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" 1961 VW Beetle dim tail lights"

Eddie Davila

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2 hours ago, Larry Schramm said:

      On ‎2‎/‎13‎/‎2018 at 9:38 PM, Larry Schramm said:



If you have a 6 volt system and are using a 12 volt bulb, that is the problem.  Your light output will be 1/2 what it should be. 


What voltage system does your car have?  If you do not know, put a volt meter across the two batter terminals when the car is running and take a reading.  The bulbs in the car should be the same value as the electrical system of your car.



Let's see,  If the resistance in the bulb stays the same, 1/2 voltage should be about 1/2 the wattage which in an incandescent bulb is light.  Maybe I left out "about" to make you happy.


The math:  The big power formulas are: Formula 1 - Electrical (electric) power equation: Power P = I × V = R × I2 = V2 ⁄ R where power P is in watts, voltage V is in volts and current I is in amperes (DC).




Been a long a while, but:


W = I * E (power (watts) is current times voltage)




E = I * R (voltage is current times resistance). This can be written as I = E/R


so, substituting I in the latter equation into the first we get:


W = E2/R


Bulbs/lamps were designed for a specific wattage. In the 50s it was probably about 35 to 40 watts for your low beam. Lets go with 35 watts though the math is similar what ever wattage number you pick.


For a 35 watts you get 1.02 Ohms on a 6v bulb and 4.1 Ohms for a 12v bulb. (That will be the resistance when hot, cold resistance will be much less.)


Now put that 12v 4 Ohm bulb into a 6v circuit that ought to have a 1 Ohm lamp and you get 62/4 = 9 watts of output.


So your light output has been reduced to 9 watts from the desired 35 watts: 25% of what it should be. (Ignoring that the temperature and thus the resistance of the 12v bulb is going to be lower than if running at its design voltage.)

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  • 2 months later...

I have obtained highly reflective  aluminum body repair tape. The surface is almost like a mirror.  The adhesive is quite strong and durable.

Cut it to size and shape  then installed it into tail light on reflector surface.


I have also used it on my trailers, that have the black plastic tail light housings and black  insides and no real reflector surface.

The level of light that is visible is much better.


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  • 4 weeks later...
On 2/13/2018 at 1:21 PM, Grimy said:

Hi Eddie, here are a few ideas:


1.  You have a SIX volt system, because VW did not go to 12V until about 1967.  Make sure your battery cables are size 0 (aught); they may have been replaced by thinner 12V cables (perhaps 4 gauge) which will greatly reduce lamp output.  Clean all connections on both ends of the cables.


2.  Remove your taillight lenses and clean them inside and out, removing any accumulated dust and dirt on the inside.  If the outside is crazed or cloudy, get replacement lenses, but you can try scrubbing them with toothpaste, a mild abrasive, then polish with a microfiber towel.


3. Stop and taillight bulbs are probably dual filament (dimmer filament for tail, brighter for stop), likely 3 candlepower (cp) for tail and 21 cp for stop.  These days everyone is used to seeing super bright LED lights.  And the original 3 cp taillights emit about as much light as a votive candle in a goat's a$$ at midnight during the dark of the moon.  Almost ALL of us with 6V systems and low-output 12V bulbs have been upgrading bulbs to meet expectations of the ignorant ones behind us, who are used to seeing light shows from the back of modern cars.  You can do one of two replacements:  (a) buy replacement LED bulbs for 6V, but you must specify NEGATIVE ground (I think that's what your car is, but verify); or (2) buy aftermarket high-intensity bulbs (14 cp tail, 50 cp stop) from one of the Ford or Chev repro companies.  I bought those 14/50 bulbs from Bob Drake (repro Ford parts) in Medford, OR for $3.00-$3.50 each + shipping, BUT buy twice as many as you need--two of the ones I bought for my Jeepster had quality control issues and got tossed.  Consider the possibility that someone before you but **12V** bulbs in, which would be operating at half output on your 6V electrical system.


4.  Search the TX motor vehicle statutes online (in CA it's the Vehicle Code) for equipment requirements, such as a provision which says that Original Equipment lamps are OK for continued use.  Print a copy of an applicable section and carry it with your registration to show any officer in the future.


5.  6V systems require a little more maintenance, periodic cleaning of grounds.  To me, it's NOT worthwhile to change to 12V.


Good luck and please keep us posted.

 Good post, however the last Beetle in Europe with a 6 Volt system was a 1975 111 1200 & 1200L models.


Edited by Pfeil (see edit history)
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On 2/13/2018 at 3:27 PM, Eddie Davila said:

I looked the Tx veg code and it states 547.32 states at tail lamps emit a red light plainly visible at a distance of 1000 feet from the rear of the vehicle. I will clean the lenses and get brighter blubs. My lights are 12v


I have used aluminum foil to line the inside of the taillight bucket to better reflect the available light. LED bulbs also help, but you'll need an additional draw on the circuit so that the flashers work properly.

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  • 7 months later...
On 6/19/2018 at 10:17 PM, Larry Schramm said:

If the resistance in the bulb stays the same, 1/2 voltage should be about 1/2 the wattage


Right there is the math error, the resistance DOES change! The cold resistance of tungsten filament is low, and goes higher as the heat goes up. Measure the resistance of a typical (or used to be typical) 100 watt 120 volt incandescent bulb, it will be about 9 or so ohms. If it was still 9 ohms with 120 volts on it after the filament was warm, it would draw 13 amps, and not be called a 100 watt bulb!  😁  So unless you think the filament is the same temperature with either 6 or 12 volts, then the math is wrong. You must use test results to get the answer.👍



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