Solution for new cooling fan issue

[Bender-san]

I searched through the various cooling fan topics and couldn't come up with the particular issue I was having, so I thought I would post this as a solution in case anyone else finds themselves with this particular problem. As this has to do with the ECM, I posted it in the Engine section as opposed to Electrical. Mods feel free to move this if it seems it should be elsewhere.

The 1993 Legacy wagon I purchased had exhibited a particular cooling fan issue that I had not come across on a Subaru. When the vehicle would reach hot operating temperature, the fans wouldn't ever come on unless the A/C was used. There were no codes for cooling system or fan operation in the ECU. The coolant temperature sensor had been replaced, but I double checked and verified proper operation of the entire coolant temp circut. I found that when I grounded the cooling fan main relay circut at the ECU (F47, pin 17 at ECU; red/blue wire) that the fans would operate properly. Since I had never heard of an ECU internal failure issue with the 1st generation Legacy, I thought I'd better borrow a known good ECU to check the interal circut. It turns out I didn't even need to get that in-depth. With the new ECU the fans operated properly, so I connected the green check-connector to see if the fans would even come on under a self-check...they didn't. It turns out that the fan main relay ground isn't operating properly inside the ECU (possibly a cracked solder?). Now this legacy has fairly high mileage (340,000), so I'm not certain if it was a longetivity issue or something else particular to this car. However, with a different ECU I now have proper fan operation. Hopefully this may help someone stuck in a similar situation sometime in the future.

[Legacy777]

Thanks for sharing your experience. Yeah sounds like you got some miles out of that Legacy.

[Bender-san]

I haven't put more than 500 on it, but the previous (and original) owner averaged 20,000 per year for 17 years.

[Bender-san]

UPDATE!!!

Today, during some testing, my fans came on...with the old ECU in the car. I then installed the known good ECU and the fans wouldn't come on unless the main fan relay control circuit was grounded separately. Upon further inspection I found the ECU harness terminal at F47, pin 17 to have almost zero pin tension. So the issue on my car was NOT with the ECU itself, but with the ECU harness terminal. After some difficulty removing the terminal from the ECU connector, I found it was actually simple to pry the tension spring in the terminal back into place, so a new terminal was not required. I hate pin tension issues...makes a bear out of a simple diagnostic procedure.

[Legacy777]

Glad you got it figured out though.

[Bender-san]

So am I. Coincidentally, I now have a known good ECU for any further diagnostic testing I may need to do in the future (not that these things ever fail).