Hi @Brinkdog1,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Some timers can reduce pressure by 5-15% due to the internal mechanisms the water has to flow through, but clogged ones may cause a more significant drop. In your case, the pressure reduction is enough to stop the sprinklers from functioning, which suggests either a notable restriction or that your flow rate before the addition of the timer was right on the limit of what would allow the system to run as intended.
Is it this Holman QuikDial 4x Outlet Tap Timer that you've used? If not, please let me know the specific model as it will help narrow down our troubleshooting efforts.
This timer has a max flow rate of 25L/min which you can use Holman's DIY Flow Rate Test with Bucket Calculator to check. Assess the flow rate once with the timer attached and again with it removed to gain insight into what is going on.
If you find the timer is allowing the stated 25L/min flow rate, and this is not enough for your system, then you may need to alter the irrigation system to accommodate this flow rate or remove the timer from the setup altogether.
Let me know if you have any questions.
Jacob
