Why does my trunk lamp have a diode feeding the bulb, can I eliminate it?

I put a VOM on resistance and connect the meter to both ends of the diode, diode side of the spot welds and get infinite resistance in both directions. A 12 bulb will not light trying to pass voltage through this diode, again just touching the leads not the spot welds. Both directions.
Some thoughts. This is for the trunk, could this diode be for preventing the interior dome lights from lighting when I open the trunk?
Last year I replaced the back up lamps with LEDs, related?
If I replace the festoon s 8w bulb with an LED bulb and eliminate this diode, does the led act as a diode allowing current only one way? Solving this problem?
The pics show only 2 terminals. I don't see a place for a third connection. ?
 
Yes, that is a diode/rectifier. It likely failed when the bulb burned out (an incandescent bulb will use maybe 20 amps for a thousanth of a second when it burns out, enough to easily take-out a 1 amp diode). It is beyond me why it is there. The only thing I can think of is that the computer it is connected-to may not like that the wire is grounded (even with the added resistance of the light). The diode will isolate the power wire from the ground.
Since it failed "open" you don't even have to remove the old one. Just obtain a 1N4001 (or 4002, 4003, 4004, etc, it doesn't really matter) and solder it across the old one. You can also solder a second one in parallel which would double the current carrying capability. Problem solved. Too bad Radio Shack is closed, this would have been an easy sale for them. If you lived by me I would give you a few.
BTW, on Hyundai/Kia cars, if you pop the trunk the light will come on and stay on even if the trunk lid is not raised and it will stay lit until the battery goes dead. Ask me how I know.
 
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What VOM do you have? You need to put it into diode mode. A good meter should not make enough voltage to turn on a diode in resistance mode.

View attachment 133304
Earlier today I used a Fluke to measure a diode in resistance mode before making my post. It measured about 4 megaohms.

This corresponds with what Fluke states on the page linked below, quote: "The forward-biased resistance of a good diode should range from 1000 Ω to 10 MΩ"


At the same time, meters have diode mode to test diodes for a reason, to be sure to produce enough voltage in that mode to exceed the forward drop and then measure that, since a low resistance measurement could mean it's failed shorted.
 
There are a couple of places around here that say they repair TV's . Maybe they can provide a diode.
The diode will not conduct 12v to power a bulb in either direction.
I could buy an LED replacement bulb. If I jumped the bad diode and put in an LED, do they not also function as a diode?
 
There are a couple of places around here that say they repair TV's . Maybe they can provide a diode.
The diode will not conduct 12v to power a bulb in either direction.
I could buy an LED replacement bulb. If I jumped the bad diode and put in an LED, do they not also function as a diode?
The LED will most likely act as a diode, and won't light up if you hook it up backwards.

Note, there are LED's which will light up in either direction, with different colors, and I doubt you'll accidently come across those for an automotive application. These LED's would need a voltage dropping / current limiting resistor, and any sort of LED automotive retrofit bulb would have that built in for you already.

I suppose it's possible that there are retrofit bulbs out there with diode bridges in them, allowing them to work in either direction, just seems like an easy corner to cut for them.
 
I "might" have uncovered what this diode is doing, acting as an OR-ing diode in conjunction with one in the BCM that has its own voltage sensing to detect a pull down, these two diodes isolating each branch, to sense when the trunk is open, since the trunk latch switch ("might".... probably?) serves double duty for sensing the trunk is open, and to turn the bulb on.

This is not a schematic specific to that vehicle, but shows the principle. Even if you elect to switch to using an LED bulb, I would still replace the diode, though technically yes, a little festoon type LED retrofit bulb is just going to have a few LEDs in series and will do the job of the separate diode, but will have to be installed with the right polarity. I wouldn't even chance it though, better to put it back to OEM state with the diode included even if you swap in an LED bulb, so at any time in the future, any tech or owner won't have to guess what has been cobbled together and how to resolve any issues.

LED retrofit bulbs (especially the dodgy/cheap designs) don't always have long lifespans, often not as long as the incan bulb they replace because they are driven too hard and heat up too much, so it's entirely conceivable that at some point in the future, someone might try putting an incan bulb back in to replace a failed LED bulb.

