TheSamba.com :: View topic - Brake Lights and Tail Lights Same Brightness

Fri Aug 29, 2014 11:22 am

Hello,
So here is my situation. With my headlights and tail lights off my brake lights work as expected (when I drive during the day). However, I discovered with my headlights and tail lights on if I press the brakes, nothing happens with the light brightness (the tail lights are the same brightness). When I turn my headlights and tail lights on, I can measure 12 volts on both sides of the master cylinder switches with the key on (I have 2 prong switches). Headlights and tail lights work whether the key is on or off, but brake lights only if the key is on. I took my tail light covers off, I do have 2 prongs for the tail lights but I get 12 volts on both sides with just the tail lights on. Other than trace the whole wiring diagram, does anyone have a likely culprit as to where the circuits might be unintentionally merging (at least that's what I think is causing the problem)?
Thanks.

Fri Aug 29, 2014 12:03 pm

astromechanic wrote: Hello,
So here is my situation. With my headlights and tail lights off my brake lights work as expected (when I drive during the day). However, I discovered with my headlights and tail lights on if I press the brakes, nothing happens with the light brightness (the tail lights are the same brightness). When I turn my headlights and tail lights on, I can measure 12 volts on both sides of the master cylinder switches with the key on (I have 2 prong switches). Headlights and tail lights work whether the key is on or off, but brake lights only if the key is on. I took my tail light covers off, I do have 2 prongs for the tail lights but I get 12 volts on both sides with just the tail lights on. Other than trace the whole wiring diagram, does anyone have a likely culprit as to where the circuits might be unintentionally merging (at least that's what I think is causing the problem)?
Thanks.

are you sure you have a dual filiment bulb in?

Fri Aug 29, 2014 12:12 pm

Yes, I'm getting 12 volts on both sides though with just the headlights and tail lights on, so it's illuminating the higher watt filament with just the head lights and tail lights on instead of only when the brakes are applied. This isn't a problem during the day since I don't have the tail lights on.

Fri Aug 29, 2014 12:48 pm

astromechanic wrote: Yes, I'm getting 12 volts on both sides though with just the headlights and tail lights on, so it's illuminating the higher watt filament with just the head lights and tail lights on instead of only when the brakes are applied. This isn't a problem during the day since I don't have the tail lights on.

were the tailights off the car, ie paint, maint. etc. recently? sounds as if it is like you said a wiring issue. the two wires that go to the tailight/brake light might have got switched.? do you have a wiring diagram in your manual? if so double check that the wires are connected to the correct terminal. just a couple of thoughts.

Fri Aug 29, 2014 3:02 pm

astromechanic wrote: Yes, I'm getting 12 volts on both sides though with just the headlights and tail lights on, so it's illuminating the higher watt filament with just the head lights and tail lights on instead of only when the brakes are applied. This isn't a problem during the day since I don't have the tail lights on.

the first place to look would be the bulbs' socket itself. if you remove the bulbs, does it change or you still have voltage on both tabs?

Fri Aug 29, 2014 4:51 pm

subhuman wrote: astromechanic wrote: Hello,
So here is my situation. With my headlights and tail lights off my brake lights work as expected (when I drive during the day). However, I discovered with my headlights and tail lights on if I press the brakes, nothing happens with the light brightness (the tail lights are the same brightness). When I turn my headlights and tail lights on, I can measure 12 volts on both sides of the master cylinder switches with the key on (I have 2 prong switches). Headlights and tail lights work whether the key is on or off, but brake lights only if the key is on. I took my tail light covers off, I do have 2 prongs for the tail lights but I get 12 volts on both sides with just the tail lights on. Other than trace the whole wiring diagram, does anyone have a likely culprit as to where the circuits might be unintentionally merging (at least that's what I think is causing the problem)?
Thanks.

are you sure you have a dual filiment bulb in?

You don't have a voltage problem; you have a wiring problem. If your answer to subhuman's question is "yes", then it's unlikely that you have 12VDC on BOTH contacts in the taillight socket. If you did, the higher wattage filament in the bulb would illuminate brighter than the tail light filament. I suspect that a PO did some creative wiring since the brake lights (read: higher wattage filaments) only work with the key on and the light switch off. Are you certain that the brake light filament is illuminating with the light switch off? Could you be seeing the tail light filament at that time?

Fri Aug 29, 2014 4:59 pm

Always go to Speedy Jim's website. He makes everything simple.

http://www.speedyjim.net/htm/elec.htm

...Rich

Fri Aug 29, 2014 6:04 pm

It is most likely your problem is at the taillights or the wiring to the taillights because that is about the only place where brake lights and parking lights come together.

