Electric current needed to stop hyperflashing turn signal bulb?

beewstew · Post by **beewstew** » 19 Mar 2025, 06:04

Wondering if anyone knows what the minimum electric current (or power) is to stop the "failed position lamp" and hyperflashing turn signal error with LED bulbs.

I've seen some LED's suggest a 50 Ohm resistor in parallel with the bulb would be sufficient, but the heat sink on the commercial "load resistors" seems much bigger that what would be necessary for Power=Voltage^2 /Resistance = 12v^2 / 50Ohm ~= 2.9 Watt. Related, the 50 Ohm resistor would mean the Volvo's ECU thinks a lamp bulb is ok if it draws at least Current = Voltage / Resistance = 12v / 50 Ohm = 0.24A = 250mA?

Are these electrical limits in the factory service manual somewhere?

Post by **volvolugnut** » 19 Mar 2025, 09:55

I don't know the resistor needed, but you should be able to calculate if you measure resistance of incandescent bulb and LED bulb. Then calculate the resistor needed for either series or parallel load resistor to add.
volvolugnut

Post by **abscate** » 21 Mar 2025, 04:20

It has to be a parallel load so that the full voltage is on the LED. If you mimic the regular bulb draw though, why go to LEDs?

beewstew · Post by **beewstew** » 21 Mar 2025, 05:46

abscate wrote: ↑21 Mar 2025, 04:20 It has to be a parallel load so that the full voltage is on the LED. If you mimic the regular bulb draw though, why go to LEDs?

Can a parallel resistor that draws 5W trick the ECU, which thinks there's should be a 50W bulb in that socket? That's what I'm wondering. There must be some minimum current that the ECU interprets as a functional bulb. Knowing that spec would allow me to use a pretty big resistor with lower power draw than standard.

Post by **scot850** » 21 Mar 2025, 10:31

As you already found out the Volvo test system is very sensitive to bulb loading. The dealer service manager I know says they have lots of issues with people coming in with bulb errors even after they changed the bulb. Fitting a new Volvo supplied bulb fixed the issue.

Personally I have not had an issue ever with this using bulbs form well known manufacturers like Sylvania. On the P2 cars though I have had several issues with the actual bulb holders burning out are failing. One of those had been damaged by the owner fitting a huge LED COB bulb and it had burned the contact area of the holder. I have also had issues with those bulb holders also failing. Not often on that one but it has happened.

I understand the attraction of LED bulbs, but on Volvos they don't always work. Headlight bulbs I can see the attraction as they can make a huge difference especially in older housings where the reflectors are aging.

Good Luck!

Neil.

JoeBar · Post by **JoeBar** » 22 Mar 2025, 15:34

HI, I have used 8 Ohm / 25 watt resistors parallel to the LEDs in my daughters car. Its no Volvo, but you need to simulate the current used by the bulb.
An indicator bulb of 21W uses +/- 1.75A at 12V. (system is more like 14V so 1.5A). 14V / 1.5A = 9.33 Ohm. 8 Ohm/25 Watt i had laying around here so used that. The power consumption for the resistor = @14V/ 8 Ohm= 1.75A > P= 14V x 1.75A = 24.5 Watt
The 50 Ohm resistor you mentioned is much to high and won't work I think.
I will test it tomorrow. I have some LED for my headlights that needs a resistor as well.

I do not know how many amps the system minimal needs to detect that bulb is good. The system is measuring it with shunts if you look at the schematics.