biturbomax

I have one easy question for us.
I have a 928 s4 '88 with flap cooling system.
I noticed that if at operating temp with the fan on i turn of the engine the fan shut off but after if i turn the key on without start the engine the fan come on.
after if i start the engine the fan shut off. Why the fan come on with the key in position on but without engine running and after with engine running shut off and operate and come on only after.

jpitman2

After shutdown you will get heat soaking out of engine core into the coolant, and this can trigger the fan sensors. If you turn the engine off, leave the key at accessory and wait, the fans might come on for a minute or so and then go off as they remove heat from the radiator as the coolant is being moved by convection. Mine does this most shutdowns.
jp 83 Euro S AT 52k

biturbomax

ok but seems to me a strange operation a strange way to work for the system.
Another question because i have some doubts on my ecu controlled coolant system.
For example today i have tried a functional test on the intake temperature switch.
with the key off i pulled off the two wires on the intake temperature switch connected with a piece of wire pressed the hood switch but my fans don't come on.
-intake switch it's functional infinite ohm with cold engine
-hodd switch it's functional infinite ohm with open hood and good continuity with hood closed or switch pressed.

biturbomax

Yestarday the intake switch worked ok but i'm quite sure only after that i had turn te key on after the switch worked always with key on and strange but i'm quite sure that before that i had turned the key the intake switch was not working after was workin ok with the key off and removed from the dashboard.
Another question in the process of testing the coolant system i have make some test on the flap and fan control uniti and on the fan output stage.
The workshop manual says on section 19-25 function-testing output stage
point 3.) plug I and plug II must be connected on the control unit, connect voltemeter to pin 3 (ground) and pin 2 (plus) at the output stage plug, with ignition on reading battery voltage.
I had performed this test and my voltemeter had read battery voltage with ignition on but after i had performed this test voltemeter had read always with ignition on and off battery voltage the battery voltage remained always present at the pin 2 and 3 of the output stage. It's normal?
Only when i had disconnected the plug I and II at the control unit the voltage at the output stage plug pin 2 and 3 returned at zero with ignition off.
It seems that the control unit have some capacity of memories or self-learning.
who has studied very well the elettronic board aof the fan and flap control unit?

SeanR

If you have the A/C turned on, the fans will run with the ignition key turned on.

dr bob

The primary battery voltage at the final stages is there regardless of key position. There are dedicated power feed wires for each fan from the battery, through fuses 28 and 29, to the final stages.

Poor connections at the battery and damage in those fuse sections of the CE panel are common causes of fan malfunction. Check for battery voltage at those two fuses --while the fans are running normally--, and you can tell a lot instantly about primary power problems like these. The fuses have places where you can stick a small meter probe in the back above each pin for the testing.