Flashing indicator woes

[Alan]

A friend has a 34 tourer which he has fitted with flashing indicators. They work well when the car is standing still, but as soon as the engine is switched on they flash at twice the rate. We have tried all we know. New flasher units, suppressed plug leads, verified the dynamo voltage, even mounted the flasher unit in a faraday cage (tobacco tin).  Nothing works. Anybody got any ideas?

["Slack Alice" Simon]

A large value capacitor - maybe an electrolytic one - connected to the power lead of the flasher module and the nearest earth point?

[John Cornforth]

Hi Alan

It's almost certainly caused by voltage spikes coming from the ignition system, which modern solid-state flashers take exception to. Modern "inductive" suppressed ignition cable doesn't work very well on its own at the lower frequencies involved as it doesn't have much resistance. I suspect it's really intended for use in conjunction with plugs already having built in suppression resistors. For Austin Sevens I'd recommend keeping the copper cored leads and adding 5000 ohm suppressor caps at the plug ends. This will considerably reduce the spike energy at source.

If this still doesn't work because the noise is being conveyed along the wiring, then "Slack Alice" Simon's suggestion is a good one. Traditional capacitors for this role were something like 0.5 microfarad in value. You can still buy ones of 2.2 microfarads with a spade terminal to bolt to ground and a short flying lead. I'd suggest you leave on the suppressor caps and try adding the capacitor next to the flasher unit.

[joe]

Bet your using 6v, if so it’s not likely to work well, 12v is best. Step up units are available at about £10 each.
Joe

[Dave Mann]

I'm sure I know the car and yes it's 6 volt, I also know of another with a similar problem. What you need is an old non electronic flasher unit from the 1980s, they are bomb proof.

[John Mason]

You can get them from AES for 6 volt neg earth.

John Mason

[Reckless Rat]

If it's one of those 6v/12v electronic units with a built in buzzer (sold on t'Bay of E) they don't work very well on 6 volts or with LED bulbs (don't ask me how I know that)

[John Mason]

Go on Reckers  tell us how you know. On second thoughts you don’t need to bother they are probably the same  or similar things that we didn’t know at the time but now know now and right off as experience.

John Mason

[David Stepney]

I had some difficulty when I first fitted flashing indicators to the RP. The electronic 6v flasher that I got off flashed at a prodigious rate and was very voltage sensitive. I eventually got a flasher unit from Paul Beck. It wasn’t cheap, at about £17 including postage and packing, but it cured the problem. It flashes at about 90 beats a minute and doesn’t vary with the voltage.
I should perhaps point out that I am using incandescent bulbs rather than LEDs. LEDs seem to be a lot of bother judging by these annals.

[Alan]

Thanks all, still working on it but I think you have pointed us to the answer. For the record it is the car Dave is thinking of, it’s 6v and on conventional bulbs.