355 the cabin blower turns on by itself at maximum speed when engine is running

Try this one, just found it:

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It's in one of two places.

If you have a Bosch braking system, it's here, next to the right hand frunk latch:

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If you have a Teves braking system, it's here, closer to the cabin bulkhead/behind the Teves multicoloured ABS relays:

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(EDIT: Disregard... I see you've already found it).

 

As you've already discovered, you pull on the black clip which is saddling the yellow plug.

 

I'll just stay it. It won't help you but the entire HVAC system on these is rubbish. From the ECU, to the motors, gears, heater valve.. junk all of it. There I feel better.

 

You may need to replace IC number 2 within the ECU to fix this. I'm not 100% sure since I haven't had to reverse engineer the blower motor circuit yet on mine, but I suspect that could be the issue. It takes specialized surface mount soldering equipment and know how to replace. Image Unavailable, Please Login

 

The control panel uses a basic resistor network. It's not likely to fail. Either the driver chip in the ECU has failed or another component in the fan speed controller has failed. The guys who refurbish the fan speed controllers professionally, do mention this.

Driver chips here:

https://www.ferrarichat.com/forum/posts/144810828/

I wouldn't recommend replacing it yourself without some practice. Some folks recommend a product called "Chipquik SMD1" to help desolder the chip without special chip desolder tools. It helps distribute and hold heat in the chip legs. I've used it on smaller chip, but I'm not sure I would attempt these larger chips without special tools.

Regarding the chips, you might want to fit some heatsinks

https://www.amazon.com/dp/B07WR3S8CZ?psc=1&ref=ppx_yo2ov_dt_b_product_details

 

I have a hot air soldering station. I used a combo that and a traditional iron to get it all done. I wouldn't try it without both on hand. I agree if not familiar with this type of work you need to practice as it's a bit of an art to do properly and not do more damage. There are practice board kits pretty cheep though. I can't remember exactly since it's been so long but basically I was able to lift the IC off carefully and patiently. Then you'll need solder removing braid to clean up the board used together with an traditional soldering iron. Once everything is cleaned up nice, resolder the new chip with paste and the hot air station.

Oh you'll need to clean the potting material coating off the area first. That's a messy job.

 

Remember mine? some previous owner put an potentiometer inside the panel to control fan speed, probably it was the ECU that was at fault from the start.
I had a nightmare fixing the system but it works now, i replaced the power transistor, control panel, and repaired the ECU and also replaced the whole cable that went from the control panel to the ECU as previous owner cut it in several places to bypass the ECU for fan control.

 

Bizarre. So you know your control panel and your fan speed regulator works... sometimes.

What differences are there between engine running and engine not running? How does the HVAC ECU know the engine is running?

1) There is an orange/white wire going from the HVAC ECU to the Motronic ECU. There was a pinout for the ECU posted on the forum. I don't know how accurate it was (I noticed some errors), but pin 69 (5.2 car) was labelled "Orange White AC Turned On". To me, that only sounds one-way communication. Under hard acceleration on the F355, does the AC cut out? I know my Lotus Esprit had this feature.

2) Also, the HVAC ECU tells the Motronic ECU that the AC compressor has been commanded on (via a relay), but that communication is definitely only one way. The Motronic ECU can't send data back to the HVAC ECU in the opposite direction.

3) There is a wire coming from the ignition key telling the ECU that the car is being started. On earlier cars, there was direct wiring. On later cars with a start relay, the wiring goes via a start relay. There would have to be some sort of memory function in the HVAC ECU to

I did notice yesterday on my car that when I turned on the ignition, the fan seemed to have a mind of its own. I recall a delay before the fans started. I can't recall button settings and sequences though.

 

I had a look at the original Bosch Motronic diagram (in German) ... and the HVAC ECU internals look like this:

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Klimabereitschaft translates as "climate readiness". Looking at the symbology, it does look like the HVAC ECU is telling the Motronic ECU something and not the other way around.

The '69' is referring to the Motronic ECU pin it is attached to.

 

When i was trouble shooting my heater valve i was chasing my tail and only after car was started would it change positions.. key on engine off wasnt enough.

 

We just have to figure out how the system knows the engine is running

And can it be simulated.

 

Ouch. Expensive fix.

Thanks for the follow-up

 

Did the repair company tell you what they replaced in the ECU. My car is now doing the very same thing. The fan only changes speed on the last few speed settings.