I've read several threads about brake light issues and all seem to be pretty unique and varied, so here's my story.


I recently purchased a '91 240 Sedan and noticed brake light problems. Initially the guy at Advance Auto said I had brake light switch issues, so I got one and was getting ready to do that job and saw what a project it could be. I dug a little deeper into threads and found how problematic the tail lights can be, so started digging into them and realized that they were in horrible shape: contacts in light sockets loose, circuit board ripped and burned out in spots, generally loose fitting all around.


I replaced the tail lights with some aftermarket ones and seemed to have the problem solved but not quite.


When car is off, brake lights work fine; left, right and center all light up. When car is running with lights off, brake lights work but bulb out light on dash lights up when stepping on the pedal. When lights are on, bulb out light is on continuously, left brake light is always on, and right and center brake lights work. All other lights are working fine.


Can this be explained to a newbie with limited electrical abilities.


Have multi-meter, willing to learn.

 

sounds to me like a wiring issue, where maybe the left brake light and left parking light wiring is somehow interacting (insulation failure somewhere?) other possibility is a marginal ground at the left side tail light.

those bulb out detectors get increasingly flakey with age. they measure an imbalance between the current drawn by the left and right sides.. unmatched bulbs can set them off. marginal grounds, or wiring/connection resistance can set them off.

btw, don't use brass base american style bulbs on these cars except in a pinch, always find the chrome base bulbs, usually in the sylvania/osram green hang cards labeled OEM or European... the brass ones will corrode in the sockets I think the correct lamp is a Sylvania 7528 instead of the US 1157.. and a 7506 instead of 1156 for the single filament 21W bulbs.

 

The bulb itself could have the dual filaments ( the little coil wires in the bulb ) shorted to each other.

 

How would I go about tracing this interaction? The ground was intact but I did strip it out when installing the tail light so I refurbished the ground connection and it's back to being a solid ground.


I think, but will have to verify that the brake lights work as they should when the lights are off, meaning they are at the same intensity but when the lights are on is when the left brake light filament is lit along with the tall light, so there is no on/off when the pedal is depressed.


I will check the bulb more closely to see if there is a possible short inside.


I'll snoop around for the interactions but any additional insight is greatly appreciated.


Thanks for the advice and info.


Signed, electrically challenged/MJ.

 

I have seen several times, where the brake light filament will detach on one end, and touch the tail light filament within the bulb.When the taillights are on, current flows through the tail light filament and shorts across to the brake light filament simultaneously,thus lighting both filaments.

To isolate the two circuits : remove the suspect bulb at the left brake/tail light. Turn on the light switch,and with a test light check to see if you have power at the brake light terminal. If NO power on the brake circuit,then problem is right there at the socket/bulb. If there is power on the brake light wire,then problem is elsewhere,like the wire harness or under the dash.


89 245 wagon
318,000 miles