I am completing a swap of a 86 TPI/305 From a trashed 86 Iroc into an 84 Cutlass. Everything was taken from the donor car (harness/ECM/inside connections), but the fan stays on all the time and the car won't keep running when the fan relay is connected. I also can't get the ECM to go into closed loop. I've gone thru the harness at least 10 times, and I am not getting any trouble codes. Any thoughts?

 

Hmmn, since it's a swap, it's possible you have the combination of the CTS, the temp sender for the gauge, and the sender for the 2ndry fan relay mixed up.

Closed loop requires the O2 sensor to be outputting 400+ mvs (maybe crosscounts too?), the CTS reporting back to the ECM the coolant is at 105*F.

 

Oh, I messed part of your post...also very possible that the Alt is toast too, not being able to keep up with the fan's current draw.