Remove both tail/brake light bulbs from both the left and right sides. Make sure both bulbs are dual filament bulbs. If one of the bulbs was a single filament bridging the contacts the other side would produce the results you mentioned. Test the contacts at the bottom of the sockets while both bulbs are out. Do you still get 12v on both contacts while the parking lights are ON?

If you still have the problem, look at the wiring junction behind the tar paper on the left and right sides of the engine compartment. See if the black/red and grey wires are somehow bridged.
The fact that the brake lights work while parking lights are OFF and there is power on the output side of the brake switch while the parking lights are ON indicates the power is coming from somewhere at the rear off the car where the wires come together.

Fri Aug 29, 2014 6:29 pm

astromechanic,

What year and model (Type 1, 2, 3, 4?) are you dealing with? If Type 1, standard or super?

Fri Aug 29, 2014 11:26 pm

Thanks for all the replies. It's a 1969 Beetle (Type 1) standard. The car shows clear evidence of being completely apart repaints, different engine, etc. So I'm sure a PO goofed something (I've only had the car a couple months). Fortunately, I did buy it as a 3rd car (weekend fun project), so these things are not too big of a deal. I have a wiring diagram, so I will take a look at the rear wiring as suggested and see if I can spot anything awry. However, the one thing I wouldn't get about that though, is why am I getting 12v on both sides of the master cylinder switches with only the head lights and tail lights on?

Fri Aug 29, 2014 11:35 pm

Sounds like some good tips, I did take the bulbs out before measuring voltage, so I'm going to look and see if the wiring is bridged somewhere.

ashman40 wrote: It is most likely your problem is at the taillights or the wiring to the taillights because that is about the only place where brake lights and parking lights come together.

Remove both tail/brake light bulbs from both the left and right sides. Make sure both bulbs are dual filament bulbs. If one of the bulbs was a single filament bridging the contacts the other side would produce the results you mentioned. Test the contacts at the bottom of the sockets while both bulbs are out. Do you still get 12v on both contacts while the parking lights are ON?

If you still have the problem, look at the wiring junction behind the tar paper on the left and right sides of the engine compartment. See if the black/red and grey wires are somehow bridged.
The fact that the brake lights work while parking lights are OFF and there is power on the output side of the brake switch while the parking lights are ON indicates the power is coming from somewhere at the rear off the car where the wires come together.

Sat Aug 30, 2014 12:40 am

astromechanic wrote: why am I getting 12v on both sides of the master cylinder switches with only the head lights and tail lights on?

the + side of the lights circuit is contacting the brake lights circuit somewhere thus bringing all at the same potential.

Sat Aug 30, 2014 3:36 am

GArBa wrote: astromechanic wrote: why am I getting 12v on both sides of the master cylinder switches with only the head lights and tail lights on?

the + side of the lights circuit is contacting the brake lights circuit somewhere thus bringing all at the same potential.
Astromechanic, it sounds like you are asking how current could be flowing through the brake switch while it is OFF? It probably isn't.

GArBa has it right. It is more likely the current is coming from the wiring at the rear.

Sat Aug 30, 2014 10:50 am

This may be a long shot but...the thing bothering me is that you read 12VDC on both of the master cylinder switches with the key on. Have you checked those switches for continuity with the key off? Disconnect the switch plugs, put your continuity tester or multimeter (set for ohms/continuity) across the two pins on the MC switches that are oriented the same direction. Then have an assistant push the brake pedal. You should read an "open" with no pedal depressed and a "short" when assistant depresses the brake pedal (this will rule out shorted MC switches). Reconnect the MC switch plugs and verify that you have no 12VDC with the key and headlight switch off, and that you get 12VDC with the key on. If this is the case, I would look at the fused black wire to the coil to see if PO spliced into it to get 12VDC for the taillights. When you turn the key to the "on" position, you get 12VDC to the coil (and fuel cutoff solenoid and automatic choke). If PO tapped into that wire for taillight power, it would explain power to the MC switches with key on. Granted, it doesn't explain the lack of power to the brake light filament with headlight switch on, nor does it explain why the brake light filament illuminates properly with the headlight switch off (are you certain that the brake light filament is the one illuminating when the brake pedal is depressed with the headlight switch off? Could it actually be the taillight filament?). At any rate, if you perform the above checks and everything appears normal, you will have eliminated the MC switches, at the very least. If PO got creative with the wiring, the chances of the rear light assemblies conforming to wire colors on Speedy Jim's site are very slim, and you're looking at tracing the rear light's wires back toward the fuse panel. Good luck.

Steve