Strange Electrical Issue w brake lights / front markers / and dash lights

[mcwho]

I pushed the roadster out to do an initial Headlight alignment using tape on the garage door. With the ignition off...

As I was backing up I noticed when I hit the brake that the brake lights as well as the front running lights came on, as well as the dash lights. If I pulled the Headlight switch out one notch the running lights front and back worked as normal. Since I used a Ron Francis wiring loom I don't know what the heck happened. Not a serious problem, but why...

[Melb-Mike]

A bad ground will cause unusual things to happen. It is possible the switch is causing issues but unlikely in my opinion.

[Derald Rice]

I have had the same problem, and in my case, one of the brake light filaments had sagged down (inside the bulb), and was contacting the park light filament. So, when power was sent to the brake lights, the park lights and the dash light circuit was powered through the filament contact. It drove me nuts. Out of desperation,I pulled the rear bulbs out, the problem went away, and I found the defective bulb.

[mcwho]

That's a likely issue, I was looking at my RF wiring diagram and saying to myself, somewhere the hot lead for the brakes (Purple) from the Fuse box the hot lead for the brakes go to the switch on the brake pedal, NO, Normally Open, it goes thru one connector and back to the brake lights where the other side of the brake light is grounded.

Pretty Straightforward, this means that aft of the brake switch there must be a short somewhere.

Since I converted all these front/rear lights to weatherpack connectors, on the rear I have a 3 wire connector for brake lights, and another 3 wire weatherpack connector for tail lights.

I just went out and disconnected the brake lights on both sides, the problem has disappeared. Looks like I may have bad bulbs as you did, very strange.

I alos noticed that the tail light feed (Yellow) terminates WITH the front marker lights at the headlight switch. I am still looking why the dash lights would go on.

Thanks for the input Guys,

Bob

[Derald Rice]

My dash lights were activated since they are part of the park light / headlight circuit.

Do your dash lights still come on after you pulled the bulbs ??

[edwardb]

Sounds like a good time to switch to LED bulbs. 1157 style LED's are available in most auto parts stores and multiple on-line sources.

[conger]

I pushed the roadster out to do an initial Headlight alignment using tape on the garage door. With the ignition off...

As I was backing up I noticed when I hit the brake that the brake lights as well as the front running lights came on, as well as the dash lights. If I pulled the Headlight switch out one notch the running lights front and back worked as normal. Since I used a Ron Francis wiring loom I don't know what the heck happened. Not a serious problem, but why...

Are the park / headlight cct on a relay? It sounds like the lighting cct is finding a ground path via the brake switch.
That's where I'd start looking.

Good luck
Gerry

[mcwho]

Today I removed the bulbs for the brake lights and the problem went away. Tested the two pins on the bottom of the bulb with my VOM (Multimeter) and yes they are somehow connected together.

[boat737]

Today I removed the bulbs for the brake lights and the problem went away. Tested the two pins on the bottom of the bulb with my VOM (Multimeter) and yes they are somehow connected together.

I think that would make sense. Both filaments connect to the ground, the same ground. So they would have continuity (although maybe some resistance) through that common ground element. I'm thinking that is normal for the bulb.

[Jeff Kleiner]

I think that would make sense. Both filaments connect to the ground, the same ground. So they would have continuity (although maybe some resistance) through that common ground element. I'm thinking that is normal for the bulb.

No, with a dual element 1157 you should not have continuity between the two pins. You should have continuity between each pin and the metal bulb base (unless the bulb is burned out of course).

Jeff

[Big Blocker]

Jeff's right as usual - the two pins on a 1157 bulb are two separate circuits. One pin is low current tail lights and grounds thru the bulb base sitting in the bulb socket.
The other pin is for high current Brake/Turn Signal and also grounds thru the bulb base sitting in the bulb socket.
The two circuits are NOT connected together at all except at the grounded base.

Continuity (Ohms) checks should give different values when tested from pin one to ground and pin two to ground . . . pin one to pin two should give an "open circuit" reading (infinite Ohms).

HTH

Doc

[boBQuincy]

Actually most meters will show continuity between the pins due to the low resistance of the cold filaments (about 0.3 and 3 ohms) and their common connection. Two of my digital meters showed continuity, I don't know where the cutoff is for non-continuity but it must be above 3 ohms.

The current could go to the brake light filament, not find ground, go back through the taillight filament, to the gage lights and then find ground, lighting them all up although at reduced brightness. Grounding issues are a real pain!

boB

[boat737]

The two circuits are NOT connected together at all except at the grounded base.

Doc

That is what I said. They ARE connected through the ground, in this case the bulb's base. It may not be a true, pure, continuity, but there will be a current flow, through the resistance of both filaments. That is, from one of the positive socket pins, through that element, to the common ground, from that common ground back through the other filament and finally to the other socket pin.

Try this: Take a dual filament bulb (out of the car, by itself), put +12V dc (your car battery voltage) to one of the positive pins on the bottom of the bulb. (Don't ground the bulbs base, otherwise the bulb will light up.) Put one lead of a DVOM on the other positive pin on the bulb. Now the other lead of the DVOM goes to ground. You should read darn close to the car's battery voltage on the DVOM. (the bulb filaments will not light because the meter has more resistance, thus preventing enough current to run through the circuit.) The two filaments are connected through the common ground base of the bulb.

[CraigS]

Another thing to look at is those pure crap sockets. Often the small spring loaded base that has the two contacts in it, is not located well compared to the grooves in the walls that the pins on the side of the bulb base fit into. So the contacts on the bulb don't match well to the contacts in the socket. Also be sure you are installing the bulb correctly. Unfortunately they can be installed 180 degrees off

[GSides9]

Did you install new bulbs to see if your problem went away? If it did, the bulbs were probably shorted internally. Make sure there wasn't an 1156 in there initially, that would have caused your issue. Not a short, so no blown fuses, but a backfed circuit. 180 degrees off would have the bright filament lit when you have the parking lights on, and the turn signal/brake would be dim. But not shorted. Use a test light on each of the contacts with the turn signals on and see if both contacts flash. I hope this helps.

Glen

[mcwho]

I put in new bulbs into the brake circuit, and now I am stumped...

With the new bulbs the problem goes away, BUT measuring the resistance as mentioned before the mesurements are similar, BUT the problem is gone.


Measurements

Old Bilb

Pin A to ground 2.8 ohms
Pin B to ground .7 Ohms
Pin A to Pin B resistance 3.3

New Bulb


Pin A to ground 2.2 ohms
Pin B to ground .7 Ohms
Pin A to Pin B resistance 2.7

I was convinced before I put the new bulb in that grounding the socket would be the fix, now I am more confused.

[Derald Rice]

You're overthinking this. It only takes one of the brake bulbs to backfeed the park light circuit.

Enjoy the fact that your problem is gone, sit back and have a beer.

[Boydster]

As said before, it could have been a contact alignment issue. It may have been fixed without realizing.

[GSides9]

Bob,

Keep in mind that the filament in the bulb lights because power going into the contacts on the bottom of the bulb pass thru the filament on their way to ground, so they are connected. The different readings are why one filament glows brighter than the other. And lots of people here are smarter than me.

Glen

[GSides9]

and the difference in reading is why one filament glows brighter than the other