I've already made a few comments on this already. But I'll make more of a list of findings so far. In hopes others have ideas or ideas to add:
The fan is a Delta FFB0412UHN-SP4Z (
FFB0412UHN-SP4Z) Which has the 4 pin wire info.
The connector on the board looks like a 4 pin JST connector. I did not want to bother trying to find the right one. So instead I cut the wires from the board to the fan and inserted normal PC fan headers.
The pin out is not the normal PC fan layout. The normal 12v PWM pin out is this:
https://noctua.at/pub/media/wysiwyg/faqs/noctua_pin_configuration_12v_fans.png
Delta uses different colours. Which can be seen in the PDF for the fan or here:
Pulse-Width Modulation - PWM - Technology
So I swapped the wires around to the normal PC Fan PWM pinout. And did the reverse on the original fan. So i could always just plug the old fan back in.
Wire swap to convert to a normal header:
EC200a - Fuction - Noctua colours
Black -> Ground -> Black
Yellow -> PWM Signal -> Blue
Blue -> PWM Tach -> Green
Red -> +12v Power -> Yellow
So with a bit of soldering and/or adding new headers. You get a normal PWM header coming off the board. But if you then plug in another fan. iLO will complain and say "No Fan is connected" preventing the system from booting.
My guess is iLO checks the PWM tach to get a RPM from the fan. And since the Delta fan spins a a lot higher RPM than any fan we would want to replace it with. iLO sees that low RPM as a fan simply not connected.
What I want to try to do, is sending a fake PWM tach back to iLO. Which I think can be done fairly simple with a PWM Signal Pulse Generator (This or similar:
Amazon.com: Pulse Generator, DROK 2pcs LCD Display PWM Frequency 1Hz-150kHz Duty Ratio 0-100 Percent Adjustable Square Wave Rectangular Wave Signal Generator Board: Home Audio & Theater) From the pdf of the original fan it seems the PWM control signal runs at a preferred 25k hz. So setting that along with a duty cycle. Might trick iLO into thinking a fan is attached. Now how this will impact PWM usage on a replacement fan. I don't know. My plan was just to run the fans at full speed or a little less., turned down by normal fan controller.
I should also add. That I don't think a drop in replacement 40mm fan is enough to cool the chip on its own. I wanted to put in a Noctua NF-A4x20 40mm (Which is not as thick as the Delta. but does fit otherwise) And cut a hole in the lid of the case. Above the headsink the 40mm is on. To then have a Noctua NF-A8 80mm blowing in fresh air on both the heatsink and help cool the drives in the box out as well.
The default setup in the EC200a is just the single (loud) fan sucking in air from the other side of the server, across the drivebays and out the case. That is ask a lot of a 40mm fan. Which is likely why it can go to 23000 RPM and ~60db.