Weird brake light problem

[carguybradd]

Good point about the brake switch, I didn't think of that.
I'll have to double-check the bulbs, too!

[carguybradd]

Well, I'm reviving this thread as I'm still fighting these electrical gremlins. 
The stoplight fuse blows no matter what I do. I've replaced the stoplight switch, turn signal switch, and all four tail light bulbs. The wiring is original and unmodified, so "hacking" by previous owner's isn't a problem. I have good grounds at all of the tailight bulbs, and good continuity from the turnsignal switch plug to the wire ends at the switch itself. I compared the wiring on the new TS switch to the old one, and it appears correct (although with different colored wires). The factory harnesses under the dash and the fuse block all appear original and in good condition. There is no corrosion anywhere in any of the wiring on this car.

This one stupid problem is keeping me from driving the car, and I'm at my wits end. I've never had this problem with the multiple cars I've owned over the last 25 years, including other Mopars. I consider myself a capable mechanic, but this isdriving me nuts!

I now suspect something is defective with the new TS switch as I didn't have this problem before I replaced it. It's either that, or I'm going to have to start replacing the original wiring harnesses, which I'd like to avoid. I'm now trying to talk my father (a retired engineer) to come over and have a look.

In the meantime, has anyone experienced this? Any thoughts! 

[Chryco Psycho]

unplug the TS switch at the column & test all the wires to ground both up the column & in the main harness , if the fuse is blowing you have a short , locate the short , if the short is in the switch remove the switch from the column & make sure no wires are damaged

[340_6pak]

I have replaced my turn signal switch as well. The first one I bought was from Year One and lasted for about a year.  I noticed when I ran the wires down the steering column there was a chance that that they might short out on the column if care was not taken on installation. A good volt/ohm meter here will do wonders.

[carguybradd]

Thanks for the input guys!

I already pulled the steering wheel back off and there is no visible evidence that the wires have chaffed and are grounding out. I unscrewed the TS switch from the column and it looks good too. No telling if anything's amiss in the sealed portions of the switch, but since my turn signals and flashers are working normally I think they're probably OK.

unplug the TS switch at the column & test all the wires to ground both up the column & in the main harness , if the fuse is blowing you have a short , locate the short , if the short is in the switch remove the switch from the column & make sure no wires are damaged

I did this already with the body side of the plug, noting the resistance readings with my meter (before I began to suspect the new TS might be the problem). Here are the readings I got, with everything in the "off" position:

BLK (horn ground) = no continuity
LGN (LH signal lamp in cluster) = 1.5 Ohms
Tan (RH signal lamp in cluster) = .9 Ohms
Pink (emergency flasher) = no continuity
Red (turn signal flasher) = 72.4 Ohms
DGN (to rear stop lamps) = .9 Ohms
BR (to rear stop lamps) = 1.0 Ohms
White (stop light switch) = no continuity

I find it interesting that the LH signal lamp circuit is .6 Ohms more than the RH side, but I don't think that's my problem. The high reading for the turn signal flasher I attribute to the coil windings in the flasher unit itself. My meter reads .2 Ohms when the two leads are crossed, so one needs to subtract that amount from the numbers above to get the actual resistance.

I was also wondering if something might creating "feedback" through the circuit, creating a situation where two positive voltages are being "seen" at the fuse. I have a reproduction factory manual and know how to read wiring schematics, but it doesn't show everything (such as the inner workings of the TS and emergency flasher switches).

I'll go back out today and check the resistance to ground for the TS switch.

Thanks again guys!

[MEK-Dangerfield]

Look out...I'm jumping in again.   


I read all of this again. One issue we didn't talk about was your rear wiring harness. Even though original and unmolested, something could be wrong there. Something heavy in the trunk slides up against it and smashes a few wires together...also look behind the driver's side kick panel. That's where the rear harness connects to the turn signal harness. See if something is funky with the connector. Also, the rear wiring harness runs along a groove under your door sill plate and floor panel. Maybe something got squeezed down there?

I have an original shop manual, but I suspect it is like your's. The turn signal switch is a big round circle, but they don't show you what actually is going on inside it.   



  Mike

[quapman]

If you haven't already, check the body ground inside the trunk. I forget where it is and it's too dark out to check mine, sorry.

