Interior dome light noticeably dims when the brake lights are activated

[MaximRecoil]

(1969 Charger)

This is something I noticed yesterday while testing the brake lights with the engine running, and I happened to have the door open (so the dome light was on). This isn't a problem in and of itself, but why are the brake lights drawing so much current that the dome light dims? Does it indicate a poor ground to the dome light and/or the brake lights? None of the other lights (e.g., headlights, parking lights, instrument cluster lights, blinkers) dim when the brakes are applied.

[flyinlow]

The 4 brake/turn filaments burn about 8 amps. At idle with a stock alt. some diming would be normal.  If its a dramatic dimming might be a poor ground. The dome grounds thru door switches. Does it dim the same with the pass. door open.? (different ground)

[A383Wing]

all my Mopars do this...it's real noticeable when you got 6 brake lights and 4 interior lights

Bryan

[MaximRecoil]

The 4 brake/turn filaments burn about 8 amps. At idle with a stock alt. some diming would be normal.  If its a dramatic dimming might be a poor ground. The dome grounds thru door switches. Does it dim the same with the pass. door open.? (different ground)

8 amps at 12 volts, so about 96 watts. That's quite a bit. Typical wattage for a sealed beam halogen headlight (H5001 / H5006) is 35 watts low beam / 50 watts high beam, so the brake lights pull more current than having your low beam headlights on, and almost as much current as two high beams on. Additionally, that's a long run of wire (the length of the car) and the wires to the brake lights are only what, 18 gauge?

The dimming of the dome light when you press the brake pedal in my car isn't dramatic; it is just enough to be noticeable. I just tried it with the passenger door open, and the dimming is the same.

My alternator is stock Mopar, but a later '70s style dual field one for use with an electronic regulator. I'm not sure what the amperage is; 60 amps maybe? I get 14.4 VDC at the battery at idle with it, and there is also an 8 gauge wire from the alternator post to the starter relay, which combined with the stock 8 gauge wire from the starter relay to the battery effectively forms a direct 8 gauge path between the alternator and the battery.

If this is par-for-the-course for old Mopars, it must be a bad circuit design issue. Maybe it is caused by the hungry brake lights combined with a long run of thin wires feeding them. 

[A383Wing]

It's pretty much normal...don't sweat it

I think the "math" is off for the brake light amp-volt-watt thing....I'm pretty sure 4 brake lights don't make 96 watts...but I could be wrong

Bryan

[MaximRecoil]

I think the "math" is off for the brake light amp-volt-watt thing....I'm pretty sure 4 brake lights don't make 96 watts...but I could be wrong

If they draw 8 amps then that = 96 watts of power @ 12 volts (watts = amps × volts). Maybe tomorrow I'll measure the amps of the brake lights with a meter. I hope they are not more than 10 amps, because that's as high as my meter is rated for.

[A383Wing]

might be more than 10a....bet you got a 20a fuse for the brake lights

[flyinlow]

OK ,you made an old man stumble out to the  garage late at night ,get all the dogs in the neighborhood barking, dig up an 1157 bulb. My 10 amp charger shows about two amps with the bright filament lit. Not a real accurate check, I admit.

[MaximRecoil]

Well, my meter can handle 10 amps for 15 seconds, so it should be alright. If the brake lights draw more than 10 amps, that would be ridiculous. Just 10 amps would be 120 watts.

The car's fuses just protect the wires in the circuit, so as long as they are set to blow at a significantly lower amount of amps than would melt the particular size of wires in the circuit, it is fine. The fusing current of 18 gauge copper wire is about 83 amps, so a 20 amp fuse has a good safety margin, plus if the brake lights only draw 8 amps, a 20 amp fuse also will allow for a certain level of spikes without blowing, which is good.

[MaximRecoil]

OK ,you made an old man stumble out to the  garage late at night ,get all the dogs in the neighborhood barking, dig up an 1157 bulb. My 10 amp charger shows about two amps with the bright filament lit. Not a real accurate check, I admit.

That's funny.

So with 4 of them lit, that would indeed be ~8 amps / ~96 watts, which is surprisingly high. And for people running 6 1157s instead of 4 1157s and 2 1095s, that would be ~12 amps / ~144 watts. Maybe that's why Chrysler didn't put brake lights in those two inner sockets.

