Power Up Voltage Test Fails on Second Step

I’m new here, so please bear with me…

I inherited a 1966 Volvo P1800S with a Spartan 2 / LSU 4.9 connected to a MaxxECU Mini. I’m not getting lambda readings, so I have started troubleshooting.

I pulled the pin for the green wire (linear output) so it’s disconnected and connected a meter to it. I get 1.65v on key on (good), but then almost exactly 5 seconds later I hear a click from my IAC valve (which is controlled by MaxxECU) and the voltage drops to zero.

Any idea what could be causing this?

Thank you.