What is proper operation of electric cooling fans
 

Here is what I think is correct:
fans are 2 speed: high and low
fan speed is variable, dependent on engine temperature
when AC is engaged, fans are on high (only high speed?)
if necessary, fans will run on after key is off, on low

just replaced my fans and while they "work" and there are clearly 2 speeds,
both low and high do not seem to be "fast" enough. I understand the amplifier
can be problematic, but how can I determine if my fans are running at ample RPMs?

New or used fans? Can you compare to a car that is known to be working properly? You can swap "amps" and see if the cars trade behaviors.

Workshop manual has a good description of operation.

There are pressure and temperature vs fan speed charts that show the contributors to fan operation. Start there. If the fans will run on high speed with the AC on, they are running correctly. The temp sensor in the radiator, bottom-left-forward in the end tank, does teh same thing, varying speed based on water temp returning to the engine. There's an aux function that cools the intake when it's in a hot-soak condition, fan on low speed, switch on the top of the manifold. Auto gearbox cars have the same slow-speed fan when ATF is too hot. Switch is in the banjo fitting on the right side forward and low on the gearbox.

Here are a couple of pages on S4+ cooling fan theory of operation from the 1987 Service info book.

https://webfiles.uci.edu/redwards/pu...%204-19-18.pdf

Thanks for advice and input! I have more to add.....

Capt Slow, fans are new OEM Porsche units.

here is what I have determined: fans work properly without AC on. As soon as I turn on the AC
the passenger side fan shuts down. Shut the AC off, and there is a delay (maybe 5 minutes?) before the pass fan will come
back on, but it does return to proper function. Turn AC on, and same result???

Amplifier, or control unit?? or something else???

Sounds like the high speed circuit for the passeger fan is bad, inside the amplifier.

I have a test bench for both the amplifier and the control unit, if you need them tested.

I also stock and sell rebuilt amplifiers.

thanks Greg. If AC is off, and coolant temp gets hot, will fans go to high speed?? And with my issue,
will the RH fan shut down as if the AC is on?? Trying to determine if there are separate circuits within the amp
that control the fans differently with or without AC on: wondering if it is possible for the RH fan to work
properly in high speed with AC off???

AND YET MORE: lol I let the car cool down, started the car, and put the AC on. Both fans worked for about 10 minutes.
After that, the passenger fan shut down and the drivers fan revved up to a higher speed. So.....

with the AC turned on, both fans initially come on, but NOT at high speed. when AC is on, should the fans work at both
speeds, or only on high?

since they both come on at low, is there a default within the amp that forces both fans to operate at the same speed?

and......after waiting a bit, both fans work: does that mean there is a thermal or time delay within the amp? or is there
any other sensor that might create this situation?

System actually has more than two speeds...it is way more complex than most people realize.

I'll get Jake to explain how it works.

OK, here we go!

(this is not counting the flap control)

The system has 4 inputs for control
Coolant temp
Freon pressure
AC on
IAT/Trans temp

at normal operating coolant temp, the fans are on "50" percent
at normal operating coolant temp with AC on, LOW freon pressure, the fans go on "80" percent
at normal operating coolant temp, AC on, HIGH freon pressure,
at low coolant temp, IAT/Trans HOT, fans are at "50"
at low coolant temp, AC on LOW freon , no fans
at low coolant temp, AC on , High Freon pressure "80"percent fans
High coolant temp, AC on or OFF, FULL fans

The amp has a "check" mode for the fans. if one fan goes out it senses that add make the good fan go full tilt. every min or so, the amp "checks" to see if the bad fan will work. you can hear this by the good working fan slow down, it checks, then spins the good fan full bore again if the bad fan does not work.

Jake,

Thanks for that. It explains something I was told many moons ago in that the stock system does not run both fans at full speed but it does run one flat out when only one fan is available. I had this happen a few weeks ago in that on my Spal twin fan set I heard what seemed to be an "unusual" fan noise after a run when the motor was still running after I parked up. Did a bit of checking and found one of the fans was out- one of the Spal supplied connectors had failed.

Do you happen to know what the coolant temperature and freon pressure set points are?

The other point of note is that the item on the radiator is a temperature transmitter so one wonders why the controller does not have an option to run both fans flat out at a "high high" coolant temperature.

great stuff! starting to grasp the system with all this information not as "simple" as I thought.

coolant temp sensor works between 1000-4000ohms.
60C = 3862 ohm +/- 150 ohm
85C = 1582 ohm +/- 54 ohm
100C = 967ohm +/- 36 ohm

Freon switch is 20-150ohm range
1.5 Bar = 22 ohm +/- 4 ohm
5 bar = 53 ohm +/- 4 ohm
10 bar = 92 ohm +/- 5 ohm
15 bar = 125 ohm +/- 5 ohm

intake temp switch
> 87.5C = 0 ohm
< 82.5C = infinite

auto trans temp switch
> 110C = 0 ohm
< 105C = infinite

You missed one.

Third one down:

From the rest of the list, I'm guessing this is supposed to be "FULL fans", but it would be nice to know for sure.