I'll check my car with a meter tomorrow to get an "as-installed" measurement.

Edit: The bright filament on an 1157 (or the only filament on an 1156) is 26.8 watts, which = 2.23 amps @ 12 volts (or 2.09 amps @ 12.8 volts) - link. So the "about 8 amps" figure from flyinlow is correct for 4 of them.

[A383Wing]

both my 66's have 6 lights total..and I figured if they could work, I did the same on my 70...3 brake lights per side

the 66's have 4 interior lights, two #1004 bulbs, and two #90 bulbs...even above an idle, when door is open and brake lights applied, the interior lights flicker dim for a split second.

Bryan

[flyinlow]

Sylvania says 1157 is a 26.9 watt bulb . I found my son's '70 was seeing a 1.5 volt drop between the battery and the brake
light socket with the brake lights lit. We converted to a six light brake light set up and relayed the taillights. It has a trunk mounted battery so it was easy. Much brighter. Old wiring can always use some help.

A local Cop had pulled up to him once and told him that he could hardly see his brake lights in the daytime. They where pretty dim. After the mod the same guy saw him and pulled into where he works as he was arriving saying "great brake lights now". They talked about  the car for a few minutes.

[MaximRecoil]

Sylvania says 1157 is a 26.9 watt bulb.

Yes, I found the same thing (we probably looked it up at about the same time). See my edit to my previous post.

Relays are an interesting idea; it could work good even without a trunk-mounted battery. Just run a good size power wire back there to the relay, 10 gauge should be enough. Now I'm curious about what kind of voltage drop I'm getting between the battery and the tail lights in my car. I'll have to check that tomorrow too.

[A383Wing]

....... We converted to a six light brake light set up and relayed the taillights........

did you "relay" the park lights or brake lights?

[flyinlow]

Both . One relay for the taillights, one for each side brake/turn lamps. I was concerned that the relays would delay the brake lights. I did one side first and compaired. The relays must be very fast. LEDS are about a 1/10 of a second quicker to light than regular bulbs . That's about 10 feet at 70 MPH.

[MaximRecoil]

I'd probably "relay" both the parking lights and the brake lights if I did it. The biggest benefit would be to the brake lights; the parking lights are only about 8 watts for each filament. Brighter brake lights is a good safety measure.

[Dino]

Very normal.  Does it still dim when you keep it at 2000 rpm?

Bigger alternator and some relay mods and there will be no more dimming...but it won't hurt anything either so don't worry about it.

[2Gunz]

I'd probably "relay" both the parking lights and the brake lights if I did it. The biggest benefit would be to the brake lights; the parking lights are only about 8 watts for each filament. Brighter brake lights is a good safety measure.

IF you guys are using relays.... make sure you are using fuses for the relays!

[Nacho-RT74]

I have had that simpthom... ended being the brakes/hazzards light fuse box holder, which is not able to feed good the fuse, at the point it gets heat and melts the fuse box. This resistence makes a feedback on power through the dome light circuit

[Ghoste]

What if you have a 1st gen with two C pillar interior lights the underdash passenger light and a console with the two interior lights and then all of the bulbs in the full width taillights?  I can tell you its normal for the interior lights in my 67 to dim noticeably if you step on the brake at idle with the door open.

[A383Wing]

[Nacho-RT74]

guys, is not normal... trust me

[Ghoste]

Nacho, I've owned this 67 for a long time, it has new wiring from one end to the other and I maintain the connections religiously.  Further to that I have been in a LOT of fastback Chargers over the last 35 years of playing with them and they all dimmed noticeably.
Every single one of them.

[A383Wing]

guys, is not normal... trust me

uh, yea, it is...both my 66's do this as well....along with my 73 Pontiac...the dome lights "flicker" or "dim" a bit when brakes are applied and door is opened

[Nacho-RT74]

well, I don't think when new they did it. Honestly I still don't think is normal. Now imagine you got a car back in the days from dealer and notice some dimming light when pressing brakes and your dealer tells you... THATS NORMAL!!! would you believe that ?

Eat shit, Million of flyes can't be wrong   ( dunno if you get the idea on the phrase )

please check your fuse box holders... press brakes a while and notice if the fuse gets enough heat to burn your finger.


in fact, once you locate the brake lights fuse,"wiggle" it a little bit and check if the dome dimming light goes away at certain point