LED rear lights on V90

[martin calva]

(I thought I posted a question like this yesterday but I can see no sign of it now.)

I changed the 5/21W rear lamp bulbs for LED equivalents (pure red and brighter, with brake lights lighting instantaneously).

The "Lamp bulb defective" warning light on the dashboard came on as soon as the LEDs were installed. I assume the system thinks that the LED lamps are defective because they draw much less current than the incandescent lamp bulbs.

Is there any solution other than either replacing the LEDs with the original incandescent bulbs or fitting resistors in parallel with the LEDs so that the system sees what it thinks is the correct current?

[classicswede]

Sounds like dud cheapo ones. Good LED bulbs are fitted with a resistor to emulate the resistance of a regular bulb

[Laird Scooby]

(I thought I posted a question like this yesterday but I can see no sign of it now.)

I changed the 5/21W rear lamp bulbs for LED equivalents (pure red and brighter, with brake lights lighting instantaneously).

The "Lamp bulb defective" warning light on the dashboard came on as soon as the LEDs were installed. I assume the system thinks that the LED lamps are defective because they draw much less current than the incandescent lamp bulbs.

Is there any solution other than either replacing the LEDs with the original incandescent bulbs or fitting resistors in parallel with the LEDs so that the system sees what it thinks is the correct current?

I'm working on exactly that on my 760 at the moment among doing a million other things thanks to life in general!

A 6 Ohm 50W resistor across the brake lamp terminals will stop them triggering the BFU, i'll let you know on the tail lights. Don't be fooled into thinking those with the resistor built in to fool CANBus will fool the BFU, it works on different resistances and currents to the I2C system, later known as CANBus.

Don't know where you bought your LED bulbs from but you can buy those resistors here, primarily intended to prevent hyperflash on indicators, as the indicator bulb is also 21W (as are the brake filaments in a normal bulb) they will spoof the BFU the same way. There's even a handy diagram of where to connect on the bulb to fool the BFU on the brake lights.

https://www.classiccarleds.co.uk/products/2-x-50w-6ohm-led-indicator-ballast-resistors-fix-hyper-flash-turn-signal?_pos=1&_psq=resist&_ss=e&_v=1.0

[baggy798]

Never had much luck with the 5/21w LED bulbs 8 years ago, there wasn't that much difference between the 5w and 21w circuit so it almost looked like you had the brake lights on constantly. They didn't last long either before one started flickering/failing. Maybe they've got better since then.

The one in the high level brake light position had a faint glow even without pressing the brake, maybe because the rest of the bulbs were LED?

Went back to incandescent, although the indicators and side lights are still LEDs and been fine.

[Clan]

Never had much luck with the 5/21w LED bulbs 8 years ago, there wasn't that much difference between the 5w and 21w circuit so it almost looked like you had the brake lights on constantly. They didn't last long either before one started flickering/failing. Maybe they've got better since then.

The one in the high level brake light position had a faint glow even without pressing the brake, maybe because the rest of the bulbs were LED?

Went back to incandescent, although the indicators and side lights are still LEDs and been fine.

you cant beat a bulb, fit new volvo bulbs and another 10 or 15 years trouble free operation. LEDS will fail or flicker, are the wrong brightness, either too bright or too dull and look odd. and don't like damp, I've been trying for the last 10 years on various cars as each new generation comes out and given up.

[Laird Scooby]

you cant beat a bulb, fit new volvo bulbs and another 10 or 15 years trouble free operation. LEDS will fail or flicker, are the wrong brightness, either too bright or too dull and look odd. and don't like damp, I've been trying for the last 10 years on various cars as each new generation comes out and given up.

You can beat a bulb, they make a lovely sound when beaten with a lump hammer!

The LEDs that fail are generally cheap Chinese ones that are usually over-driven because they forget most cars charge at 14V or above and do their sums for the current limiting resistors on 12V and not 14V resulting in failure because they insist on driving them at Imax on 12V resulting in (usually) about 33% over-current on 14V when the vehicle is charging.


Stop/tail on ^^^^^


Stop only ^^^^^


Tail only ^^^^^


Old filament bulbs tail only ^^^^^


The LED bulbs i used ^^^^^

https://www.classiccarleds.co.uk/products/2-x-bay15d-red-stop-tail-led-4014-30-smd-brake-rear-light-glb380-p21-5w-classic-car-negative-or-positive-earth?_pos=10&_sid=02f5fc7f1&_ss=r

Those were fitted in 2016 after already equipping my other 827 with them a year or two previously, those LED bulbs are still working and must be ~8 years old now. No problems with failing, flickering, brightness (or lack of) unlike filament bulbs where i've recently had several intermittent failures because of poor quality bulbs (and still have it seems!) and have (temporarily) replaced them with more filament bulbs until i sort out the load resistor to spoof the BFU.