Check and re-check the 1157 bulbs, even the FRONT one (parking light). If a filament is broken, it could be shorting across to the other circuit (the other filament) and increasing the resistance. My 73 used to light up the turn signal bulb in the DASH when I hit the brakes. New bulbs all around did the trick. Can't hurt to check the sidemarkers, too.

Check that the cigar lighter is not depressed or loose or who-knows-what? This is connected to one of the light circuits (I forget which one) and can cause problems. A red/pink wire splice is also the connector to the lighter.

Good luck. Electical demons suck.


Steve

[MEK-Dangerfield]

If you haven't already, check the body ground inside the trunk. I forget where it is and it's too dark out to check mine, sorry.

Check and re-check the 1157 bulbs, even the FRONT one (parking light). If a filament is broken, it could be shorting across to the other circuit (the other filament) and increasing the resistance. My 73 used to light up the turn signal bulb in the DASH when I hit the brakes. New bulbs all around did the trick. Can't hurt to check the sidemarkers, too.

Check that the cigar lighter is not depressed or loose or who-knows-what? This is connected to one of the light circuits (I forget which one) and can cause problems. A red/pink wire splice is also the connector to the lighter.

Good luck. Electical demons suck.


Steve

You made a good point about checking all of the bulbs. One in my front went to heaven, and the turn signal light in my dash would stay lit and not flash when I turned on the turn signal.

As for body ground in the 70 trunk..The tail light buckets need to be tight to the body. Are all the speed-nuts on the studs? Are the tail light buckets tight against the body? The studs from the tail light bezels into the body is where the ground contact would come from if you haven't repainted recently.

[FY1Cuda]

One more suggestion:
Check that the little fiber disks on the bottom of the lightbulb sockets are in the right position. They have the two contacts poking through them.  The disks can get easily rotated out of position.

[carguybradd]

Wow - lots of good feedback here!

My father (the retired electronics engineer) came out today and we spent several more hours on this, and have come to the conclusion the problem doesn't lie in the bulbs or wiring from them back to the dash. We then turned our attention back to the new TS switch, and here's where it gets weird again:

After performing multiple checks, I put a new 20A fuse in, ostensibly to test whether it would blow if the 4-way flashers were on. It didn't. And now it doesn't blow at all, whether the flashers or turnsignals are on or not, so the problem just fixed itself? 

But wait, now there's a new problem: When the flashers are on and the brake is applied, the brake switch is supposed to override the flashers and hold the stop lights steady until the brake is released (according to the manual). Mine isn't doing this (the rear lights keep flashing). While this doesn't render the car undriveable, it's not correct and tells me the problem lies with the emergency flasher switch (part of the TS assembly). I'm going to buy another TS switch (from a different source this time) and see what happens. Anybody have a brand they'd recommend?

As an aside, I did find a couple of interesting tidbids during this misadventure:
One, the manual shows the flashers as being on the same circuit as the dome light, but on my car the dome is definitely on the same circuit as the stoplights. Makes me wonder what else might wrong in the manual.
Second, there is a warning in the manual to avoid operating the turn signal and 4-way flashers at the same time, as feedback through the "accessory" circuit might cause unwanted operation of accessories plugged into that circuit. 

[Chryco Psycho]

well from my research there may only be one mfg left for the TS switch , the Mopar & Year one are Identical to the 1/3 price Standard Blue Streak part TW7 so I buy the TW7 , I would bet Mopar & YO are getting them from Standard or the company that supplies Standard

[carguybradd]

Well, I swapped out the TS this past weekend and everything seems to be working fine now. I checked it against the "old" one I had recently put on and noticed two of the wires going to the plug were swapped! The new switch was a Standard TW7 as recommended, and works fine (I don't recall the brand of the other one). The other TS switch matched what came out of the car, which I deduced wasn't original either as the 4-way switch had been ground down to fit the column, so maybe they were both incorrect?

After reading Chryco's comments about there being ony one manufacturer, I carefully compared the two. The plastic parts look like they might have come from the same molds, although they had different numbers cast into them. The wiring and connector plug were also different colors. The Standard part said "Made in Taiwan"  on the box, so who knows...but at least it works now!

[72 cuda]

Check the wire travel by tugging on the wire while the signal is on.
this may help isolate the problem.
burnt wire ,stripped insulation,etc.
posibly grounding out on a chafe (wire to the body somewhere)
hope this might help
Mark