I noticed something strange about the lights on my '66 this weekend.

It seems that now if I signal a right hand turn everything is fine.

However, when I signal a left hand turn...both turn signals flash almost like turning on the hazard lights. The left signal is brighter where as you have to look harder to see the right one but sure enough they are now both flashing. As a side note, when signaling a right hand turn the right signal is as bright as ever, just flashes dimmer along with the left side when signaling a left.

Obviously I don't follow myself very often. I know when I was working on the car about four months ago all was fine. I have noticed however that the turn signal switch seems to be getting progressively worn when signaling a left turn.

Also of note is that none of the turn signals work when the lights are on, even after putting in a new light switch. Does it never end, LOL......

I have had it on my short list to replace the turn signal harness in the column anyway...is it anyone's hunch that replacing the turn signal harness will fix the issue or is there something else I should be looking at?

 

ground issue I'd guess.

put it this way. on one side of the lamp you need to have 12V+ and on the other side minus.
Now when the lamp doesn't get proper ground it will try to pull ground from wherever it can find. In your case it seems to be over the other indicator light.

check on this diagram:
http://www.midlife66.com/wiring/66exter.jpg

you will see in the top left corner a picture where there is 2 (one for each side) "Tailight stop and turn signal). in the 64-66 mustangs it's only one bulb doing all 3 jobbies.
As you can see in the picture the lamnp gets it's ground from the chassis.

what to do: unscrew the bezel and remove bezel, the red plastic.

make sure the outside of the lamp (the metal cylinder) gets proper contact to where it is hooked in your tail light housing.
Then make sure the taillight housing itself is properly connected to the chassis.

Can't guarantee that this is what's wrong, but it sounds very much like it

 

Thanks for the advice Kalli....I'll check it out next time I get a chance.

I did replace the backup lights a few weeks ago and maybe I didn't get something grounded right in that process as well.

Electrical issues drive me nuts sometimes. Still haven't figured out why my park lights on the front come on when the breaks are applied but I am pretty sure that has something to do with the PO's wiring mess. Has something spliced into the wiring coming off the brake switch, pretty half *** jerry rig type of thing.

Hoping to slowly rewire most of the car and eventually fix all these issues in the process.

Park lights coming on when the brake is applied I can live with. In the short term I am more worried about fixing things that will get you a ticket.

At least I finally got the head lights working properly on both high and low beam but I guess I shouldn't say that too loud...LOL!

 

hehe :-)
best of luck !!

 

I had a very brief amount of time to look at some things last evening on the car. Big picture, everything appears to be wired correctly since in most cases that is the only way things can go.

One thing dawned on me.

There are four screws / nuts holding on the tail light housing and the ground wire coming from my reverse light is attached to one of these.

My screws are pretty old with some corrosion / light surface rust on them.

Wondering if I wouldn't go a long way towards resolving some issues by at least changing out the bolts / nuts if not just replacing the entire tail light housings all together?

 

Thanks for posting that link. That'll be most helpful to me as well.

 

28HopUp: check out teh FAQ section.
JamesW has a link there with several wiring diagrams. So there a pictorial ones as well as schematic diagrams. Lots of em :-)

SJs 66coupe: In a 64/65 model I believe the lamp get's its ground via taillaight hosuing to chassis, but from the diagram the 66 should get it via a different wire.
so there should be one wire from each taillight going directly to the chassis with screws.
make sure those are good and replace the screws there maybe

as well while you are at it check at the battery. you should have a strap from battery minus directly to the chassis of the car. If that is missing or bad it will cause all sorts of havoc as well. So make sure you have a proper big connection directly from bat- to chassis.

 

Just working totally from memory, but I believe there is a negative battery cable that goes from the minus terminal on the batter to the engine block (I think). When I changed out my headlight harness and alternator harness I did notice that the negative cable was attached to a different spot on the block then what appeared on the wire diagram in the shop manual. Does that sound even remotely correct? Of course the plus terminal off the battery goes to the starter relay.

 

you will need both
battery minus to chassis
and
battery minus to chassis

this way the chassis and block is grounded.
You will need the engine grounded for the starter, the oil light switch etc so if that's all working then your ground cable to engine should be fine

if you have any trouble with electricals that get ground from the chassis (like lights), then you need to check on the batteryminus to chassis strap

Kalli

 

"if you have any trouble with electricals that get ground from the chassis (like lights), then you need to check on the batteryminus to chassis strap"

You wouldn't happen to have a picture of said strap handy would you?

Not totally certain what I am looking for on that one, but that is likely due to me not being the sharpest knife in the drawer.