"CANBUS" LED bulbs in older cars.

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Jim Hearne

Jul 8, 2021, 5:30:39 AM7/8/21
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Has anybody seen any articles on why you shouldn’t use “CANBUS” compatible LED bulbs in older cars.
Or, at least not without modifying them first.
If not i may write one.

Ian Harrison

Jul 8, 2021, 5:58:43 AM7/8/21
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From what I’ve seen, the only thing that makes them CANBUS compatible is a built in resistor fooling the computer into thinking it has a working filament bulb.


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Ian Harrison

Jul 8, 2021, 6:00:59 AM7/8/21
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aaaand I missed the point of your post completely.

No I’ve not seen an article about this either way.


Jim Hearne

Jul 8, 2021, 6:31:39 AM7/8/21
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Indeed, the problem is that that resistor is dissipating a lot of heat.
In a real bulb that heat is mostly dissipated though the glass and into the air around the bulb with very little conducted back into the connector via the thin wires inside the bulb.
A “CANBUS” LED bulb on the other hand has this extra resistor (or more than 1) mounted on a PCB, often right next to the connector and with thick copper tracks conducting heat back into the connector.
I’ve seen bulb holders overheated far more with LED bulbs than normal bulbs.
This extra resistor is only fitted to “CANBUS” compatible LED bulbs, just to make the LED bulb take extra current to fool the bulb failure monitoring system in some cars that otherwise would think the bulb has failed.
Now, the bulb monitoring system has absolutely nothing to do with CANBUS, there were bulb monitoring systems long before CANBUS, i’m pretty sure my Sierra had it.
CANBUS is a digital communications bus system that links different ECU’s in the car.
You can have a CANBUS system without bulb monitoring and bulb monitoring without CANBUS
I don’t know how LED bulbs with the extra resistor came to be called “CANBUS” compatible , probably some Chinese misunderstanding.
Even if you car does have bulb failure monitoring it’s only going to be on the exterior lighting, not the interior light or dash illumination.
So, for using CANBUS LED’s in a older car (or indeed any car that doesn’t have bulb failure monitoring) i always remove the extra resistors.
They are usually the largest ones and , right next to the connector and connected directly to it, there will be smaller resistors for limiting the current to the actual LED’s themselves.
As an example i just bought some T5 bulbs with 12 LEDS on them in total.
When i measure the current on a 12Volt supply they take 0.173 amps which using ohms law is 2 watts
Once i remove the 2 x 24 ohm resistors that are on there just to fool the bulb out systems, the current is only 0.073 amps, only 0.876 watts
The bulb will work exactly the same in a car without bulb monitoring, it just doesn’t waste a lot of energy as heat which will damage the connectors and also the LED’s themselves.
The situation is even worse on larger bulbs where the extra resistors have to draw enough current to fool the bulb failure system into thinking that a 21 watt bulb is still there.
I appreciate not everybody is good with a soldering iron but you just need to heat up one end of the resistor with a blob of solder, once it melts move to the other end of the resistor, alternate a few times so both ends are molten at the same time and then the resistor should slide off.
There are quite often one on each side of the PCB.
Or, you can carefully break up the resistors with a pair of wire cutters.
In the attached picture the resistors you want to remove are labelled 221  (interestingly the picture from Amazon shows them as 221 = 22 ohms whereas the ones i received are 241 = 24 ohms)
One on each side.
If you have any bulbs and you aren’t sure which resistors are which , send me a picture.
Your bulb holders will thank you.
Rant over.


Jul 9, 2021, 7:06:26 AM7/9/21
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Thanks for that, Jim.


I’d always had the vague nagging doubt that Canbus bulbs probably did their thing via resistance, rather than electronic cleverness – and therefore entirely negate the biggest benefit of LED bulbs.

I shall remove resistors with impunity on all my new LED bulbs. 😊



Ian Harrison

Jul 9, 2021, 9:23:07 AM7/9/21
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Question Jim, do these ‘just’ provide load for the car side bulb monitoring or does it pull the nominal 12v down to a working voltage for the leds, which is likely to be much less on small bulbs?



Jul 9, 2021, 4:50:04 PM7/9/21
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From my experience the load resistors are not just to provide a load to cancel the bulb out monitoring but are there to prevent the canbus connected light control modules constantly switching the supply voltage on and off & going into bulb out function where the car powers up another bulb in the cluster on what it thinks is bulb failure due to the lower power consumption of the LED over a bulb.

Mercedes and GM have been using the above for quite some time, amongst others now, & I suspect is the reason the led bulbs are marketed as canbus types.

Still doesn’t excuse the lack of provision for proper heat dissipation though.


I do agree with Jim as the Chinese, amongst others, who make cheaper end throw away electronic based products seem to have little regard to how the heat is going to be dissipated & quite often see things failed simply because the components used require some sort of heatsink to dissipate the heat they produce in service but are very rarely manufactured with any provision for the heat produced so fail in short order quite simply due to overheating.

One of the most annoying failures I am coming across recently is where people have replaced the old bi metalic voltage regulators for the gauge clusters in older cars with one of the cheaper end solid state devices sold for the job but the problem is most of them fail after a sort time.

After opening a few its quite simply the component that is doing all the supply switching requires a heat sink to dissipate the heat & I have yet to come across one with even a basic heat sink fitted.

I have fitted a few with small heatsinks and so far all of those are still going strong.


I tend to make sure the LED type bulbs I buy are the non canbus type if fitting to a older car or do as Jim suggests and remove the loading resister just leaving the resistors that are required for the LED to work.




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