LED turn flasher lamps

A 12 volt LED lamp sold for vehicle use should already incorporate a suitable current limiting resistor, or an electronic regulator circuit.

You dont need to add a series resistor to a ready made lamp. You only need to add a series resistor to a home made array of bare LEDs, not to a ready made lamp.

The current drawn by a ready built LED lamp, or by a home made array will be much less than that drawn by the originally intended incandescent lamp. It may be insufficient to correctly operate the flasher unit.

A PARALEL resistor may need to be added across the lamp in order to increase the total current to a figure sufficient to work the flasher. These may be obtained from dealers in vehicle electrical parts.

The old fashioned arrangement for vehicle indictors was a good one in that the flasher load was matched by the two or three indicator filament lamps. If a lamp blew, the driver would be notified by the indicators not flashing correctly, normally the remaining lamps would stay on permanently when the indicator stalk was operated. This fault would show by the dashboard repeater light.

Some old style flasher units made a satisfying (or annoying) clicking sound when in operation, thus indicating that they were working normally when called for.

Do modern electronic flasher units have a failure notification feature, or a similar one?

Car Turn signal flasher circuit explained - YouTube

Z.

Some of the more expensive electronic flashers go to great trouble to monitor the current to mimic the failure warning modes you got for free with the old designs using a bimetalic heater and strip.

 (The 2 lug ones had heater is in series with the lamps but much higher resistance, so no light, then the bi-metal strip curls and shorts out the heater, and the lamps come on, robbing the heater of power, and then the strip relaxes….) If the lamp load changes, so does the off time, so it flashes very fast or not at all - no good for trailers,. though 

(The 3 lug ones had the heater in parallel with the lamps and the bimetallic strip cut the supply to lamps and heaters. This sort was less load dependant and turned up on commercial vehicles, tractors etc )

In short you may or may not need to add load in parallel to mimic a real lamp to make it work properly.

Mike.

If you have an old style flasher unit (it sounds like it) you will need the load resistor, and it will be big and get hot. The usual load is 2 x 21W and 6W which take about 4 Amps so a 3.3 Ohm 50 W resistor will be needed. This should work normally, either electronic or bimetal flasher. Ignore the led current (a few hundred mA).

Fit an LED flasher unit.

Note that strictly speaking, depending upon the age of the vehicle, the replacement lamps may not have a wattage which complies with the Road Vehicles Lighting Regulations 1989.

Chris has an interesting point. If the replacement lamps are not compliant, your vehicle may fail it’s next MOT. 
 

Regards,

Alan. 

It does sound as if the LED doesn't draw enough current to satisfy the lamp failure unit.

Going the other way this why they fit an extra “ticker” when you wire up a trailer socket.  With three bulbs the failure unit can't tell when one fails.

Alan Capon: 
Chris has an interesting point. If the replacement lamps are not compliant, your vehicle may fail it’s next MOT. 

I doubt it. I cannot imagine a tester either removing a lamp to see its wattage, or measuring the consumption. More likely would be an inadequate headlight beam because it is impossible to put all the leds at the focal point.

Thanks for all the advice given your all ?
I've changed almost all lamps to LED apart from the H4 as I'm still trying to find a suitable small but powerful lamp. 

Beware also of the generation of vehicles designed very roughly 1990s to 2000s. In my limited personal knowledge Volvos & Mercedes. The flashers are electronically controlled by an ECU, no flasher unit, and pretty much current insensitive. But the (I forget the exact term) smart MOSFETs used as drivers have monitoring built in for O/C, low current, S/C. So your LED bulb (sans shunt) works perfectly. But the dash shows a fault code. Annoying, masks real faults, and I suspect an MOT fail. 

On my MB Vito I substituted one number plate bulb (5W) for an LED in order to squeeze a camera alongside. Worked but fault reported. Had to shunt it, accurately. And yes, even a 5W shunt required careful placement to avoid overheating (I had to make a void in the motorhome insulation).

I worked on the Volvos & Astons - later versions may have garage settable options to tell them that LEDs are fitted and suppress specific fault detection/reporting. Don't know of other brands.

HTH