Powerdynamo, purpose of the relay

Powerdynamo brings new ignition & light
to your vintage motorcycle



Technical Help



What is the relay in many Powerdynamo Systems for? Do i need it?

Starting to say that he who fits the relay but has no battery in the system will probably never get spark. Same for those who use a standard vehicle relay without pin 87a.

We talk of this little fellow here.  
And please note that though this is a vehicle relay, it is still special as it has a  closed-circuit pin 87a (the one in the middle), most standard relays do not have). 

To explain the purpose of the part, a little excursion into ignition:

A battery based system is switched on by bringing voltage to the ignition coil and switched off by taking that voltage off again. 

The main switch in such systems therefore has a position which, with engine off opens a bridge between 2 terminals (depriving the ignition coil of voltage) and closes the bridge when switched on  (giving way for the voltage to flow to the ignition coil).

A magneto based system is switched off by short-circuiting the voltage the magneto brings to the ignition coil and switched on by eliminating this shortcircuit.

The main switch therefore has a position which, with engine off closes a bridge between 2 terminals (short-circuting the magneto) and opens it when switched on.


Most Powerdynamo ignition Systems are magneto based. They need the arrangement as explained above for magnetos. As long as the stock system had been magneto this is fine, the main switch will function as it did before.

If you had a battery based system however that your switch will not normally sport a facility for magneto switch off (some BMW singles have both, but this is an exception).

In order to be able to use the stock switch and the original switch ON/OFF method the relay comes to work, inverting the switch function from the Run with closed to Run with open connection.
Pins 86 and 87a are to ground. Applying voltage to pin 85 will make the relay contact switch the status of pin 30 from position 87a to 87. That is from ground contact to nothing. With Pin 30 open, the system can run. 

Taking voltage off pin 85 (by switching main switch off) makes the relay snap back and close  pin 87a (ground) against pin 30 which via the blue wire is connected to the CDI, short-circuiting it.

A typical Powerdynamo wiring diagram with relay would look something like this here

The whole thing surely only works with a battery present. How else could you activate the relay to give way to ignition? If you drive without battery you will have to skip the relay and connect the blue wire to some manual switch. Say a push button one at the handlebar.

Alternatively misuse the headlight flasher knob to do the job or any other switch. You might use e.g. the terminal provided on many old main switches for pushstart

Please note, the blue wire will carry some 300 volts (at very low amperage) which, even if not dangerous is quite painful.

A typical Powerdynamo wiring diagram would look something like this here

It can be, that you can use the rely option even without battery. This depends whether your engine (good kick ratio) and your physical ability to kickstart rather rapidly permit using the relay without battery.

You need to test this however individually, we can not tell you from  afar. For this, just wire the relay as instructed, no battery. Make sure that the relay with its pin85 (black wire) is connected to your main switch at a terminal that normally has voltage when switch is on.

Switch on and kick. Might be generator produces enough voltage during kick to operate the relay. if so, than you may use the relay even without battery.

But also check that there is enough voltage for the relay at idling with headlights, blinker and stoplight on. IF there is too little, the relay would close, engine would stall. Test this.

Here mistakes customers make again and again which lead to non-function of the ignition
  • relay is fitted, but no battery is present
  • a standard vehicle relay without pin 87a is used. It has to be a relay with a pin 87a, that is the pin in the middle of the relay bottom. Should there not be a pin there, the relay can not be used in PD systems as switch off relay.

Please take note, that the relay we supply is not a standard vehicle relay as it has this pin 87a - closing during non-activity - which most vehicle relays do not have.