An electrical work-around for brake lights?

[JimBee]

Surfing around the net and forums seems I'm not the only one to have the brake light gremlin that can be so hard to find, though I haven't seen any clear solutions posted.

It's tough to trace and test the wires because from the brake light switch they go into a loom that goes into the electrical center (fuse and relay box), then out through various pathways and into the light control unit near the rear tail light THEN finally into the tail light socket panel.

I thought I had it solved b/c I had brake lights with the socket panels out of the tail light housing. But when I put the panels back in and moved the car, the fuse blew again.

SELF TALK: Since I know I have power into the brake light switch, that's not the problem. And the switch seems to work and technicians tell me an internal short in the switch is very unlikely (if there was one of course that would blow the fuse).

The light control unit in the fuse box doesn't seem to be the problem b/c if the fuse is otherwise going to blow it does whether or not that module (relay?) is installed.

The rear-most bulb failure sensor box (near the rear tail light — didn't know you had one? Me neither until this issue cropped up) does seem implicated, but is it the module itself or a loose wire? So far, unable to tell. One poster with an V 70 replaced that module, thought he had it fixed, and then discovered the problem persisted.

A lot of that meandering brake wiring circuitry seems related to the bulb failure sensor(s).

SO WHY NOT JUST RUN A WIRE FROM THE BRAKE LIGHT SWITCH DIRECTLY BACK TO THE WIRE THAT FEEDS THE BRAKE LIGHTS AND FORGET ABOUT ALL THE FANCY TECHNOLOGY. That's the green wire that exits the light control module (failed bulb sensor?) near the left rear tail light. i've lived for decades with many cars that didn't have a bulb failure indicator so if it doesn't work on this one, no problem.

Electrically savvy people, any ideas on this work-around (or suggestions for fixing the gremlin)?

[Ozark Lee]

I thought I had it solved b/c I had brake lights with the socket panels out of the tail light housing. But when I put the panels back in and moved the car, the fuse blew again.

What that is telling me is that the problem is in the wiring from the bulb failure sensor to the tail lights or possibly in the tail light socket assembly. There really isn't that much going on in the failure module itself and its typical failure mode is for the lights to not work at all or for the bulb failure indicator on the dash to either not work or never turn off. A place to look closely at is the third, high mounted tail light. The wire to that light might actually follow hinge arm along with the locking and license plate lights. That harness is a trouble spot, particularly if the trunk gets slammed on a box or some luggage. The same thing applies to a wagon (I'm not sure which you have) except that the failure is usually at the hinge itself where the wires flex when the tailgate gets opened and closed.

...Lee

[BEJinFbk]

Maybe we could start with your year and model?

[JimBee]

Update. This problem is (was?) in my 93 854 (I had stated that originally but in a hurry I guess I edited out that line).
I now have brake lights; the other rear lights, except for the rear window light, had been working through all of this.

I've been this far before with the brake lights and then when I put the tail light socket panels back in place and clipped the bulb failure module back and everything was working but as soon as I drove off and used the brakes, the fuse blew again, so I'm kind of holding my breath.

This go-around I unplugged the bulb failure module, again, to look again at its electrical contacts, everything looked clean, no corrosion on the module's pins or its connector. Installed a new fuse, touched the brake and, of course, with the bulb failure module out, the brake lights didn't light. Also, the fuse didn't blow. I'd tested that before but wanted to be sure there was no short upstream of that module that I might have missed.

Plugged the bulb failure module back in, touched the brake pedal and blew the fuse.

One thing I'd thought of earlier today is that somebody mentioned making sure the bulbs are the same on both sides. This is curious because on the back of the tail light one of the holes in the plastic plate for the bulbs to go through has a little tab that's cast into the plastic. It juts into the bulb hole just less than a 1/4" at 9 o'clock. That tab prevents installation of a standard 21 volt bulb in that respective socket, so I had to leave that bulb out. (At the junk yard last week, I noticed a 94 854 had a plastic strip all across one of its brake light bulb openings on the other side.)

In both my 93 and the 94 the concept was the same—there was a plastic barrier that prevents installation of a bulb in one of the brake light sockets on the socket panel. Those sockets are both live when the system is up, but if you wanted a bulb in that socket, it would need to be smaller. The thing is, 21 W is stamped right into the steel plate next to 4 of the 6 bulb sockets (2 21 watt builbs at the bottom are for the brake lights, the 2 in the middle are for backup and signal lights, the other 2 at the top indicate 5 watt tail light bulbs). I did find some 1156 (26.8 watt) bulbs in several of the 21 watt sockets.

So during all my testing and blowing fuses I had no bulb in that one brake light socket, but there were bulbs in the other 3.

I got some Wagner #BP17635's from NAPA which are supposed to be 21 watt, O'Reilly only had the 1156 26.8 variety. I put 1 21 watt Wagner in the outer-more brake light sockets on the left and right sides, leaving an empty socket on each side. Since my backup lights have never worked, I left one of the 21 watt middle sockets also without a bulb.

For now, the brake lights are working and through multiple stops have continued working. Since I suspected a loose wire from the bulb failure module to the brake lights, I've left the module sort of hanging rather than twist the wires to clip it back in place. I still think it's possible that's at least part of the problem.

I'll be really surprised if with all the fancy electronics built into these cars, one socket, each side, for the brake lights is supposed to remain empty. But, well, if it works, I won't be complaining.

Perhaps Lee will comment on this curious design to fill out the discussion and maybe help others to a quicker solution. AND, maybe reply to my idea of hacking the system and just running a wire directly from the switch at the brake pedal (which is hot key on or off) to the green wire that carries juice to the brake lights. Would that fault out the bulb failure module and cause problems? Probably a bad idea but why?

[JimBee]

After several days of driving around the city the brake light fuse is not blowing and the brake lights continue to work, so I'm assuming using just one bulb in the brake light socket on drivers side and one in the brake light socket on the passengers side solved it. Those are both 21 watt bulbs bought from NAPA.

SO EQUAL BULBS OF THE SAME WATTAGE ON BOTH SIDES WOULD BE THE FIRST THING TO CHECK ON YOUR 850 IF THE BRAKE LIGHTS ARE NOT WORKING. If one of your tail light housing's bulb holes is blocked so you can't use a bulb in that socket, like one of mine is, do not use two bulbs on the other side.

BTW: The bulb failure light on the dash lights with all the others with key-on but does not come on with the other two (inboard) brake light sockets and the two backup light sockets empty.