1939 La Salle Taillight LED Conversion Problem

[NH LaSalle]

I'm having a problem with the converting of the taillights on my `39 La Salle to LED bulbs.

Basically, when plugged into the sockets, the incandescent and halogen bulbs (which I want to replace due to the current that they draw) work perfectly.  However, when the LED bulbs are plugged into the sockets, they do not work.  Here's what I've done to try to solve this problem (apologies for the length).

The LED bulbs were bench-checked and are, indeed, 6-volt positive ground, so the polarity is correct.

The socket's hot terminals have 5.7 volts so the available voltage, as I understand what is necessary for the LEDs to light, is adequate.

I was concerned about an inadequate ground so a wire was soldered to the base of the LED light to provide a direct ground to the chassis.  To assure a functional solder joint, this grounding wire was bench-checked and the bulb works when current is applied to the contact buttons.

On the car, with the LED bulb out of the socket and with its soldered grounding wire connected to the chassis, when a jumper wire is attached to the socket's contact buttons and then to each of the light's contact buttons, both phases (taillight/brake light) of the light works perfectly.

However, with ground wire still connected to the chassis, when the bulb is placed into the socket, the bulb does not work.  (The solder from the added ground wire does not make contact with the socket).

The sockets were rebuilt and there is a new compression spring in place and there is good tension when the socket's contact plate is compressed.  Furthermore, the contact buttons on the bottom of the LED bulb show mild scuffing so it would appear that they are making contact with the buttons when it is in the socket.

In short, the polarity is correct, the voltage is adequate and there is a direct ground from the bulb to the chassis.

Thanks, in advance, for any advice that will help me solve this conundrum.

[AdamS]

@NH LaSalle,
The contacts in the car-side sockets likely aren't making good contact to the connections at the base of the LED bulb. The socket contacts each need a rounded blob of solder that sticks out past the insulating wafer. You may also need similar solder blobs at base of the bulb. Try the socket side first.

Perhaps you can share close-up photos of the socket contacts and the bulb contacts?

[NH LaSalle]

Adam

Thanks for responding and for your suggestion.  There are scuff marks on the contact buttons at the base of the light, which made me think that contact was being made between the bulb's contacts and those located in the socket. Your suggestion prompted me to take another look.

The light's contact buttons are convex shaped relative to the horizontal surface of the bottom of the light's base - i.e. - high in the middle but tapering to the horizontal plane on both ends.  But at the center, they do not project very high from that horizontal surface.  Based on your comments, what I wondering may be happening is that as the bulb is placed into the socket and turned to lock it into place, socket button contact is made with the high point on the bulb's contact button causing the scuffing but as the bulb is turned to be locked in, the socket contact buttons ride down on the contour of the bulb's contact buttons and at some point either contact is broken or the contact diminishes enough for the voltage being transferred to drop.  As I understand the LED bulbs, they are more sensitive to dropping voltage than either the incandescent or halogen bulbs are, which would explain why they illuminate and the LED doesn't.

It's a bit more of a process to remove the socket to build up its contacts with solder.  So, what I'll try first is to build up the solder on the light's contact buttons first to see if that will solve the issue.

Getting photos of the bulb's base is not too much of an issue but getting an accurate one of the socket's contact buttons are a bit more problematic due to the depth of the socket.

[AdamS]

@NH LaSalle,
If you look at a conventional tail lite bulb, it would have solder blobs as contacts. The socket, similarly, would have solder blobs on the contact buttons. It's rather crude, but these mating solder blobs make a good connection.

Let us know how it goes.

[Jw4]

If the bulbs work connected to the car battery, and you did not mess-up the ground (or these are ground-insensitive), are you sure that 5.7 volts is enough? How much do you get at the battery when bench-testing?
Never dealt with 6-vold LED bulbs, but a quick google-search points-out that these might be highly voltage-sensitive. Did you test these with the car running?