Why is only my passenger taillight out?

[Hotwatermusic]

So this a very late follow-up to the problem I had in the first post, that as of now is still unresolved. My passenger side tail light doesn't work. It's not the bulb. Bulb has been checked and known good bulb doesn't work in the socket either. So far, this is what I've done:


Checked the voltage at the socket. Only getting ~1 volt at the socket. Weird part is, after pulling the wires from the socket I checked the voltage straight from the wires, and it jumped to 6.43. That happens to be exactly half the voltage I need. I wonder if there's something to that.

I've checked the voltages at the fusebox and all seem to be at ~12-14 volts at the top and bottom of the fuses.
Can anyone make anything of this? Bulbs are good. There's obviously not enough voltage to the socket. But if the fuse were blown or bad, NO voltage would be getting there, right? How would I check continuity from the tail light all the way to the fuse box with my little multi-meter? Any help is greatly appreciated. Thanks!

[RSBob]

Fuses do go bad. Try swapping them with the good side and see if the good side goes out and the bad side lights up. Yeah its a long shot but it's quick.

[targa80]

This is half of the schematic and legend for the 1973 and should apply to your vehicle. The whole schematic is available from our host on this web site if you need it.




You can copy both pictures to your computer and enlarge them to see the circuit. The luggage compartment fuse panel consists of two fuse banks the first has 8 fuses and the second has 10 fuses. The legend indicates fuse six on the bank of 8 fuses supplies power to the Legend 19 (rear light and side marker light) using a wire with the color code of Gray/red (the red is the stripe).
If you are using a volt meter and reading battery voltage ~12 volts at both sides of fuse 6 to ground and then are reading ~6 volts to ground at the disconnected contact to your right rear light and side marker light then you have a high resistance somewhere between the fuse and the contact at the light socket. There is only one multi-terminal (6 contacts) between the fuse and the light contact terminal. This would be the most logical place for the high resistance to occur. Disconnect the six pin connector and try to ascertain which pin is the Grey/Red wire. verify that you have ~12 volts at the pin. If you have the ~12 volts and the contacts are clean then re-mate and check the light contact again.

my preferred method of troubleshooting is continuity testing with a multimeter, you can pull the fuse and using a long lead do a resistance check from the load side of the fuse in the luggage compartment to the light terminal at the rear of the car. You should see a low resistance ~0 to 5 ohms. if you see a high resistance you can recheck the terminals at the fuse, multi terminal connector and at the light contact. moving the wires may give you a visual indication on the meter where the problem is located.

[targa80]

The long lead can be made up from a wire with two insulated alligator clips (radio shack has the parts). I have had two 10' cables in my tool kit for the past 40 years. One is red and the other is black with insulated alligator clips on both ends. They come in handy to jumper out any wire in a circuit or to do resistance tests between to distant points. The continuity check is the method I used to verify my wiring harness circuits before applying power as I re-installed them into my 80SC.

[Hotwatermusic]

So fuse 6 in the bank of 8 has the gray/red stripe wire. I checked again and it has 12 at *the bottom* but isn't registering a voltage at the top. But a lot of the fuses weren't registering a voltage at the top. I swapped out fuse 6 anyway but still no light in the back. I tried looking for the 6 wire connector but can't find anything inside the luggage compartment that looks like it. There is a silver box deep inside the cowl area, just behind where the oil level gauge would be on the dash. I took a picture, is this it?

[timmy2]

The lighting harness wires run directly from the front fuse panel through the car to a six pin connector in the engine bay with no other connectors. It is to the left of the rear motor mount for the left side tail lights.



The running light wires run first to the driver side lights and then doubles back through the main harness over to the passenger side. You may have a break from the driver side to the passenger side.



If you want the correct color coded electrical drawing for your car, PM me your email address and I will send you a link to my files on Dropbox.

