Light bar issue

[Dk1997]

I recently installed a 40 inch hella light bar with a 40 amp relay and tapped into the high beam for trigger, strangely it works absolutely sound with the ignition on then with engine running the relay is buzzing like mad and flickering when you put highbeam on

Anyone any ideas ?

 

[mmi]

Which year Crafter?
What's the voltage between battery positive and the tapped connection when highbeam on
a) engine off
b) engine running

 

[Dk1997]

73 plate, so live going into relay is 12.4 when engine off and 14.8 when running, the trigger wire seems to drop massively when engine running, drops to 7.4 volts with highbeam on but I’ve also got after market highbeam bulbs, I changed these back to the original bulbs and it works as it should (no buzzing) I’m thinking the trigger wire can’t handle the load not sure what else I could do to get around it.