Cooling Fans are Cycling
 

Hi All,

I'm still new to Porsche ownership, but enjoy driving and tinkering with my Guards Red '90 944 S2 Cab (aka "Scarlett"). I'm lucky insomuch as the PO of my car was my meticulous brother who kept it sorted and campaigned it at Sebring.

Since owning the car, the fans decided to run continuously. I disconnected the battery until I could change the thermoswitch at the radiator. All was good for several months. Recently the fans want to cycle on for about 10-15 seconds then off for another 10-15 seconds. Very annoying and I doubt this is healthy in the long term.

I have read a few posts, here and elsewhere. Since I recently changed the thermoswitch, I'll exclude that for the time being. I pulled and opened the relay, which looked good. Closed that back up and reinstalled it. Problem remains.

Any ideas? I know visual inspections of electronic components are unreliable, so I believe the relay could still be the culprit. All I can say about that is there is no obvious signs of burning or frozen armatures. Again that may not mean anything.

Looking forward to any words of wisdom. Also if a relay replacement is imminent, what are my best options.

Thanks in advance!

I have read on other forums issue with the fan switch going bad sometime pretty quickly....so maybe your new fan switch is bad...

Really a few things it could be:

Thermo (Replaced not likely in my mind this went bad so soon)
Relay (I am not sure you could see problems with the contacts - consider replacement)
Wiring at the Thermo including the connectors (these wires are very thin and can get frayed and break - this happened on my 944 and had to patch in a section and redo the spade connectors.)

There are some great 944 experts on this board which I am sure will chime in.

Thanks for the reply. I will check those wires to the thermoswitch! Great idea. I also read about the wiring at the (hi/lo speed) resistor that can corrode... I will have a few things to check on this evening.

I sure hope not, but since the price of the switch is an order of magnitude less than a fan relay, I would be okay with that :-)

BTW, I have my front plates mounted PROPERLY now, thanks for the bracket!

Actually I remember having to tap on the Thermo/Fan Relay a few times to get them shut up before I replaced the Relay (after the key was removed). Your strongest candidate for this issue is most likely the Relay at this point.


Good Luck

Glad it works, you shall post picture of your pride!

The 944S2 uses a large 4-in-one relay to control both of the fans. It looks like this:

Attachment 51089

The coolant temp switch doesn't switch the fans on directly, it provides input the the relay to turn them on. The "4-in-one" part comes from the fact that each fan has two relays (high speed and low speed). Looking at the schematic for this mess:

Attachment 51090

we can see many inputs/outputs to the relay. The pins in the red box are controls for Fan Motor 1:

• 30M1: +12V input for Fan 1
• V1: Low speed +12V output
• M1: High speed +12V output

Likewise, the pins in blue are the i/o for Fan Motor 2:

• 30M2: +12V input for Fan 2
• V2: Low speed +12V output
• M2: High speed +12V output

The rest of the pins, in the green box, are the control inputs to the relay:

• TS: Signal from lower (92 degrees) side of the coolant temp switch. Should turn the fans on in slow mode when switched to ground
• X: +12V power to the relay
• 31: Ground
• AC: Signal from A/C freeze control and intake temp switch
• TF: Signal from higher (102 degrees) side of the coolant temp switch AND the A/C pressure switch.

Assuming you understand how to use a multimeter, here's where I'd start - WHEN IT'S HAVING THE PROBLEM pull the fan relay and check continuity to ground on the TS and TF pins. You say the fans are cycling 15s on/15s off so if the fan relay is good you should see the same cycling here. If you see TS cycling with the fans, then your new coolant temp switch is bad. It happens. If the TF pin is cycling, then it's either the coolant temp switch OR the A/C pressure switch. Disconnect those at their respective locations and check their outputs directly.

If both of these are OK check the AC input. I read over the schematic quickly but it appears that this input is switched to +12V to turn the fans on, not ground like the TS and TF inputs.

Chris,
Thanks for this.

I have used my multi-meter before and shouldn't have too much trouble with this. Do you have any experience with LiON battery packs? I have a very small one which I have used to jump start my car (after a long snowy spell that I didn't feel a need to disturb her...oops). Do you think this would this be ok for testing purposes?

Well that went from pretty good to fuching awesome quickly...