1987 928 S4 rad fans
 

1987 S4:

Making some changes to console, and I wanted to add a switch to manually activate radiator fans. (Separate from rad temp sensor)

What is the best location to connect the power wire from the switch ??
- to the temp sensor in the radiator ??
- to the relay in the fuse block ??
- or ???
.

One or both?

You could simply tie into the AC input for the fans and that would get you 1 fan on high with that switch.

The S4 and later models have their fans powered via a PWM module that is in effect a high speed solid state switch. Power to the module is direct from the battery and there is no relay in the system [as I recall]. The module takes its control signal from the ac control unit in the cabin which is the unit the radiator sensor unit sends its signal to and the PWM module then switches the fans rapidly to simulate the output fan speed the control system wants. If the system is healthy both outputs will be modulating as required however if the system detects that one fan is faulty it automatically drives the healthy fan at full speed. For its timeline the system was rather sophisticated.

What is not so clear [to numb nuts me at least} from your post is what exactly do you want to achieve as I can read your post in different ways viz:
1. Switch the fans manually completely independent from the stock control system [I suspect this is your intent].
2. Keep the existing system functioning as normal but with some kind of manual override capability so they run at full speed as and when you want
3. Control the fans directly using the radiator sensor independent of the current control system or any other concept you may have in mind.

Perhaps you can explain in more detail what functionality you want to achieve over and above what the stock system is doing which is pretty comprehensive and [I suspect] nigh on impossible to improve upon.

On my 81, I out a manual switch in the ashtray, and hooked the fans up via a relay I mounted at the front of the car. The switch was powered by my swicdaet fuse box and it sent a 12v trigger signal to the front mounted relay. I supplied power to that relay from the original wiring that went to the fans and then ran a new wire from the relay to the fans that would power the fans when the relay picked.

With this setup I could run the fans when sitting at a drive thru or in traffic or when I turned on the AC to make it more efficient.by moving more air over the condenser

see post #244 here - https://rennlist.com/forums/928-foru...o-5-speed.html

When i had controller and electrical issues miles from home on a roadtrip, I put in temporary fan control via a relay take off the hot post, triggered when I switched the side lights on. Worked a treat, saved my car from boiling over in stationary summer traffic until I got home as was able to swap out fan motors

Thank you for the replies.

Permit me to clarify:

Keep the existing system functioning as normal but with some kind of manual override capability so they run at full speed as and when you want

If you unplug one of the connectors to the silver controller near the passenger door sill-
it will run both fans on high

A 928 has no controls that only run one fan.
The two fans always work together.
If one fan runs, there is something wrong: Fuse, fan, or amplifier.

However, I would agree that simply "interrupting" one of the inputs to the control unit, inside the car, would be the easiest way to add a switch.
And the A/C temperature switch is as good a place as any other.

I would interrupt the input or output to the radiator temperature switch.

I believe that the best place to manually turn on the fans is with the radiator temperature "switch" wiring. If you use the AC compressor circuit, you have to account for complexities with various operating scenarios with AC on/off while in manual fan mode.

The coolant temperature sensor located on the lower front left of the radiator is (thanks to Fred's input) a variable resistive sensor. It tells the coolant fan and flap unit when the fans should be turned on and by how much. It connects the signal line to ground when the fans should be on.

So, to summarize:

1. Cut the Brown/Green wire running from the radiator temperature switch to the coolant fan flap unit (or pull it off of the temperature switch and create a mini-wiring harness to connect to the wire and the temperature switch).
   -I'll call the side of the wire going to the switch the "original wire"
   -I'll call the side of the wire going to the control unit the "control wire"
2. Use a SPDT (Single Pole Double Throw) toggle switch:
   -Connect the single pole "input" (which you will be using as an "output") to the "control wire"
   -Connect one of the double throw terminals to any convenient ground but running through a 900 ohm resistor.
   -Connect the remaining double throw terminal to the "original wire".

If you don't like a toggle switch, there are push button style SPDT switches as well. And even some fancy touch pad style buttons, wireless controls, etc. Whatever suits your aesthetics and tastes.

When the toggle switch is in one position, it will connect the temperature switch to the fan controller and operate as normal. When it is in the other position, it will ground the wire and signal the fan controller that the fans should be turned on.

Obviously, test this before cutting wires. Just pull the connections off of the temperature switch, identify the brown/green wire and briefly short it to ground (with ignition on and preferably thought the resistor) to confirm that it does signal the fans to turn on as expected.

It is a transmitter- variable output needed to position the fans as per factory settings.

After looking in the service manual for the specs and testing, it looks like you're right. The coolant sensor for the radiator is, in fact, a variable resistive sensor.

The intake manifold temperature sensor, though, IS a switch. So, that would be the place to tie into for simple on/off to run the fans at 8v (not the full 12v).

See my next post...

Looking at the service manual, 19-15 and 19-23, it gives a chart for the sensor reading versus fan voltage and expected resistance values. I would infer that if the sensor connection is shorted to ground, the control unit will interpret that as very high engine temperature and turn the fans on full. So, my above post stands.

To be on the safe side and meet factory specs, instead of shorting the connection to ground, you could run it through a 900 ohm resistor, and that would simulate a temperature just warmer than 100 celcius, which would turn the fans on full.

That's what I would do...use a resistor and send the sensor signal to ground through the resistor. Should be effective, reliable, and within factory specs.

Good that you figured this out. I considered doing something like this some time ago but never followed through on it as I was not convinced it was worth the effort.

There is another potential dimension to this discussion that might be relevant to some extent. The PWM controller is without doubt a very clever device and any limitations regarding capacity of such a system will no doubt have been taken into consideration by Porsche. Many years ago there was a discussion about whether our fans ever reach full speed potential when such a PWM controller is deployed- possibly an eifficiency thing? Whether or not directly connecting the power supply to the motor might make a small difference I do not know. What I can say with certainty is that the amount of air moved varies as the square of the fan speed and the head developed varies as the cube of the speed so a little extra speed on top end might be "desirable"..

Funny things do happen with PWM systems- that I am sure of- I have a Spal twin fan setup and when the fans run at slower speed such as when they are driven by the switch on the inlet manifold after shutting down the motor they appear to "rattle" some. I changed the bearings- tested them on full battery voltage- ran perfectly- put them in the car and they still rattled- durrgh! I eventually purchased a couple of fans locally to harvest the motors and they did the same thing. Not strictly on topic but loosely related - wondered if anyone else has ever noticed something similar?.