997.1 Cooling fan control unit theory of operation?

[jchapura]

I'm trying to understand the outputs of the 997.1 cooling fan control unit. I understand that the fans are a continuously variable type and run in proportion to the cooling water temperature.

 

How is the fan speed adjusted? For example, is the voltage adjusted by the control unit? Maybe 0v to get 0 RPM, to 12v to get max RPM. Or, PWM?

 

I'm interested in using this info to add a manual override to be able to run the fan at max speed by a dash switch (but maybe needing some "smarts" if PWM is involved).

 

[RFM]

The control of the fans is realized by a PWM signal via the fan control box, however the operation  is more complex, DME 7.8-CAN (high speed and comfort)-GATEWAY-AC-ENGINE/COOLANT/ENGINE ROOM/OUTSIDE temp. sensors and the board network control box are involved.

[jchapura]

Thanks RFM.

 

Since PWM is meant to (electrically) safely give the device effectively "less" voltage, or more precisely, less "time" at the nominal voltage, to get the fan the run "wide open", one could just supply it with straight/unmodulated nominal voltage - does that seem right?

 

To do this, I could tap into the 12v that's supplying (because it's got the proper current carrying capacity) (in other words, on the input side of the controller) the controller, and add my dash switch in between this tap-in point and the termination at the fan.

 

This effectively gives the fan 12v 100% of the time, whereas the normal circuit gives it 12v between 0% and 100% of the time depending on what the controller is commanding it to do.

 

[jl-c]

IF this works, I would add a diode in line between your input and the controllers output to stop any back feed of your 12v into the controllers output. This is on the (not necessarily safe) assumption that Porsche do not use this line for data transfer back to the controller.

[jchapura]

Thanks jl-c. That's a great idea.

 

Here's a different possible approach. How could I fake out the controller to think that the coolant temperature is "too high"? Could I not feed that voltage (or signal? or resistance?) into the controller and then the fans would run on high? Does anyone have some specifics on the voltage and controller terminal for the coolant temp sensor?

 

[floatingkiwi]

I don't think the algorithm uses coolant temp, only engine bay temp possibly in conjunction with speed and outside temp.

[jchapura]

15 hours ago, floatingkiwi said:

I don't think the algorithm uses coolant temp, only engine bay temp possibly in conjunction with speed and outside temp.

Howdy floatingkiwi, is it possible you're mixing up the engine bay purge fan operation with my need for information about the coolant/radiator/evaporator fan control? I'm needing info or a technique to be able to manually turn on the fan with a switch.

[floatingkiwi]

Opps so I am!! my very very bad, sorry....

[Rob001]

What I had done is drilling a hole in the right radiator, tapped thread into it and added a Mercedes temperaure sensor (85 C /185 F ) and spliced it into the fan whirring. 

 

All is working as normal except when the coolants get 85 degrees at radiator, => L +R fan full speed. 

Edited October 6, 2016 by Rob001
pictures

[jchapura]

8 hours ago, Rob001 said:

What I had done is drilling a hole in the right radiator, tapped thread into it and added a Mercedes temperaure sensor (85 C /185 F ) and spliced it into the fan whirring. 

All is working as normal except when the coolants get 85 degrees at radiator, => L +R fan full speed. 

 

 

Thanks Rob - this sounds like an idea worth pursuing. Can you draw and take a close-up pic of the MB radiator temp switch tie-in to the fan electrical supply line(s)? I want to make sure I cut/tap into the correct lines. Is your car a 997.1? I may just substitute a SPST switch in the cabin (for the MB temp switch). Did you use a diode to keep the current flowing in only one direction?

[Rob001]

997.1 just tap into the yellow/ blue wire, no diode needed.

When cut, fan goes to full speed both sides so when connected to sensor the "cut" will then at 85 C 

[Seth Cusumano]

On 10/6/2016 at 8:17 AM, Rob001 said:

What I had done is drilling a hole in the right radiator, tapped thread into it and added a Mercedes temperaure sensor (85 C /185 F ) and spliced it into the fan whirring. 

 

All is working as normal except when the coolants get 85 degrees at radiator, => L +R fan full speed. 

 

This exactly what I need. Do have a year and model of the Mercedes that the sensor originally came from?