Brake Lights and Tail Lights Same Brightness

[astromechanic]

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.

[subhuman]

[astromechanic]

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.

[subhuman]

[GArBa]

[mainexile]

[bugbyte]

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

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

...Rich

[ashman40]

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.
_________________
AshMan40
---------------------------
'67 Beetle #1 {project car that never made it to the road }
'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}

[mainexile]

astromechanic,

What year and model (Type 1, 2, 3, 4?) are you dealing with? If Type 1, standard or super?
_________________
First, make sure it will stop; then make sure it will go; finally, make it pretty.

1974 Super Beetle Convertible, 1600 DP.
2016 Honda Accord Coupe (wife's)
2006 GMC Canyon Crew Cab (mine)
1978 Grady White Pacer 174 Mercruiser 470

[astromechanic]

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?

[astromechanic]

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.

[GArBa]

[ashman40]

[mainexile]

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
_________________
First, make sure it will stop; then make sure it will go; finally, make it pretty.

1974 Super Beetle Convertible, 1600 DP.
2016 Honda Accord Coupe (wife's)
2006 GMC Canyon Crew Cab (mine)
1978 Grady White Pacer 174 Mercruiser 470