GU TD42 cab chassis turbo timer

What brand etc of Turbo Timer is it?

Wazza, I suspect you have picked up an unsuitable feed source for the timer, as it is interferring with the control module that ECU's the sub-fuel pump. You probably thought you had a patrol without a computor.Wrong, the sub-fuel switching has it's own dedicated ECU had will throw a wobbly if you induce an incorrect voltage through it, ie when welding without disconnecting the battery. You need to POSITIVE, that you have tapped into the fuel shut off solenoid by determining the colour code of that wire(can't tell you off-hand). Source that wire and connect to that for the timer. That should do the job.Cheers