second element is switched on.???????? Anyway the wires are ok (otherwise the braking-light will also not work) How can I fix this???
There is an option to connect it directly to the bolb above it, but that is not the way to go I think, any suggestions before I go to the dealer????

Ps I changed all bolbs around (also at right rear light) to make sure is not the bolb, and every time the lower left position was not working so I am out of ideas.

Greetings Fred

 

To make more clear I have taken some pic. First lights switched off, no brakes on.

First bulb is connected red+yellow/black. Second bulb is same red+yellow
Image Unavailable, Please Login

 

second pic. Lights switched on, all bulbs must be on. As you can see one is not working= red+yellow/black.
Also interior licht (middel) is on as it should be.
Image Unavailable, Please Login

 

last pic. lights on and brakes on. As you can see it works fine, so wires are connected. Also I exchanged bulbs and have same result????
Dominick (D. Mauro) explained that sometimes the bottom of the fitting can be cracked, I checked not the case. Also I found out if you turn the bottom of the fitting (on which the connecting points are made) clock wise; the light ulluminates twice as bright, so I turned it back.
As I understand red=ground, yellow is +12dc on the working bulb, on the broken one red=ground, and yellow/black is +12dc but is also powered when parking lights are switched on. this is why I cannot make a bridge between yellow and yellow/black.
What could it be????
Image Unavailable, Please Login

 

Do you have a voltmeter/multimeter?

 

No, this is not correct -- there are actually three wires going to each bulb.

red = +12V when the brake pedal is depressed (and key "on")

yellow and yellow/black = +12V when the the headlight switch is "on" (any mode)

black = ground = the whole metal bracket is the ground connection, and this connects to the base on the bulb. The bulb base is connected to one side of each filament inside

The two solder balls on the bottom of the bulb are the "+" side of each filament -- when you rotate (and push in) the bulb (and get the "double brightness)" you are just making a connection to the other (brighter) brake filament -- which is actually a sign that +12V is present on the GN wire contact inside the socket, but somehow its signal is not making it onto the solder ball on the bulb (for the running light filament) when you are not physically pushing down on the bulb.

Since you only have this problem with a single bulb in the lower part of my left rear light, the problem must be in the socket itself or in the wiring nearby to that socket (i.e., since the RH light is working correctly this confirms all signal are present leaving the fuse-relay panel).

Follow the GN (yellow/black) wire to the nearby connector and make these two voltage measurements as shown in the jpeg when the running/headlights are "on" and everything is plugged in:

Test 1. from the metal terminal holding the GN wire on the lamp side to the metal bracket (ground)

Test 2. from the metal terminal holding the GN wire on the harness side to the metal bracket (ground)

both should be +12V with the running/headlights "on".

If you pass both tests = the problem is downstream in the socket-balls-bracket-terminals themselves

If you fail test 1 and pass test 2 = the problem is in that connector and/or terminals

If you fail both test 1 and 2 = you have an upstream wiring problem somewhere.
Image Unavailable, Please Login

 

thanks I will start testing and let you know.

 

Steve, I did the tests and both gave 0 Volts so it must be an upstream problem. Also I checked the first en second connectors (transfers into full yellow wire) and measured to ground, noppes/nada
Could this be related to the parking light switch? As you put on parking lights only the lower part of rear lights will be on.

Thanks for your time, I showed my wife what you writen/posted and she was impressed, How do they do that is her first reaction.
Nice engine by the way, 12 cil in overhaul, doing yourself?

Fred

 

As you already noted, this is why they separated things with the G (yellow) and GN (yellow/black) wire -- so that for the "parking" light function it only runs the lower light. However, when the external running lights or headlights are "on", both the G and GN wire should go to +12V. If you have nothing on the GN wire at the LH lamp connector (on the harness side), but do have +12V on the GN wire at the RH lamp connector, it has to be a wiring/connector problem somewhere (you might consider making the same measurement on the working RH side just to confirm your measurement equipment and method are OK). The US 328 schematic shows a connector 131 where the GN wire coming from the fuse-relay panel splits into two wires with one going to each rear side. I'm not sure where this connector is located, but they call it "connector between dashboard and front section".

PS I'm a little surprised by your result because you said that when you rotated the bulb with the light switch "on", the brighter brake filament did light up, yes?

Not an overhaul -- just a photo from an earlier 2004 project replacing some leaking water hoses -- but typical of TR work -- almost doing anything is a huge project

 

It is possible this lighting up occurred by pushing/towing the wires, so the problem cannot be far away?! If I cannot find this break I will install a new wire from Right rear to Left and disconnect yellow/black of Left. I know it is not original but to find this problem can be a little to bigger fight for me. In this way it all works as it should and it is safe. Can you send me a pic of the wiring diagram indicating this part of the installation? I would appriciate this very much
Thanks for your help, when you come to Holland give me a call/PM to have a drink.

Fred which is out of ideas....

 

I have some of the lamp bridge assemblies from old lights I replaced, but I'd have to check I don't recall the backwards mounting of the center bulb, maybe my 1976-77 assemblies are different?

 

I'd be glad to send you what I have (the US version 328 wiring diagram in a .pdf file) -- send me a PM with your email address.

 

The backwards facing bulbs shine through clear lenses to illuminate the trunk. Since you're not familiar with these I'm assuming they debuted on the 328 (or QV?).

I reversed the middle sockets on my 328 in order to add a third bulb for additional brake light illumination.

 

Ah...thanks for explaining, no the inside of a 1976 trunk is....................dark! LOL!