Hey all, long time reader first time poster. I've spent hours reading and researching here but I've finally got one I just can't figure out.

Background info: 05 R52 dedicated track car. Mostly stock engine wise, gutted interior, upgraded and refreshed suspension, etc.

Got a little overzealous and overcooked a turn, going through the gravel pit and into the tire wall. By the time the impact happened I wasn't going all that fast, so I thought it was mostly just going to be body damage.



Got the car put back together with basically a new front end. Core support, radiator, hood, etc. Starts up and runs fine, but noticed the cooling fan wasn't coming on. Started the basics checking for fuses, relays, etc. All seemed good. Found a few threads about fuse F5 under the hood and that seemed likely since the power steering fan also wasn't working. That was also fine.

I ended up tracing it all the way back to the ECU. From what I can tell the ECU supplies the ground (for the fan high circuit anyways). If I ground at that pin the fan comes on high as it should. So this seems like the ECU isn't sending the ground when the temps gets high enough (I let it get up to about 210 read on the obd port with a scangauge).

The low side circuit is only powered with the ECU connected so I can't try grounding at the ECU for that side. But I was able to jumper from +12v to the low side at the relay and the fan comes on low and the power steering fan comes on, as it should.

To me this is pointing in the direction of a bad ECU, but it seems very specific that it would only fail the cooling circuit and everything else works fine? Car fires up, revs, and idles fine. Haven't tried driving yet since it's still not completely put back together.

The only other thing I can think is maybe the ECU just isn't seeing the correct input in order to turn on the fans? I'm able to read the coolant temp fine at the obd port so shouldn't that mean the ECU is seeing the temps and should be sending the signal to turn on the fans?

As far as I can tell I couldn't find any damaged wiring harness or any pinched wires or broken grounds.

Any opinions would be appreciated. Getting a new ECU is always a massive pain and not cheap so figured I'd ask around for any last ditch efforts 🤣

 

I thought I had an ECU fault when my 2004 S would crank but not start. Code said throttle body so I bought a new one and it didn't change anything. Turned out a wire in the bundle from the ECU plug had become pinched somehow and the insulation broke letting moisture in. The wire was corroded and not triggering the main relay. But in your case, it sounds like the harness is good since you can trigger the fans by grounding the pin socket in the plug. I found a place called The ECU Pro that can check/repair your ECU for about $300. I was going to go that route until I found my problem. Certainly cheaper than a new ECU and they can tell you what was wrong so you're not left guessing.

 

Thank you for the information!

Turns out I did all this troubleshooting for nothing. The fan doesn't come on until 221 for the low side (ridiculously high in my opinion), and I had only let it get up to 210. After I had given up for the day, I was doing some further research and found that tidbit of information. Let the car get that hot and everything worked as it should.