Flasher problems

DavidThompson · Posted: Wed Jan 19, 2022 3:41 pm Post subject: Flasher problems

Having fitted bulbs on all four corners, wired up so that all four wires arrive behind the dashboard, I have been testing the proposed arrangement before final fitting of the flasher and switch.
The bulbs are all 21w 12v and the flasher unit is marked 2 x 21w + 1 x 5w (for the dashboard warning light).
When connected up the bulbs flash very rapidly, but if I connect up the two front ones (half a hazard) they flash normally. Connecting front and back on either side they flash rapidly.
I don't understand why this should be so as the bulbs are all the same capacity.
Can anyone tell me where I'm going wrong?

Nicholas Simons · Posted: Wed Jan 19, 2022 7:04 pm Post subject:

Is this a new installation or are there previous wires already fitted to the car for indicators? The wiring for flashers is straightforward. Supply goes to the flasher unit, then from there to the two way switch, and from each side of that goes to the pair of flasher bulbs on each side of the car, wired in parallel.

DavidThompson · Posted: Wed Jan 19, 2022 9:29 pm Post subject:

It is a new installation, wired exactly as you describe. The problem is that the flash frequency is very rapid.

Charles R Simmers · Joined: 19 Apr 2000 Posts: 117 Location: Arizona, USA

Are any of the new lamps LED? Or all are incandescent?

John Murch · Posted: Thu Jan 20, 2022 6:33 am Post subject:

I have experienced the same problem in the past. It was caused by poor earthing at one of the lights.

DavidThompson · Posted: Thu Jan 20, 2022 10:15 am Post subject:

They are filament bulbs, not LED's.
I tried eliminating a possible poor earth by wiring direct to the earth (positive) side of the battery, but no change.
The interesting thing is, that if I wire up the two front bulbs (as if they were back and front) it works fine.
This suggests the fault lies somewhere in the rear units or wiring, but is all brand new and fitted in the last couple of days.
Testing so far does not reveal any fault, and it's very simple anyway.
When I get chance today to get in the garage, I will eliminate the new wiring and see what happens.

Thanks for your replies so far.

John Murch · Posted: Thu Jan 20, 2022 10:33 am Post subject:

How annoying, may I suggest drawing a wiring diagram - not what you think has been done, but from inspection of what is actually there. You may find that somehow something is shorting.

DavidThompson · Posted: Thu Jan 20, 2022 4:52 pm Post subject:

Having eliminated the possibility of issues arising from the wiring, earth, bulb holders, I wired in a bayonet type bulb to replace the festoon bulb at the back. Hey Presto it worked!
I then measured the resistance of the two bulbs and discovered the festoon gave a value of 48 ohms, whilst the bulb that worked gave a value of 0.7 ohms. Both bulbs are marked 12v 21w. Both rear festoon bulbs gave the same readings.
Now out of my electrical depth. No idea why one type should be more resistant than the other.
The solution lies in binning the rear units and fitting others allowing me to avoid the festoon bulbs.
However, hopefully on here someone will come along and explain all.

Nicholas Simons · Posted: Thu Jan 20, 2022 5:06 pm Post subject:

You have some weird bulbs. Use the equations V = I.R, and W = V.I.
A 21 Watt bulb on 12 Volts will take 1.75 Amps. A bulb on 12 V needs a resistance of 6.86 Ohms to give 1.75 Amps.
Of course, the cold resistance will change when hot. Your festoon bulb is clearly not rated at 21 Watt, unless it is designed for a much higher voltage.

DavidThompson · Posted: Fri Jan 21, 2022 1:52 pm Post subject:

Thank you Nicholas. Very helpful.
The 0.7 ohm reading should be 7. This is just due to the scale I was using on my multimeter. Of course, this makes the festoon bulb 480 ohm. As you point out, very weird.
Today I replaced the festoon bulbs with new ones, and it all now works as it should.
A bit miffed that I have spent the better part of two days tip toeing through electrics that I don't fully understand, only to discover the problem lies with the Chinese bulbs supplied in the amber holders by Car Builder Solutions.