Mercury dawn-to-dusk won't shut off

[rossow (mn)]

The mercury-vapor dawn-to-dusk lamp outside my shop won't shut off on its own. It has worked flawlessly until recently, when it decided to stay on all day and night. I figured a bad sensor so replaced it, but still doesn't shut off unless I turn off the power. The bulb is the one with a mogul base. The sensor is the common three-prong, twist-on type on top of the fixture, and it goes in and locks in place smoothly. New sensor specs match the old one. The receptacle for the sensor is clean and dry. So is the bulb socket. The lamp goes on quickly after dark when I turn on the power but stays on all day. If I cut power in daylight, the light gradually comes on after a minute or so but never goes off on its own. What besides a sensor might cause this?

(I'd like to avoid having to take the lamp unit off the pole because it's heavy and a bit unsafe to access without a bucket truck.)

 

[cwtech]

I have experienced new sensors which were bad.

 

[kcm.MN]

If you've already replaced the photocell, then it's either a bad new photocell, or there is a short inside the light fixture that's keeping power on all the time.

Don't know what wattage your light is, but I'd recommend replacing with an LED. They're smaller, lighter, use less power, and a LOT brighter light than our old Mercury Vapor. Amazon has lots of them. Ours is from local electric utility. Seems like they said ours is 67watts or something, but puts out more and brighter light than the old Mercury Vapor. Also, they're instant-on!!

Check here if you like:
https://www.amazon.com/s?k=led+yard+light&ref=nb_sb_noss_2

https://www.amazon.com/s?k=led+street+light&ref=nb_sb_noss_2

 

[Bob]

One of three things... new photo control is "bad", black and red wires have shorted together inside the fixture from years of heat and vibration in the wind, bypassing the photo control, or the white wire inside the fixture to the photo control socket is "open"/disconnected.
<img src = "https://www.paigewire.com/specs/images/K4021_diagram.gif">

To check for a short in the fixture, remove the photo control and see if the light (stubbornly) still comes on when powered up.

If it doesn't, try another photo control.

If the STILL doesn't fix it, likely something has happened to the white/neutral wire to the photo control, it can't function without a "neutral", and the light will stay on in daylight.

I have a number of "dusk to dawn" lights and have disabled the photocontrols on most of them and installed a separate one in a weatherproof outlet box down at a level where I can get to them, seems like they never fail when they are easy to get to!

 

[K Effective]

Since there are no other parts related to the switching, the sensor must not be opening, or is not sensing the light. While they need to be aimed a certain direction to sense the approaching darkness, they should open at midday regardless of direction. The sensors typically fail closed, leaving the light on all day. It sounds as if the new sensor is the place to start.

 

[Barts Dad]

Had a new sensor late last fall that worked fine for a month then random times wouldn?t turn the light off. Company online suggestions, tap sensor, sensor will reset and turn off. Got an old telescoping fish pole and reached way up and tapped it, yep went off. Would randomly stay on once, twice a week. Ordered new sensor from Menards, same brand as light fixture. Waited for snow to melt, ladder up and changed sensor, works like a champ.

 

[dr sportster]

If you put electrical tape over the sensor you can test it. Wait a minute then test.

 

[Geo-TH,In]

The old school sensor works backwards for what most think.
At night the sensor, CdS cell increase its resistance dropping out the relay, which turns the light on.
Day light decreases the photo sensors resistance pulling the relay in turning the light off.
Just backwards from what most think.
Take sensor apart and see if switch on relay isn't stuck in the in on position.
George

 

[rossow (mn)]

UPDATE:

First, thanks to all of you for such quick replies to my dilemma. Second, based on your advice, I removed the sensor and found that the lamp still lit up without it. So I decided to dig in. I managed to unbolt the pole from its concrete base and tip it far enough to access the lamp from the roof of my shed. Inside I found a mess. The heat from the bulb had baked the insulation off of much of the wiring. ( I had rebuilt the unit with new wiring many years ago.) I'm amazed that everything hadn't shorted out and exploded in a shower of sparks. Be that as it may, I cut out all the old stuff and am in the process of figuring out how to put it all back together with new wiring. Time will tell if it ever works again, but it's fun to try. Thanks again to everyone for advice.

 

[Bob]

(quoted from post at 14:50:16 04/07/20) UPDATE:

First, thanks to all of you for such quick replies to my dilemma. Second, based on your advice, I removed the sensor and found that the lamp still lit up without it. So I decided to dig in. I managed to unbolt the pole from its concrete base and tip it far enough to access the lamp from the roof of my shed. Inside I found a mess. The heat from the bulb had baked the insulation off of much of the wiring. ( I had rebuilt the unit with new wiring many years ago.) I'm amazed that everything hadn't shorted out and exploded in a shower of sparks. Be that as it may, I cut out all the old stuff and am in the process of figuring out how to put it all back together with new wiring. Time will tell if it ever works again, but it's fun to try. Thanks again to everyone for advice.

AHA!

Second scenario on my list!

Glad you got it figured out.

Years of heat and dust and wind and bugs sure can make a mess inside those units.

 

[miangus]

I had one do that. Installed a remote sensor at the switch worked good and I did it from the ground.

 

[Janicholson]

That far into it, put in a LED bulb for way more light, and 1/5 the electricity/heat. You need no ballast, the nsw sensor will work, and a mogul to edison medium base can be had. Jim
corn LED

 

[K Effective]

That was the same scenario I had found from my light, I replaced the whole fixture with an LED unit, much smaller, and brighter. it seems to shine more light on the ground and less just "out there".

 

[rossow (mn)]

Yes, Bob, I tried your suggestion to take the sensor off to see if lamp would still light up, and it did. That's what told me it was time to dig into the guts. I haven't been able to reassemble the thing yet -- too much wind to be dancing around on a 15-foot roof. But I soldered new wire to all three wires coming from the sensor socket, cobbled them up to a regular bulb and everything worked -- on when dark, off when light, multiple times -- all as it should be. Now I just need to get some better weather to do the reassembly, hoping all the while I don't fall off the roof. I may end up replacing the wiring inside the pole, but that's not a problem because the pole has two access doors near the bottom and a few feet up. Thanks again for help.

 

[Bob]

(quoted from post at 18:28:04 04/08/20) Yes, Bob, I tried your suggestion to take the sensor off to see if lamp would still light up, and it did. That's what told me it was time to dig into the guts. I haven't been able to reassemble the thing yet -- too much wind to be dancing around on a 15-foot roof. But I soldered new wire to all three wires coming from the sensor socket, cobbled them up to a regular bulb and everything worked -- on when dark, off when light, multiple times -- all as it should be. Now I just need to get some better weather to do the reassembly, hoping all the while I don't fall off the roof. I may end up replacing the wiring inside the pole, but that's not a problem because the pole has two access doors near the bottom and a few feet up. Thanks again for help.

You are most welcome, glad you got it worked out, and good for you for posting back, it may help someone with a similar problem that finds this thread in and archive search.