[targa80]

Sorry , I should have been more specific on the location of the 6 pin connector located in the engine compartment. If you read 12 volts to ground on one side of a fuse you should also read it on the other side. It should be noted that you need to have the key in the ignition to on and the light switch to the parking lite position to check for the correct voltage of this circuit. Verify that one side of the fuse has the gray/red wire attached. The information I have indicates that the schematic that Timmy2 has provided is for a 1978 series 911 and the one I provided is for a 1973 series 911. The 78 schematic shows fuse 3 as the one for the rear lights, where as on the 73 schematic fuse 6 is the supply. You need to locate which fuse has the Grey/Red wires attached and go with the schematic that corresponds to your wiring. In most cases, the wire color coding has stayed the same throughout the model years. but the fuse being used has changed i.e. in a earlier post for a 89 series used fuse 1 for the parking lights.

i just looked at my car and i have two grey/red wires. one is from the light switch to fuse 3 supplies 12 volts when light switch is on with ignition on. and the second wire is on the other side of fuse 3 going back to the light assembly.Once you locate the grey/red wires I would suggest using the continuity test with a multimeter to isolate the wiring and locating the problem.

there is also one other check the 911 has a european feature that when you park the car and shut off the ignition but leave the directional signal in the left or right positiion the parking /side marker lights for that side will illuminate. due to the narrow streets in europe this allows on coming cars to see that a car is parked on the side of a road at night.

[timmy2]

Targa80,
Zoom in on the picture of the schematic I posted. It clearly states it is for a 1974.
Similar to the '78 but different in many ways.


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[Hotwatermusic]

Thanks for all the info, I tracked the problem down to the 6 prong connector in the passenger side of the engine bay. There's a short of some sort in there, as I can wiggle it and it will work for a second, then goes back to dead. I also realized in the removing the backing plate of the tail light socket that the bulb now doesn't seat against it very well. So between those two things I think I can get this light fixed. Don't know how I'm going to attack the short in the connector though...

[timmy2]

Hotwatermusic,
PM me or email me and we can come up with a solution for your connector.
I can either have you cut yours off, send it to me to repair it and cast a new rubber molded plug for it, or make a new one for you to splice in replacing your old one. I would want your old one for parts either way.
I can give you precise instructions and supply quality parts for Splicing it back together.
Here is a picture of a connector I made.





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[timmy2]

Factory on the right, mine on the left.



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[targa80]

Originally Posted by timmy2

Targa80,
Zoom in on the picture of the schematic I posted. It clearly states it is for a 1974.
Similar to the '78 but different in many ways.


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Timmy 2 ,

My apologizes you are right, your schematic is definitely for the 1974 and not the 78. I should have look closer.

[timmy2]

Originally Posted by targa80

Timmy 2 ,



My apologizes you are right, your schematic is definitely for the 1974 and not the 78. I should have look closer.

No problem, the OP hasn't responded to my offer to send him the real print for his car or to help him with his busted up 6 pin plug. Go figure...


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[Hotwatermusic]

I'm here! I just had to work today and couldn't respond. I wanted to examine the connector a little more before asking for further help, just to see if the problem could be fixed here. Believe me, I am not taking the knowledge and help available on the this board for granted. I will report back ASA I know exactly what is broken.

[timmy2]

Lol, no problem HWM that work stuff is what let's us afford this madness.
If the failure is actually in the 6 pin connector you are dealing with a broken or cold solder joint or a broken pin. The pins are soldered to the ends of the wire before they are cast in the rubber connector and the white plastic alignment guide is installed.


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[rockabillyjay]

Any solution to this? My 78 just decided to do the same thing!

[digdug]

My car had the same issue. Issue turned out to be a mess. Totally destroyed the harness which goes through the tunnel.

[nameisbauer]

Reviving this old thread since it just happened to me. I have tracked it down to the Grey/Red wire from the female side of the rubber plug. Jumpered from tailight to Male side, lights work again. My reason for reviving this thread is now how to fix this with the molded plug. Does Timmy2 still make these? Thanks all.

[kevbo]

Re-reviving this thread because this just started on my 86. Symptoms are right side front marker and right rear tail light stopped working outa the blue. So far confirmed the fuse is good and breaking out the multimeter to begin diags. I did purchase new light weight housings so save about 10 lbs and will be converting to those for now as I refurbish the original housings.

Next move is check voltage on fuse 6 on top and bottom. Anyone have additional suggestions?

Thx

Kevin

[kevbo]

Update. Fuse 5 (working left side) 13v, Fuse 6 (non working right) 1.8v (both top and bottom of fuse). What on earth could cause that? Faulty light switch since that’s the next upstream component?