Cadillac Seville / Cadillac Eldorado Forum Discussion, 1999 STS - engine cooling fans problem in Past Cadillac Vehicle Discussion; there must be switched ground on the 85 terminals of the relays, that's how they engage. did you check with ...
I was able to cross the fan wiring so that the RH fan was connected to the LH wire feed and the LH fan was connected to the RH wire feed. I also did a test to confirm good connections. Each fan went on when directly connected to battery+.
When I put on the ac, the RH fan went on and the left one did not. This, to me, confirms that the RH fan is OK.
So back to square zero. Because both fans go on when ECT is unplugged, yet only one goes on when ac is on, I believe one part of the system is triggered for serial mode but something else is preventing the 1st fan from taking half the voltage and passing the other half to the other fan.