Hey, first FixYa post here, I love the availability of expertise, and may I add a preemptive "Thanks!" for any help. Onto the problem: Recently, my lower rear brake lights and rear turn signals have stopped functioning. The third, high-mounted brake light still responds, and the tail lights themselves work fine, as do the front signals, headlights, reverse lights, cruise control, and shift interlock (all of which narrows down the possibilities of cause of issue). Hazard lights DO activate, but flash quickly and only in the front, as there is no signal going to the rear flashers. Today, for a short amount of time after getting a tire replaced, oddly enough, my left rear turn signal (and therefore, I assume, brake light as well) began working for the drive home. Upon turning the vehicle off and returning later, it was back to being non-functioning. First, of course, I changed bulbs... no dice. So, second, I checked my fuses and everything... all good. Third, I had a mechanic acquaintance use his current tester to help narrow it down. On the Grand Marquis, the tail lights and brake lights share a socket, and testing showed that the wire on the signal/brake side was showing 0 voltage, while the tail light side read normally. So, he went up, tested the fusebox to make sure it wasn't an issue there, double checked the fuses while he was at it. Used his tester to test the signal line, and it appeared to be a faulty turn signal switch. Well, today, I purchased and replaced my turn signal switch, and am still dealing with the same issue. In the morning when the parts store opens, I plan to go and acquire a brake light/stop light switch, because while re-installing the shroud and plastic drape under the dash/column after installing the new switch, it looked visibly like some of the wiring on the brake light switch was shot, and I understand that can also produce these symptoms in the Marquis. So, the real question I have, is 1) what is the likelihood, given the details of what is and is not working, of it being the lighting control module rather than the brake light switch (I figure either way, switch is only $10 so doesn't hurt to try, LCM is closer to $200-250, which, eww haha. I just blew $60 on an unnecessary multifunction switch)? and 2) assuming I get and install the brake light switch, and still the issue persists... how best to further narrow it down from there?
Yes they are connected, but that means if there is an issue with either-one, then you SHOULD be having trouble moving the Shifter out of -Park- as well. If there is no change after the switches, I would suspect a Frayed Wiring harness somewhere down the line on the side that started working when the tire was changed.
Tallon41
Testimonial: "Well, the way I figured, the brake light switch was only 8 bucks and about an hour of work, and it's a beautiful day outside so no loss there haha. But, you are absolutely correct, it wasn't the brake light switch. After replacing the multifunction and the stoplight switch, the symptoms persist. I'm not inclined to think it's the LCM either since it's ONLY the rear brakes/signal, which share a wiring config; not to mention the odd bouts of intermittent proper functioning. Now, my next question is: how best to narrow it down to whether it is a specific wire/ground short or break, or whether it's a faulty harness, and where best to start on the process? This evening or early tomorrow I plan to finally buy a Hayne's manual, as I will apparently be needing some familiarity with the wiring diagnostics, and it's not a bad investment at $20 or so. Thanks for the advice, and in advance for any further help!"
Whoops, that text in the testimonial should've gone here haha. Still getting the hang of this place, sorry! I will probably start a new topic for the wiring itself, as electric issues are the ones I'm most unfamiliar with. In the meantime, if you've got any further helpful tips, I'll gladly hear 'em!
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SOURCE: I have a 1977 Mercury Grand Marquis the turn
Turn signal switch (also brake light canceling switch) most likely bad or bad wire to it or from it. Check, with key on run, for power into switch. Turn switch on, check for power out. No power out switch bad. Power out, trace to rear tailight harness for broken wire.
SOURCE: the tail lights on my
try reading for a supply from the back of the light switch ,i think that all the side lights are not working and the fault is the switch but as the saying a bird in the hand and if i had the vehicle in front of me i would soon sort it in a minute or two but from europe ??
SOURCE: tail lights, parking lights and
headlights are on their own so they would work, the brake, hazard, again, own circuit,, Back up same thing, now, your tail lights and parking lights are 2 in the same, sometimes the dash lights use the same fuse as those 2 ,, so check your fus box for a listing that says tail lights,, I'd bet thats blown
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Wow, sorry, didn't know fixya gets rid of line breaks in the posts! Hopefully it's still bearable to read, I didn't want to leave out info and waste anyone's time.
Oh, for anyone else coming across a similar problem in the future, this has been an ongoing, evolving issue as I've eliminated causes. I have two further "problem" postings related, see them for updated info.
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