Left / Driver's side Tail light Issue - comes on dimly by itself


PeteK

GT Owner
Apr 18, 2014
1,813
Kalama, Free part of WA State
First: Now I know who to ask if I have an electrical problem--FENZO!

Second: I'm thinking it has something to do with the hazard flasher system. I'm pretty sure you have to pull out the instrument panel to get to the flasher connector (what a PITA--as I have recently found out first-hand by changing to Speedhut gauges), but if you can or are in there for some reason, try pulling off the connector and see if that changes the results.

Third: Many German cars have an odd feature where if you leave the turn signal on, that tail light will stay illuminated even with the key off. It's supposed to be some kind of safety feature. I have no idea if this has anything to do with the FGT, but try switching that turn signal off and on with your key off and see if there's any correlation.
 

Johntpr

GT Owner
Jan 18, 2017
209
Rockland County, NY
OK, so the diagrams above are a good tool. But it reminded me of another test. With the tail light fuse (relay?) installed, try removing other fuses/relays one at a time until it goes out. If there is a crossover somewhere that will help tell the source. Then the wiring diagrams above will help find where they might be crossed. Note: There aren't too many things that have power when the ignition is "off".

Hmmm... The "hazard" blinker is one that would have power at all times. Have you tried toggling that switch to see if things change?

Yes. First of all,the hazards do have power with the car off. I tried turning on and off the hazard switch a bunch of times to see if that would change things, but no luck.
 

Johntpr

GT Owner
Jan 18, 2017
209
Rockland County, NY
First: Now I know who to ask if I have an electrical problem--FENZO!

Second: I'm thinking it has something to do with the hazard flasher system. I'm pretty sure you have to pull out the instrument panel to get to the flasher connector (what a PITA--as I have recently found out first-hand by changing to Speedhut gauges), but if you can or are in there for some reason, try pulling off the connector and see if that changes the results.

Third: Many German cars have an odd feature where if you leave the turn signal on, that tail light will stay illuminated even with the key off. It's supposed to be some kind of safety feature. I have no idea if this has anything to do with the FGT, but try switching that turn signal off and on with your key off and see if there's any correlation.

Ok with key off, moving the turn signal switch around doesn't change anything...left tailight still on.
 

Tomcat

GT Owner
May 23, 2013
1,112
North Carolina & Perdido Key
Third: Many German cars have an odd feature where if you leave the turn signal on, that tail light will stay illuminated even with the key off. It's supposed to be some kind of safety feature. I have no idea if this has anything to do with the FGT, but try switching that turn signal off and on with your key off and see if there's any correlation.
I think its a "parking/stranded on the side of the road" warning light
 

soroush

Ford Gt Owner
Mark II Lifetime
Aug 8, 2007
5,250
if nothing else works I know the number to a good exorcist
 

KJRGT

GT Owner
Mark II Lifetime
May 4, 2006
2,840
SoCal
if nothing else works I know the number to a good exorcist

That is funny!

Not so with the issue at hand. Best of luck. You will prevail.
 

paul b

GT Owner
Nov 2, 2006
810
Not much help, but many times an energized or bad ground circuit causes a dim light to glow when it shouldn't. Can you get to the light socket and test the ground?
 

Johntpr

GT Owner
Jan 18, 2017
209
Rockland County, NY
Not much help, but many times an energized or bad ground circuit causes a dim light to glow when it shouldn't. Can you get to the light socket and test the ground?

yes, tested/cleaned all the grounds. Light socket ground OK ..just has voltage coming from left turn signal feed and bulb failure feed. Looking more and more like a smart junction box circuit board or other internal failure.
 

twobjshelbys

GT Owner
Jul 26, 2010
5,645
Las Vegas, NV
Did you try to swap bulbs left to right?
 

Johntpr

GT Owner
Jan 18, 2017
209
Rockland County, NY
Did you try to swap bulbs left to right?

Well, the issue is there is voltage going through harness back to left tailight when the car is off. The rear tailights are LED and have no traditional bulbs.
 

twobjshelbys

GT Owner
Jul 26, 2010
5,645
Las Vegas, NV
OK, what I was looking for is that you stated there was power on the bulb failure line. Check the other side to see if that line is hot on the other side. It's possible that that particular line passes through the voltage through the lamps and if it goes open signals the failure (ie, a negative true)
 

FENZO

GT Owner
Mark II Lifetime
Jul 7, 2008
1,517
Lafayette, CO
Sounds like a PITA to check the voltage coming out of the SJB, but you would know immediately if that was the where the extra voltage was coming from. I guess it makes sense that C & E have the same voltage, since the "bulb fail" is what causes the indicator to fast flash when the LED is inop (I don't know how but that suggests they are connected).