That's not to mention the fact filament bulbs all blacken the glass envelope internally, resulting in reduced light output.

[martin calva]

Sounds like dud cheapo ones. Good LED bulbs are fitted with a resistor to emulate the resistance of a regular bulb

These have no resistance fitted - I checked the current they draw on 13V. I can't remember what it was but it was very small - maybe 100mA.

[martin calva]

I'm working on exactly that on my 760 at the moment among doing a million other things thanks to life in general!

A 6 Ohm 50W resistor across the brake lamp terminals will stop them triggering the BFU, i'll let you know on the tail lights. (...)

Look forward to hearing how it pans out.

I was hoping to find about a solution other than fitting resistors. If I fitted resistors I'd want to mount them either on an effective heat sink or in some fashion so that their getting hot would not involve any risk of them coming into contact with plastic items.

I bought mine from Amazon. I have bought other LEDs from Classic Car Leds, also flasher relays for LEDs, for my other cars and they have been very satisfactory

[Laird Scooby]

These have no resistance fitted - I checked the current they draw on 13V. I can't remember what it was but it was very small - maybe 100mA.

That means they have a resistance of ~130 Ohms (E/I=R) instead of the 5/12A (I=P/E) that 5W filament bulbs have - again E/I=R 12/0.4166666 (5/12 in decimal) = 28.8 Ohms.
Two ways of doing it, one is to use (R1 x R2)/(R1 + R2)=Rx and knowing R1 to be ~130 Ohms and Rx=28.8, the shunt resistor can be found.
An alternative way is to use Kirchoffs Law and make the currents add up ~100mA in one leg (LED bulb) and that means the second leg needs 316.6666mA so 12/0.31666666=37.9 Ohms, the NPV for this is 39 Ohms.

However, this is hypothetical because you're saying it was about 100mA so we dont know that base figure to start with.

Look forward to hearing how it pans out.

I was hoping to find about a solution other than fitting resistors. If I fitted resistors I'd want to mount them either on an effective heat sink or in some fashion so that their getting hot would not involve any risk of them coming into contact with plastic items.

I bought mine from Amazon. I have bought other LEDs from Classic Car Leds, also flasher relays for LEDs, for my other cars and they have been very satisfactory

The 50W resistors i linked to above won't need a separate heatsink, just mounting on something metal with some heatsink compound. They'll only dissipate ~20W each anyway.

The secondary resistors for the tail lights, assuming those figures above (which we know may not be correct) would need to be ~4W (12 x 0.316666666) and most resistors of that wattage are fine with simple air cooling.

Again, the figures may still be slightly wrong as, if memory serves, the number plate lights are also fed from the LH tail light feed, the high level brake light is fed from the LH brake light feed as well, just to add some more confusion to the mix!

[Laird Scooby]

I managed to find time today to rig up my PSU (13.8V) and multimeter and test the LED stop/tail bulbs (as linked to further up) and can confirm the following :

Stop - 155mA
Tail - 40mA

As a normal 5W tail lamp bulb uses 416.6666mA, that leaves 376.6666mA to spoof the BFU with. Using 12V and 375mA, it works out to be 32 Ohms, NPV for this is 33 Ohms and would need to be 5W rating.

https://www.ebay.co.uk/itm/264670791241

A pair of those (one for each tail light) would work well.

Double checking the workings for the stop lamp (21W) it takes 1.75A @ 12V and the stop section of the LED takes 155mA leaving 1.595A to spoof the BFU which works out to 7.5 Ohms. The 6 Ohm 50W resistor linked to above is close enough in this case, even a 9 Ohm resistor would work well as i think the ones i already have are.

