The Stovebolt Site Forums The Shop Area The Electrical Bay Under hood light repair

Have the mercury switch controlled under hood light that does not work. No matter what position the socket faces I have no voltage to the center contact and no continuity from the center contact to the end of the wire that provides 6volts. Any thoughts on repair or replacement of mercury switch? This is on a 49 3100, p/n 986642, Hobbs corp light. Thanks

Chances are that the glass vial that contains the mercury has gotten broken, or a connection inside the light housing has corroded. Look for a crimped seam or some other way to disassemble the light to inspect the inner workings and gently unroll the crimp for access to the interior. I don't think those lights were designed to be repaired, but it had to be assembled somehow originally.
Jerry

Ed, I couldn't find a picture of a 986642 so I will assume it is similar to a 986250.

You probably know this already but I'll throw it out anyway. The contact-mercury-contact part of any mercury switch rarely fails. It's the parts going from the switch (one end going to the power source, then other end going to the bulb). A mercury switch can be physically broken. Mercury is toxic and you don't want any of it contacting your body.

I suspect the light doesn't work because the wire is broken and just like hair dryers and hand drills, 99% of the time it's where the wire enters the housing or enclosure. Unfortunately there is no strain reliever in that area so the wire is free to move around due to normal vibration of the truck. How do you get the housing apart to fix it? I don't know. Since there is mercury inside, I don't want to offer a guess.

The original switches bring big bucks on evilbay. I would post it for $50 plus shipping with a note that it doesn't work. With the proceeds I'd buy a modern switch that has a built-in strain reliever.

What I was thinking was to change out the socket with mercury switch assembly from one that works into mine. Just have to find the right older light to use. I think it is doable. Will keep everyone posted.

If you can, post a picture of the one you have.

I found a 1940 Hobbs patent. No mercury.
If the 986642 switch is constructed like the patent, it would be difficult to take apart to repair the wire.

I was able to repair mine by replacing the entire socket assy with a newer one. Did require a little creative work to get the new smaller socket assy into the old light. The old lamp did not use mercury but was mechanical like the one shown above with balls in a sealed tube that make contact between the incoming power and the center contact as the lamp is moved and the balls line up. When I removed the tube and shook it I heard it rattle. Anyway, long story short repair is reasonable but takes time. The old switch is not reparable unless a new tube is used.