ORing.png
 
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Earlier today I used a Fluke to measure a diode in resistance mode before making my post. It measured about 4 megaohms.

This corresponds with what Fluke states on the page linked below, quote: "The forward-biased resistance of a good diode should range from 1000 Ω to 10 MΩ"


At the same time, meters have diode mode to test diodes for a reason, to be sure to produce enough voltage in that mode to exceed the forward drop and then measure that, since a low resistance measurement could mean it's failed shorted.
Interesting. Ok, years ago I got a 179 at work, so here is some testing with my old BK meter. 179 in resistance mode, BK to monitor voltage.
  • open circuit, I see 2.65V, much higher than I would have guessed (more than enough to turn on diodes)
  • 100k 5% carbon, measure 99k and 0.1V
  • 10k 5%, 9.85K and 0.1V
  • 1k 5%, 0.996k and 0.1V (I'm seeing a pattern here...)
  • 1N4148 measures 9.94Meg and 2.64V one way, 280.5k & 0.286V the other
  • 1N4004 measures 9.98Meg and 2.64V one way, 309k and 0.315V the other
1672535019129.png


Now the curve I saw for 1N400x didn't give forward current for such low bias voltage (no surprise).
1672534969062.png


0.3V over 0.3Meg, that's 1uA test current? Not going to see much there!

Edit: looks like the Fluke 179 uses 1mA for the diode test current. Open circuit is 7.3V, however it won't read a 5.1V zener, it shows it as open circuit. I put a couple diodes in series and did see 1.3V, not sure what the upper limit for it considering a diode as "open" is.
 
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There are a couple of places around here that say they repair TV's . Maybe they can provide a diode.
The diode will not conduct 12v to power a bulb in either direction.
I could buy an LED replacement bulb. If I jumped the bad diode and put in an LED, do they not also function as a diode?
Yes I'm sure a TV repair place would have suitable diodes in stock. IMO you are better off with the highest currrent rated diode that will fit in that recessed hole area they provided, so it might help to take your light housing with you for comparison. That's not entirely accurate, if you have clearance on the other side of it, it wouldn't have to fit in that cutout in the housing insulator sheet. Can't see that clearance from here. ;)
 
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I "might" have uncovered what this diode is doing, acting as an OR-ing diode in conjunction with one in the BCM that has its own voltage sensing to detect a pull down, these two diodes isolating each branch, to sense when the trunk is open, since the trunk latch switch ("might".... probably?) serves double duty for sensing the trunk is open, and to turn the bulb on.

This is not a schematic specific to that vehicle, but shows the principle. Even if you elect to switch to using an LED bulb, I would still replace the diode, though technically yes, a little festoon type LED retrofit bulb is just going to have a few LEDs in series and will do the job of the separate diode, but will have to be installed with the right polarity. I wouldn't even chance it though, better to put it back to OEM state with the diode included even if you swap in an LED bulb, so at any time in the future, any tech or owner won't have to guess what has been cobbled together and how to resolve any issues.

LED retrofit bulbs don't always have long lifespans, often not as long as the incan bulb they replace because they are driven too hard and heat up too much, so it's entirely conceivable that at some point in the future, someone might try putting an incan bulb back in to replace a failed LED bulb.

View attachment 133328
I don't know if I fully see it yet, but that does make sense, it's somehow diode'd together. When the trunk is open (the switch closes), the light always comes on. But the MCU can still pull the line low other times? simplified schematics sometimes omit details, so I'm not sure how much to read into this.
 
^ I suspect the BCM uses its own 12V circuit as a reference signal line where its internal diode is, and senses a drop on that signal line when the trunk latch switch closes the circuit, to *know* the trunk is open, but yes a more complete, vehicle-specific schematic would help. I just don't have that, and doubt one is available for the PCB in the BCM itself.
 
The schematic rules. It allows the lamp and the trunk switch to work and provide information back to the ECU that the trunk is open, using less wire. isolates what is likely to be different fused power sources between the ECU and map power.

the diode has a purpose.
 