[Rversteeg]

LEDs, CC/ABS

Modern CAN bus systems detect if a bulb draws a higher or lower than normal current, within certain limits. For that same reason you can no longer just piggyback the trailer lighting to the rear lights of your car.
The Volvo defective bulb system works more primitive and just compares the currents to the OS and NS of the vehicle. Any imbalance will trigger the indicator.
Using LEDs should therfore be no problem and works fine on my 940, but there is another issue playing up. The cruise control and ABS system both send a 5V signal through the brake light bulbs to ground. If the brakes are operated, this 5V signal is "pushed back" by the 12V feed coming from the brake pedal switch, signalling the systems brakes are applied.
Some LEDs are so sensitive that they already light up because of this 5V signal. They can also block the 5V signal from reaching ground, causing the CC and/or ABS not to work. For those reasons I have maintained the bulb in my third braking light. ABS and CC still work, but due to the imbalance the defective bulb warning lights up when I brake.
Replacing the third brake light by a LED will restore the balance OS/NS again, but then I would need to also place a resistor to allow the 5V signal to reach ground when not braking to keep the CC/ABS alive. This resistor (or hidden bulb) has to be placed between brake pedal and defective bulb warning relay so it does not disturbe the balance OS/NS. I still have not bothered to do so.
Rob

[martin calva]

(...)
The Volvo defective bulb system works more primitive and just compares the currents to the OS and NS of the vehicle. Any imbalance will trigger the indicator.
Using LEDs should therfore be no problem and works fine on my 940, but there is another issue playing up. The cruise control and ABS system both send a 5V signal through the brake light bulbs to ground. If the brakes are operated, this 5V signal is "pushed back" by the 12V feed coming from the brake pedal switch, signalling the systems brakes are applied. (...)
Rob

That's interesting. A couple of years ago I fitted LED rear lights to my 1994 940 with no apparent problem. The "defective bulb" detector does not notice. The car has ABS but not cruise control so, based on what Rob says, I need to check if ABS is operating correctly.

I was a bit surprised that the V90 complains about LEDs via its "defective bulb" indicator. I had imagined that, so long as the two sides were balanced, it would not notice.

I think I'll go ahead and wire a 6.8 ohm 25W resistor in parallel with each stop lamp and a 27 ohm 10W resistor in parallel with each rear lamp. I don't like the idea of potentially hot resistors hanging in the wiring so I'll mount them on small aluminium plates to act as heat sinks and fix the plates somewhere out of the way.

Dave has given precise calculations but I imagine that using "preferred value" resistors close to the theoretic ideal will suffice to stop the system from complaining.

My thanks to all for the information and advice.
Martin

[Rversteeg]

Rear lights

My defective bulb indicator doesn't complain about the rear LED lights either, just on the brake lights. Which makes sense, as it is balanced on a single bulb on one side and a double bulb on the other. With all three bulbs replaced by LEDs the equilibrium is there again, but not with the third brake light still being a normal bulb.

That's interesting. A couple of years ago I fitted LED rear lights to my 1994 940 with no apparent problem. The "defective bulb" detector does not notice. The car has ABS but not cruise control so, based on what Rob says, I need to check if ABS is operating correctly.

Apparently not all LEDs are the same and some do let 5V pass without lighting up.

I was a bit surprised that the V90 complains about LEDs via its "defective bulb" indicator. I had imagined that, so long as the two sides were balanced, it would not notice.

Theoretically it should not complain, as the equilibrium is not disturbed. It could be that the ground or feed connection on one of the LEDs is less, which in combination with the much lower current apparently tips the relay over.

[martin calva]

(...)

Theoretically it should not complain, as the equilibrium is not disturbed. It could be that the ground or feed connection on one of the LEDs is less, which in combination with the much lower current apparently tips the relay over.

Next time I look at the car (it's garaged a couple of miles away and it does not get used much, being my "collector's car of the future") I will remove the LEDs altogether and see what it thinks about that.

[Laird Scooby]

The main reson the BFU complains is because there is an imbalance caused because the number plate lights run off (i believe) the LH tail light feed so draw more than the right hand side even with the "balancing" taken into account.

What you say is right, as long as the original balance is maintained (easier on the brake lights as the left and centre form one leg, the right forms the other leg of a magnetic coil circuit around a reed switch, configured so one magnetic fieled cancels the other when they are equal and both side draw the correct current) that the BFU shouldn't react but sometimes Vovo have thrown us a curved ball.

Currently (pardon the pun!) having fun with my Jag S Type rear lights since adding LED stop/tail with CANBus resistors to fool the +ve earth system on the brake lights. By rights it's not officially CANBus but SCPBus, Jaguars name for it. No problems until the lights are on then it insists both brake lights have failed! :err:

Think i'll have to load them with some 50W resistors.

Meanwhile for the 760 i've ordered a pair of 33 Ohm 5W resistors to experiment with LED bulbs on it.