Cooling Fans won’t shut off while running

[Tom]

I did all of the following and same symptoms. Fans kick off after first 2 cycles then they just stay running until I turn car off. 92c Thermofan switch and 71c thermostat:

• Unplugged climate control unit

• Tried spare relay.

• pressed and depressed with the snowflake button switch while the fans were running

The only conclusion I can come to is that the temp at the Thermofan switch in the radiator is actually above 92c and never stabilizing below that mark once the car is warmed up. What could this mean? Reduced cooling capacity? The water pump is new. I’ve added lots of water wetter. This is the second radiator with the same symptoms. Second relay with the same symptoms. Second thermostat with the same symptoms (only difference is the gauge does show the car is running at a lower temp with the 71C). Multiple Thermofan switches with the same symptoms. Should I just decide that this is normal? Should I try to find a OEM thermostat that actually opens at 80C?

It is in the 90’s here but my 2 previous 944’s did this so this is confusing me. The fans always eventually kicked off even in 100 degree weather.

I'm still a little unclear how you know the fans are running when you are driving down the freeway at 65mph? Maybe my ears are getting old, but I can't tell in my car unless I'm stopped or in a garage, etc., and even they I need to confirm by looking.

To isolate it with 100% certainty, you could just remove the plug, short the low speed and/or high speed fan terminal to ground to force the fans on (so you don't overheat). Then use spade connectors on the disconnected thermoswitch so you can monitor with a multimeter to confirm the fan signals are actually shorting to ground and staying that way even with the fans on running -- i.e., that the fans are unable to bring the temps down enough to shut off the fans.

If you pull the plug off the thermo-switch, do the fans stop running?

'Good used radiator' gives me a bit less confidence in its cooling ability, but if you car is running on the first white hash, it sounds like it is cooling just fine.

[Glassmuseum]

I'm still a little unclear how you know the fans are running when you are driving down the freeway at 65mph? Maybe my ears are getting old, but I can't tell in my car unless I'm stopped or in a garage, etc., and even they I need to confirm by looking.

To test, I let the car idle and warm up at home then I drive it through my neighborhood around 35-45 mph for 20 mins. I then take it on a long straightaway where I can open it up to 50-60 for a good 8 minutes. I’ll stop at the end of that straight and check. When I stop they are always on. I have the mark mapped out on the gauge as well where the fans kick off. Once the 2 fan cycles are done, it stays above that mark on the gauge. It is weird too that driving with a decent amount of airflow doesn’t seem bring down the temp too much on the gauge. Maybe a little bit but not what I would expect.

To isolate it with 100% certainty, you could just remove the plug, short the low speed and/or high speed fan terminal to ground to force the fans on (so you don't overheat). Then use spade connectors on the disconnected thermoswitch so you can monitor with a multimeter to confirm the fan signals are actually shorting to ground and staying that way even with the fans on running -- i.e., that the fans are unable to bring the temps down enough to shut off the fans.

I will try this next but I have tested this Thermofan switch until blue in the face over and over in boiling water and it always closes when temp in the water goes down. I get what you are saying though to maybe verify if the fans are doing their job? I also tried the test where I ran the car without the switch hooked up and the fans never ran.

If you pull the plug off the thermo-switch, do the fans stop running?

Yes

Good used radiator' gives me a bit less confidence in its cooling ability, but if you car is running on the first white hash, it sounds like it is cooling just fine.

Yeah I am sure it’s possible that it’s not working as intended but we’ve seen the same symptoms for both radiators. I flushed the newest one and pressure tested it at 15psi for a good hour. It looked great too but I know you have to take that for what’s it worth.

I decided to throw in another thermostat today. This time a Malle Behr 80c with the 92C thermofan switch. Still not getting much of anywhere. The only difference is the gauge is now sitting more in the middle. (Between the first and second mark), which I would expect for a higher rated tstat. Only other additional difference is the the high fans kicked on at some point as well. The turbo thermostat wouldn’t have anything to do with this right?

The only thing I am thinking now is to maybe consider a new radiator. I know a new radiator can only be a positive but I hate to throw that at the car if it’s not needed. I even went ahead and did a block test and it checked out fine. Just to rule out the Hg… Going to for now double down on bleeding the system as well just in case.

[Tom]

I will try this next but I have tested this Thermofan switch until blue in the face over and over in boiling water and it always closes when temp in the water goes down. I get what you are saying though to maybe verify if the fans are doing their job? I also tried the test where I ran the car without the switch hooked up and the fans never ran.

Right, that test was not to confirm the switch works, but rather to confirm it is indeed the switch that's causing the fan to run. But if you disconnect the switch plug and the fans no longer run, that's a pretty clear indicator that the switch is turning on the fans, and since you tested it over and over, that's a pretty good indication that the coolant is above the switch point(s).

