From day one, I have encountered cooling issues with the 968. This was somewhat expected given that the heater core has been leaking for an unknown amount of time. Since it was leaking at the time of purchase, it was not possible to really troubleshoot which areas of the system needed attention. One of the primary issues was with the auxiliary fans, which would cycle when the engine got warm. The fan would cycle on and off every 3 to 4 seconds. When rebuilding the cooling system I was hoping to resolve this issue. I replaced the water pump, thermostat, heater valve, thermo switch, both temperature sensors, radiator cap, and all hoses. I was really hoping that the faulty component was the thermos switch, as when I removed it I found some suspicious anti seize on the connector. Sadly, after replacing all these components the fan was still cycling.

My search for the fault continued. One of the commonly mentioned issues are at the fan resistors. There are two resistors located on the passenger side near the fresh air vent. These resistors serve to limit the current flow to the fans which in turn reduces the fan speed. Basically, the fan works on two circuits that are switched by the thermo switch, which activates one of the circuits depending on the temperature. The circuit at the lower temperature circuit runs through the resistors. The theory is that if the resistance is too high the relay will switch to the high speed circuit assuming “emergency mode”. I have not been able to fully confirm this as the fan relay is visually very complex while the circuit map does not provide any explanation of the logic.

Back to those resistors. The common issue appears to be corrosion at the wires or connectors. In my case everything appeared relatively clean. To be sure, I cleaned the connectors in some vinegar to remove any of the corrosion that was present. The next possible issue is that there is a fault in the resistor. I removed the resistors to see if their resistances matched. The readings I was getting were not very consistent but they trended higher than the rated 0.9 ohms. I went ahead and ordered one resistor to test out. Once I received it I went ahead and tested its resistance. And of course, it also read somewhat inconsistent and slightly high. When taking these measurements I never considered what the resistance of multimeter was. When touching the leads together I assumed it would read 0 ohm. When I touched the leads together on my multimeter, I got a reading equal to my discrepancy between the rated resistance and my measured resistance. Still, I installed the new resistor hoping maybe something will change. As predicted, there was no change.

Back to square one. The last component I have not replaced was the fan relay. This is that giant relay that costs over $200 from Porsche. I really did not want to buy such an expensive component without knowing it would resolve the issue. I bought a used one off ebay for a fraction of the cost and installed it the second it arrived, and it worked. My fan issue was completely resolved.

A couple months go by and I seemed to have developed a new fault, or more so of an anomaly. When taking the 968 to Angeles Crest Highway on a winter day I started to run slightly cooler than usual.

Usually the engine temperature needle indicated slightly above the first line or slightly below. Now it was indicating between the bottom line and the first line. Looking around the web I found a handy image that indicated roughly what the temperature is at each line.

 According to this image I was running much too cool at 70C, but oddly enough my fan was turning on. The fan should turn on at 85/93C, based on the installed thermos switch, suggesting that the gauge was not accurate. When investigating this I also found that the thermo switch I have installed is not rated at the correct temperature, as it should be 92/102C. When I was purchasing the switch I used the one I removed as a guide. This means that the one I removed had been replaced, maybe even to try and resolve the fan cycling issue. Since the correct switch turns on at a higher temperature, the new switch may help with the temperature, although the thermostat should be the primary controller here.

The new switch did not make a noticeable difference. Searching the forums I found a couple mentions of the needle not being calibrated or a grounding issue. Even the possibility that the needle shifted when driving over rough terrain. I bought a variable resistor to that I could hook up in place of the temp sensor to see what the gauge reads. I follow the resistance measurements and procedure on Clark’s Garage for this. This confirmed that the gauge is indicating incorrectly.

At this point I have not figured out what has caused this, but now I know where the new norm is for my 968. If there is any doubt I just listen for the fans.