During the restoration of my 928 GT, I removed the center console and sent the climate control unit for overhaul, but I ended up installing a previously overhauled one. Now, what happens is that when I turn the ignition key, the fan comes on even though the fan speed selector is set to zero. Any suggestions on what to check? Could it be a slightly different model of control unit? Thanks.

 

Are the front panels the same, the older HVAC head units had a "0" or off for the fan, the later ones do not and the fan always runs on low.

 

Thank you a lot Dave, do you think it should be possible to modify the connections in order to have the unit working properly, or not?

 

Not sure if that can be done

Should be a part / model number on the units, also I do not know what MY it changed for Off to Low.
I do know the 86 and earlier the blower motor can be turned off.

 

There is a subtle difference between the S4/GT unit and that used in the GTS's like mine even though in my case the early GTS models came kitted out with the same system and R12 refrigerant. I think the R134 variant emerged half way through MY93 and I understand it has the same unit that was fitted to my 92 delivered GTS.
This I suspect the OP may have the GTS panel unit. There is some difference on the control buttons and when the relay went in mine initially I swapped out the GTS unit for the one I lifted from my wrecked 90S4 that was wrecked back in 2005.
If of interest to the OP I could take a look and compare the units tomorrow. Whether or not the reverse swap - a GTS unit into a GT works I have no idea but I suspect it probably will.

 

To be honest, when I turn the ignition key, I think the fan starts at maximum speed, not minimum, because the noise is noticeable 🤔

 

Put a towel under the plug for the resistor pack under the hood ,so you dont drop the cover .
pop the cover off the back of the resistor pack connector post a picture of the wire colors.
NOTE its possible the resistor pack was changed and it may not be compatible with your system
Alan may jump in to guide you.

Also post a picture of the HVAC head unit you have,
and check the CE panel relays to verify you have the correct part number relays installed,
for the blower and the HVAC system as well as the X bus.

 

First of all, thank you for your help: Here the pictures you asked me for!



On the right the previous relais I have, on the left the new ones.

 

I'm reviving this thread after doing some tests:
1) I tested all the resistors and they work perfectly.
2) I tried removing relay #14, and then the fan no longer immediately starts at full speed.
3) When reassembling the passenger seat, I mistakenly inserted the brown wire above the red one instead of the black one. At that point, NO MATTER WHAT BUTTON I PRESSED TO MOVE THE SEAT, THE FAN STARTED AT FULL SPEED ! Furthermore, the A/C control unit light also came on!
I hope someone can help me with this new information, because I'm a little depressed

Thank you in advance to anyone could help

 

FIRST disconnect the battery,
Then go look at the CE panel,
verify your putting the correct relays into the correct slots.
NOTE the relay slots have Roman letters with small arrows indicating the exact locations ,
you would not be the first person to put relays into the wrong positions.
Based on your report this is what has happened.
Fuse/ relay panel diagrams can be found at 928sRus.com,
print one off and keep it in the car.

 

I'm a little confused by "NO MATTER WHAT BUTTON I PRESSED TO MOVE THE SEAT, THE FAN STARTED AT FULL SPEED" and other descriptions.

Does the blower come on a Full speed no matter what?

Does the fan speed selector do anything?

And did you mean to write 'move the seat,' ? Because I can not fathom how the seat switch, on the Seat, could change the Fan speed.

 

You are right, but I already checked this, and the fan starts both with the original relay (53) and the newer one I have installed.

 

I try to clear better what happens: if I have the relay seated in the slot XIV, as soon as I turn the ignition key, the fan blower starts at max speed (like defrost). If I remove the relay, the fan doesnt start BUT when I pressed ANY switch of the passenger seat WITH THE WRONG ELECTRIC CONNECTION (see image) the fan blower starts at max speed, like defrost mode.
Actually I have connected correctly the passenger seat and removed the relay XIV and anything seems to work except, obviously, defrost mode