Okay I'm at my wit's end. I've gone through every forum post I can find on the internet and I swear my car has a demon somewhere. First off, the problem is that the coolant fan isn't turning on. I had this trouble a while back but I fixed it by putting a new CTS (coolant temperature sensor) in. This car has an ECM controlled fan, so that time the CTS just went out. The fan wouldn't turn on again about 8 months later, however, this time I replaced the CTS and nothing changed. So started the long and arduous process...

Here are the steps I've taken:
1.) Replaced CTS - no change
2.) Tested fan relay - relay is functional
3.) Jumpered power across relay pins directly to fan - fan turns on!
4.) Disconnected ECM and grounded fan relay control wire (E8) to the chassis - fan turns on!
5.) Bought an ALDL to USB cable and checked ECM - CTS and everything reading out normally except Fan listed as "ON" when really off
6.) Jumpered CTS connector to read max temperature with key on - Fan still didn't turn
7.) Replaced ECM and swapped in the old (stock) MEMCAL and PROM - No change
8.) Jumpered CTS again and tried with new ECM - fan still not running
9.) Checked all ECM grounds for continuity with chassis

The only thing I can really think of is that the MEMCAL and PROM are bad (how?) and/or there is a bad ground to the ECM maybe?

Please, any insight into this issue would be a lifesaver. I know I could just wire a switch but I'm selling this car in a year or so and want it to work perfectly when I do sell it. Thanks!

 

What temp is your gauge reading. Mine goes almost to 220 before my fan comes on.

 

The gauge is hard to read because it's log-scale, but the CTS reading from the ECM scan tool showed ~235 with the fan off before I turned off the car. The dash gauge was hovering around 220ish.

 

 

Finally figured it out! The replacement ECM I got from Advance was defective. Put in a NAPA 7730 ECM and that fixed all my issues! The coolant fan comes on now when the ECM and CTS read ~228 F