Cut the diode out of the unit. #IN4007 now that I can see the entire thing. There is room for a little bigger diode.
There is an electronics store about 20 minutes away although I don't go that direction often. I can put it on my list should I head out that way.
What do you suggest from this page.
 
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1N4007 is what it says? very odd, thats high voltage.

Regardless. Your link shows they have 1N4007. Just get the ten pack and be done with it. As an engineer I like to fix things until they are broken--no need to here.

If they are out, get 1N4004, then 1N4002, and if they are still out, I can’t see why 1N4001 won’t work. Otherwise just get the same part.
 
The heat from that bulb will eventually kill the diode again. It's an odd location to put that with an incandescent bulb that gets very hot. 1n4001 is rated for 1 amp at 50Vdc. The reason for it is that the BCM has a "sense" circuit to let you know if the trunk is open (dash warning light, I presume). The BCM wire is looking for 0 volts (when the switch is closed with the trunk open) to provide its warning. It's common practice to isolate the test circuit this way to make sure the voltage at the test point never goes above BCM running voltage as bad things can occur. Can see the diode in the BCM as well to provide further safety to keep the BCM from damage. Voltage drops in wires and BCM draw can vary running voltage at different parts of the car so this is to provide safety. Running two diodes in parallel right there would give some redundency to the circuit should one die. Here are some 3amp versions that your nearby store may have if you want to go with a higher current device.

 
Cut the diode out of the unit. #IN4007 now that I can see the entire thing. There is room for a little bigger diode.
There is an electronics store about 20 minutes away although I don't go that direction often. I can put it on my list should I head out that way.
What do you suggest from this page.
I didn't read through all their pages, stopped when I saw these 1N5821 3A/30V schottky's which would be suitable and not run as hot:


They are roughly 5mm diameter and ~9mm long casing, if you have clearance for that. Another option instead of putting it in the light module is put it in series on the wire to it, then you also avoid it heating up more due to proximity to the bulb, but I'd rather have it on the housing if it fits, as this puts it back as close to stock (but improved) as possible.

This would be an upgrade over the factory 1N4007 and should last a lot longer in same environment that burnt out the original. I like to upgrade/fix things so they don't suffer the same fault again, if possible.
 
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Replace the diode with 1N5821 and all is good. I hope this diode does not overhead and fail like the original 1N4007. As wag123 says, this light may stay on indefinitely with the hatch open so perhaps the heat from the 8watt incandescent bulb is a factor. We will see.
Thanks to all.
 
Yes, that is a diode/rectifier. It likely failed when the bulb burned out (an incandescent bulb will use maybe 20 amps for a thousanth of a second when it burns out, enough to easily take-out a 1 amp diode). It is beyond me why it is there. The only thing I can think of is that the computer it is connected-to may not like that the wire is grounded (even with the added resistance of the light). The diode will isolate the power wire from the ground.
Since it failed "open" you don't even have to remove the old one. Just obtain a 1N4001 (or 4002, 4003, 4004, etc, it doesn't really matter) and solder it across the old one. You can also solder a second one in parallel which would double the current carrying capability. Problem solved. Too bad Radio Shack is closed, this would have been an easy sale for them. If you lived by me I would give you a few.
BTW, on Hyundai/Kia cars, if you pop the trunk the light will come on and stay on even if the trunk lid is not raised and it will stay lit until the battery goes dead. Ask me how I know.

Maybe they want the diode to fail in case the bulb shorts. A fuse might not act fast enough to protect the body computer.
 
I spend some time googling..and ya know google is always right. The diode appears to be for circuit isolation. Hyundai uses them a lot. Perhaps without this diode, opening the trunk would also power the interior lights....who knows.
 
I spend some time googling..and ya know google is always right. The diode appears to be for circuit isolation. Hyundai uses them a lot. Perhaps without this diode, opening the trunk would also power the interior lights....who knows.
It is part of an OR circuit and the circuit needs isolation provided by the diodes.

An 8 Watt incandescent lamp needs 8W/12V = 0.7 Amps!

The 1N4XXX series are 1A diodes.

A 1N5401 diode should be more than sufficient since it is capable of 3A continuous and only drops about 0.7V.

 
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