Odd rear light issue. Thoughts?

[alec2538]

Hello all, relatively new owner here looking for some collective knowledge. I could have sworn I had an account both here and the AACA forums, however I couldn't find any record of my signing up for this amazing source of Reatta information. 

 

Anywho, I recently acquired a solid diamond in the rough: 1991 coupe, Bright Red over Flame Red with gold pinstripes, black cladding, CD, no sunroof or 16-way. 94k miles, 100% rust free. Purchase price? $800 and I drove her home after sitting for almost 3 years. 

 

I tackled the multitude of burnt out exterior lights first: I replaced every bulb in the rear light bar and almost every bulb in the front (down to ordering closeout 890's from RA for the front signals). Everything looks amazing, except for one issue I just noticed... On occasion, the LH inner-most stop/turn bulb will remain illuminated after shutdown! I know a photo would be better, but it's tough to catch as it doesn't occur every time.

 

Imagine this being the left half of the rear light bar: (x)(O)(O)(O)(y)(y). x= 194 side marker, O= stop/turn/tail, y= tail only. I bolded the offending bulb for clarity.

 

As I said, the issue occurs after shutdown. I'll park, turn her off, exit, lock the doors, and walk around back only to see the bulb of note fully illuminated. Not running light level, full brightness. I've tried everything I could think of in an attempt to narrow down the list of possibilities without any luck:

 

Cycle ignition

Cycle fog lamp switch

Cycle headlight switch

Tap brake pedal

Cycle turn signal switch

Cycle hazard switch

 

Keep in mind, this occurs with the ignition completely off. If I apply the brakes, all the bulbs illuminate as they should. Release, and the offending bulb remains lit. Activate either the hazard or turn signal, and the outer 2 bulbs flash while the offending bulb remains lit. 

 

The kicker? The problem always corrects itself. Last night, after putting her to sleep in the garage, I noticed the light glowing. I didn't bother to fiddle with it, I went inside for an hour or so then checked again. Lamp was off. Same story this morning: stuck on after arriving at work. Checked ~1hr later and everything was fine.

 

I've poured over the FSM schematics, and for the life of me I CAN NOT understand how this single bulb can possibly remain lit after shutdown... 

 

If it helps: I replaced all the bulbs in the tail light with plain old Wagner bulbs. 194's and 2157's. The sockets were all in great shape as well. All the lights function properly aside from this one trouble maker. 

 

Thanks in advance for any and all thoughts/suggestions!

[Ronnie]

Welcome to the forum!

 

Your '91 might be different from my '88.  My FSM wiring diagram shows all the stop/turn lights on the left side of the car are powered by the same wire.

 

However, the two outside lights are fed by one branch of splice S420 (in the passenger side of the trunk) and the light that is staying on is fed by another branch of the circuit. There has to be a problem at splice S420 (see photos).

 

I don't know where the voltage is coming from to make that light stay on but it has to be something at that splice or somewhere nearby. Otherwise all three wires would come on at the same time.  In other words if one light got power they would all get power.

 

The  only thing I can think of in that corner of the trunk that would have power when all lights are off is the power antenna. There has to be a short or stray voltage causing that light to stay on. 

 

 

[2seater]

I am pretty sure the 91 is as my 90 and that is the bright filament in the outer two tailight bulbs is powered directly from the turn and brake lights in the cabin. The third, or inner bulb of the three for stop/turn, is sort of like the above except there is a relay bank in the trunk on the right side wheel housing. Each side has a dedicated relay for that single inner high filament. The relay is hot at all times and the brake/turn wire trips the relay. My guess is a relay hangs up. 

[Ronnie]

35 minutes ago, 2seater said:

The relay is hot at all times and the brake/turn wire trips the relay. My guess is a relay hangs up. 

Makes sense. I knew there had to be a good answer because the way my '88 is wired it would be almost impossible for that to happen.

[alec2538]

On 5/16/2023 at 9:40 PM, 2seater said:

I am pretty sure the 91 is as my 90 and that is the bright filament in the outer two tailight bulbs is powered directly from the turn and brake lights in the cabin. The third, or inner bulb of the three for stop/turn, is sort of like the above except there is a relay bank in the trunk on the right side wheel housing. Each side has a dedicated relay for that single inner high filament. The relay is hot at all times and the brake/turn wire trips the relay. My guess is a relay hangs up. 

That makes total sense to me. I located the relay center in the right rear, I'll order an OE relay and see what happens!

[2seater]

4 hours ago, alec2538 said:

That makes total sense to me. I located the relay center in the right rear, I'll order an OE relay and see what happens!

Just switch to the two relays for the lights around and see if the fault follows the relay

[alec2538]

On 5/18/2023 at 8:36 PM, 2seater said:

Just switch to the two relays for the lights around and see if the fault follows the relay

I didn't even have to go that far. Earlier today, I had the light stick on again so I popped the trunk and tapped on the relay. The light immediately went out. Diagnosis confirmed!