It's worth noting that the thermo switch sits just under the upper radiator hose where the coolant is the hottest, and the gauge sensor sits right on the other side of the water pump where the coolant will be the coolest. So one interpretation of your system is that its working exactly as designed -- keeping the gauge temp right around the t-stat rating with help from the fans. I would expect the fans to turn off after cruising down the freeway a while though -- but honestly I can't say I've ever confirmed that. Without some kind of real-time monitoring, I don't think I could tell you if my fans were on low at 65mph. High speed yes, but low speed no. That's why I waffle a bit on whether you are chasing an actual problem. Are your fans truly never turning off -- or do they just go back on by the time you pop the hood and look? If they are not going off, is that even abnormal in hot weather?

Do your dash temps go up while cruising the highway if you unplug the fans? If so, that might suggest your radiator isn't getting good airflow and needs the fans to cool the coolant -- missing duct work, something blocking the flow, etc.

It doesn't really sound like a cooling capacity problem -- if that were the case you wouldn't see your dash temp track your t-stat rating so nicely.

[Glassmuseum]

It's worth noting that the thermo switch sits just under the upper radiator hose where the coolant is the hottest, and the gauge sensor sits right on the other side of the water pump where the coolant will be the coolest. So one interpretation of your system is that its working exactly as designed -- keeping the gauge temp right around the t-stat rating with help from the fans. I would expect the fans to turn off after cruising down the freeway a while though -- but honestly I can't say I've ever confirmed that. Without some kind of real-time monitoring, I don't think I could tell you if my fans were on low at 65mph. High speed yes, but low speed no. That's why I waffle a bit on whether you are chasing an actual problem. Are your fans truly never turning off -- or do they just go back on by the time you pop the hood and look? If they are not going off, is that even abnormal in hot weather?

Very good points. I would be very interested to know other see - if their fans kick off regularly during a long idle. The only reason I have gone down a bit of a wormhole is because my other 944’s behaved differently. I never noticed this.

With that said, you’re probably right, I am probably chasing nothing here. At least I hope that’s the case! While not scientific. I did a IR reading around the temperature sender/block and it does seem to get somewhere within intolerance of what the dash gauge is reading so I don’t think I’m overheating and I guess that is the most important thing. Anyways, I think I’m just gonna put the 71° thermostat back in because that seemed to keep the car really nice and cool and step away from this for a while.

Do your dash temps go up while cruising the highway if you unplug the fans? If so, that might suggest your radiator isn't getting good airflow and needs the fans to cool the coolant -- missing duct work, something blocking the flow, etc.
nicely.

I’m not really feeling super psyched about unplugging the fans and taking off so I might not try this one.
However, I did want to point out that I do not currently have the plastic underbelly installed since I’m doing a lot of work. Also, my car did not have a batwing so they got me thinking maybe that is affecting airflow to the radiator and hence affecting how it’s cooling while cruising. Any thoughts on that? I might try to reinstall the underbelly to see if it helps with this.

Thanks again for all the intel on this. I appreciate it!

[Tom]

It's worth noting that the thermo switch sits just under the upper radiator hose where the coolant is the hottest, and the gauge sensor sits right on the other side of the water pump where the coolant will be the coolest. So one interpretation of your system is that its working exactly as designed -- keeping the gauge temp right around the t-stat rating with help from the fans. I would expect the fans to turn off after cruising down the freeway a while though -- but honestly I can't say I've ever confirmed that. Without some kind of real-time monitoring, I don't think I could tell you if my fans were on low at 65mph. High speed yes, but low speed no. That's why I waffle a bit on whether you are chasing an actual problem. Are your fans truly never turning off -- or do they just go back on by the time you pop the hood and look? If they are not going off, is that even abnormal in hot weather?

Very good points. I would be very interested to know other see - if their fans kick off regularly during a long idle. The only reason I have gone down a bit of a wormhole is because my other 944’s behaved differently. I never noticed this.

With that said, you’re probably right, I am probably chasing nothing here. At least I hope that’s the case! While not scientific. I did a IR reading around the temperature sender/block and it does seem to get somewhere within intolerance of what the dash gauge is reading so I don’t think I’m overheating and I guess that is the most important thing. Anyways, I think I’m just gonna put the 71° thermostat back in because that seemed to keep the car really nice and cool and step away from this for a while.

Do your dash temps go up while cruising the highway if you unplug the fans? If so, that might suggest your radiator isn't getting good airflow and needs the fans to cool the coolant -- missing duct work, something blocking the flow, etc.
nicely.

I’m not really feeling super psyched about unplugging the fans and taking off so I might not try this one.
However, I did want to point out that I do not currently have the plastic underbelly installed since I’m doing a lot of work. Also, my car did not have a batwing so they got me thinking maybe that is affecting airflow to the radiator and hence affecting how it’s cooling while cruising. Any thoughts on that? I might try to reinstall the underbelly to see if it helps with this.

Thanks again for all the intel on this. I appreciate it!

You 'could' rig up a little switch to turn the fans on and off, just to see if it stays cool while cruising. If it does, then I'd lean toward this being a ghost hunt. If it doesn't, then that would support the fans coming on more than expected. But if it runs at the first hash, I'd probably be happy with that unless you know for sure they never go off on your car, but do on other 944's in the same environment. I have no under trays or batwing and have no cooling problems (though, like I said, I can't tell you for sure if the fans stay on all the time once fully warmed up in hot weather).