Pre A cabriolet map light

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Jim Wayman
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Pre A cabriolet map light

#1 Post by Jim Wayman »

I am restoring a Pre A cab light and it has been a while since I worked on one. Power to the light is thru the connection at the insulated end of the light. When it is pulled out, the circuit is completed and the light goes on. What mechanically completes the circuit when the light is pulled out.
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John Brooks
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Re: Pre A cabriolet map light

#2 Post by John Brooks »

I think if you look at the other end of the bulb, when pulled out that end of the bulb is now grounded?? pushed in power to the bulb but no ground.
Last edited by John Brooks on Sun Oct 09, 2022 1:32 pm, edited 1 time in total.
John Brooks

62 Roadster
66 912
84 Cab
getting pushed around in porsches since 1965

Jim Wayman
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Re: Pre A cabriolet map light

#3 Post by Jim Wayman »

There is a brass strip that holds the bulb. At the hot end of the bulb, the bulb is insulated from the strip. At the other end, the bulb has continuity with the strip. What causes the strip to gain continuity with the housing (ground) when the light is pulled out rather than when it is pushed in?
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Rick Albro
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Re: Pre A cabriolet map light

#4 Post by Rick Albro »

The ground is through the body of the map light and to the dash... the dash must be grounded to the chassis... Also look at and review the circuits for the idiot lights... somehow using the dash ground as part of the circuit seems to be a week link and often causes open or intermediate circuits... Road vibration, a leaking windshield seal, paint or other anomaly can also lead to incomplete connections between the dash and the chassis. Keep in mind the spacers are aluminum. while aluminum is a good conductor aluminum oxide is a very good insulator. Any surface corrosion on the spacers and the full contact then is with the attaching bolts. There are a few other straps and other pathways to ground but making a good clean connection is key to having many dash related items function.

I have a couple of these apart, the below images were off the web but were ready to post to illustrate the change and differences in the lights... the change occurred in
April of 54 from the early style to the later style... see the post on ashtray knobs... the hole spacing in the glove box door fixes the date of change to the first week of April 1954
shine a light on me.jpg

Jim Wayman
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Re: Pre A cabriolet map light

#5 Post by Jim Wayman »

I realize that the ground is from the housing to the dash. How is the brass strip that holds the bulb grounded to the housing when the map light is pulled out?
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Rick Albro
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Re: Pre A cabriolet map light

#6 Post by Rick Albro »

Look for the little spring that slides with the internal part of the housing... when you move the internal part forward the spring makes contact with the housing... perhaps your spring is missing? They do tend to break from metal fatigue.
internals.jpg

Jim Wayman
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Re: Pre A cabriolet map light

#7 Post by Jim Wayman »

Mystery solved. Spring is missing. Thanks Rick.
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Rick Albro
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Re: Pre A cabriolet map light

#8 Post by Rick Albro »

at least two paths forward...
beryllium copper shim stock is available from mead metals in both 0.032 and 0.040"
buy a copper spring part from Granger or similar supplier and repurpose the spring...

another option is to use a spring steel shim stock and have it copper plated for increased conductivity and corrosion resistance... sometimes plating results in embrittlement but the shim stock is thin and may not be significantly affected.

beryllium copper is a spring copper with high conductivity and can be formed this is the same metal that was originally used. This alloy metal is also used for gas tank sender spring on the inside (A style senders)...The shim stock has many applications including repair of many electrical items or at least the insides of many electrical items...
connection.jpg
and for completeness we should also show the very early pre A cab map light... :>)
PreA cab dome light.jpg

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