On my 1990 Renix Inline 6 I am trying to understand the O2 sensor circuit for trouble shooting, I have the factory schematic. The O2 sensor heater relay is a single pole, double throw type as seen on sheet 20 of the schematic. One throw terminal goes to the O2 sensor heater and the diagnostic connector via an orange wire, and the other throw terminal of that relay goes to an orange/black wire that splices to the fuel pump.

So when the O2 heater relay is switched one way the power goes to the fuel pump circuit, and when that relay is switched the other way the power goes to to the O2 sensor heater.

Why is this O2 relay hooked up like that, when the O2 heater is on the relay is closed to the heater ( which makes sense) but when the relay turns the heater off, that relay is connected to the fuel pump circuit. that makes no sense to me, what am I failing to comprehend here?

Thanks!

So during WOT the heater is turned off and sensor output ignored:

thank you!

What resistance should be the O2 heater value when cold? I got the O2 heater relay removed, and measuring ohms between relay socket terminal 87a and battery ground I am getting nearly a dead short.