If you are picking up a voltage from a chaffed wire or a bad connector twobjshelbys's suggestion of pulling the fuses in turn should identify that. Although it seems that whatever it is must be energized by fuse 18 so probably not. You could also check between the SJB and the tail light for both the C (102) and E (33) circuits. They diverge after C300.

From the tail light, C412 for C & E..

C E
pin connector pin connector
1 C300 4 C300
25 C210 6 C2280c (SJB)
28 C2280e (SJB)

The workshop manual also has some troubleshooting info on resistance and voltages for the tail lights, not sure that will help tho.

A bug probably took a dump on something in the SJB. That's where my money is.
 

paul b

GT Owner
Nov 2, 2006
810
any indication of mice in the garage?
 

PL510*Jeff

Well-known member
Mark IV Lifetime
Le Mans 2010 Supporter
Nov 3, 2005
4,779
Renton, Washington
Interesting coincidence today, my son was getting something for me, that I left in the GT.

He called to let me know he found what I had left. Then he called back to tell me that when he closed the garage door he noticed that my right rear taillight had a "soft glow". Tried all the switches, key on-off etc. Never happened before.
 

Johntpr

GT Owner
Jan 18, 2017
209
Rockland County, NY
Interesting coincidence today, my son was getting something for me, that I left in the GT.

He called to let me know he found what I had left. Then he called back to tell me that when he closed the garage door he noticed that my right rear taillight had a "soft glow". Tried all the switches, key on-off etc. Never happened before.

Oh no. Start with obvious first, battery condition, battery ground cable where it meets the frame etc.

In my case, I am waiting on a new smart junction box.
 

NorthwoodGT

GT Owner
Jun 12, 2009
1,172
Michigan
just my opinion but I seriously doubt it's the smart junction box (GEM module). I would change all the relays in it before I'd change out the box.
 

Johntpr

GT Owner
Jan 18, 2017
209
Rockland County, NY
just my opinion but I seriously doubt it's the smart junction box (GEM module). I would change all the relays in it before I'd change out the box.

Did that...changed relays....no luck. Traced all the wiring at every connector up to the smart junction box...don't know what else it could be besides the smart junction box.
 

PL510*Jeff

Well-known member
Mark IV Lifetime
Le Mans 2010 Supporter
Nov 3, 2005
4,779
Renton, Washington
Oh no. Start with obvious first, battery condition, battery ground cable where it meets the frame etc.

In my case, I am waiting on a new smart junction box.

Update..light is now off. Had my son put the battery charger on, and within 2 minutes the light turned off.

GT's do weird things when the electrical supply is poor. Odd that the SJB, with low battery voltage,(Longest non driven period in 11 years! would cause the non key operated tail/parking light to turn on with a glow. In my case it was on the right and johnpr's was on the left.

In my case the GT was parked for 6+ months. (Btw longest non driven period in 11 years!) Not on tender. Battery was flat. This was two weeks ago. Using the advise of our GT Guru - aka Rich Brooks - we put the charger on at a 2amp setting for two hours. The unplugged it for 2 hours. Repeated three times. No issues - gauges all worked correctly - didn't lose data so passed the required emissions check for license renewal.

Drove it everyday for the next week. Only thing I noticed that was odd - when key was turned on the gauges were a little tardy in being initialized, odometer was the last to come alive. It took maybe 30-40 seconds to activate all of them. I have always waited to push the start button until all gauges were powered up. Over the period of that week of driving I put on nearly 600 miles. No issues - other than slow gauge activation. Battery is less than 3 years old.

So this morning my son put the charger on again for two hours. And gauges and OD were all initialized in just a few seconds.

The tail/parking light issue is gone but it will be interesting to discover why low battery would cause a led twilight to turn "itself" on.

Bill Dickey - Anlogdesigner - Two Shelby - or any of you ee guys on the forum are hereby challenged to sort through all of this and hopefully find a final solution.
 

twobjshelbys

GT Owner
Jul 26, 2010
5,645
Las Vegas, NV
Bill Dickey - Anlogdesigner - Two Shelby - or any of you ee guys on the forum are hereby challenged to sort through all of this and hopefully find a final solution.

I think the original poster said he has already replaced the battery and checked the ground. However, he didn't say if he had brushed it around the cable ring to remove paint to get a good ground.

I really don't think it is the SJB causing his problem but I guess we'll see.
 

Johntpr

GT Owner
Jan 18, 2017
209
Rockland County, NY
I think the original poster said he has already replaced the battery and checked the ground. However, he didn't say if he had brushed it around the cable ring to remove paint to get a good ground.

I really don't think it is the SJB causing his problem but I guess we'll see.

Yes, i ground down the paint around the cable ring, and even added another ground wire to the battery. New oddessy battery in place with proper voltage output.