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Hi all, I'm looking for some help with making sure I am troubleshooting this correctly.
My cooling fans were working perfectly until a few days ago when they suddenly decided to never shut down, even when the car is off. They will simply keep running until they kill the battery...
If I unplug the aux fan relay (G10), the fans stop and plugging it back in will make the fans start up again. I had a spare fan relay (unknown to be working or not ), but swapping it didn't solve the problem. I'm suspecting the issue is with the radiator fan switch.
If yes, I am not clear on 1) how do I test the switch 2) how do I know the temp setting of my current switch? replacements are sold as either 85 degrees or 92-102 degrees. 3) Is there a process to test the wiring of the switch?
Yes, that pelican link shows the thermoswitch in the radiator. It's usually either the relay or that switch, and since you tried two relays, odds are it's the thermoswitch. Just pull the connector off the switch. If the fans then go off, the switch is bad. If the fans don't go off, the relay is probably bad. You could test those components and wiring with a multimeter, but 99% of the time, that simple test will diagnose the issue. The temp setting is usually stamped into the metal on the switch, though I'd just get the lowest temp switch I can find. In theory, you want to match it with you thermostat rating, etc., but in my real world, just get the lowest temp switch temp version...
mikee30 wrote: Wed Jul 23, 2025 8:56 am
Thanks Tom! I'm assuming there will be some temporary coolant loss when removing the switch even if the engine is cold?
To test it, you don't need to remove, just pull the connector off and see if the fans stop.
If you have to replace it (which I'm betting you do), then yes, you'll want to drain the coolant -- or at least enough of it to drop below the switch level... Properly vented, the radiator should be entirely full of coolant (no air).
Choosing the correct switch temp setting is closely related to the thermostat rating. The switch should be slightly hotter than the thermostat rating, so the thermostat goes full-open on temp increase before the switch closes. If you have a colder thermostat at 85º, then the 85º switch might be the right one. Hopefully the coolant has actually shed some heat in the radiator by the time it gets to the switch, and that will be the 'deadband' between thermostat-open and fan start in actual service.
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Philosophy:
Colder thermostats are installed to try and solve heat transfer issues in the rest of the system, like water pumps that are poorly adjusted or radiators that are obstructed or corroded inside. I'm not a, um, fan of those, since they keep the engine from reaching correct operating temp, at least until the other weakness in the system take control of flow in the system. In some turbo applications the 10º drop in thermostat temp -might- buy you a minute or so of initial detonation reduction under boost when the coolant temp is actually controlled by the thermostat, but again it's a momentary benefit. Once it's open, the system heat-shedding capacity becomes the controlling element. The fan is part of that capacity, but fan capacity is wasted if the thermostat is still managing coolant flow. I generally don't want the fan to run before it's actually needed, as it sucks horsepower via the alternator and battery-charging loads.
Of course, there's a faction that likes the fan running well before the thermostat runs out of control range. Ignore the parasitic load on the alternator, all in favor of letting the thermostat do all the coolant temp management itself. Wire the trigger for the fan relay from the X bus, so it's runes continuously in engine running/ key in position II. No need to worry about starting the fans for AC or a pressurized manifold, since it's always running. Maybe the temp switch rated for less than thermostat full-open temp is the easiest way to gain this functionality if in fact that's the operation you want.
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I just went through the fan motors on my 928 a couple years ago. Opened the cases, cleaned everything to like new. New brushes and new bearings. Three speeds from coolant temp, AC fridgen load pressure/temp, and a bit of heat-soak response to high intake-manifold temp with engine off. In normal driving service, the fans only come on at a stop or when the AC is on. It's like new again, quiet, with short fan-on cycles at low speed unless AC is on.
Unassailable advice and logic. Though I think the lower temp switch is still consistent with that advice on the 951, when used with a standard or lower temp t-stat.
The default switch trips at 92C and 102C, meaning there's a 19C buffer between when the stock 83C t-stat opens and the fans are running at full speed. 102C is above the top line on the gauge, meaning it doesn't kick on until you might rightfully start worrying about overheating. To compound the issue, Porsche actually put out a TSB about this, and said the switches produced before Feb '88 shouldn't be used since they run hotter than their rating. I'm not sure they ever really fixed that based on when the fans actually kick on, but that also might be due to the position of the thermo-switch vs the location of the gauge sensors. An 85/93 switch still allows a little buffer above a stock thermostat and in my experience still doesn't kick on until the gauge shows closer to 90C, giving it a pretty reasonable buffer (so the fans and t-stat don't fight against each other) and a lot more piece of mind when stuck in traffic on a 100 degree day.
As for lower temp t-stats, a 951 cooling system in perfect shape is generally able to run cooler than a stock 83C t-stat allows. As an experiment, when I was living in Miami, I ran the car with the thermostat removed (and the bypass port in the water pump blocked off). The car would actually run too cool, even after a 2+ hour round trip to Fort Lauderdale in sticky hot tropical heat. If you run a slightly lower temp t-stat, like 80C, you can see a lower operating temp if the cooling system is working well. But you need to make sure you also use a lower temp t-stat for the turbo coolant line, or that tiny t-stat may never open to cool the turbo! I think where people get in trouble is when their car is running hotter than they want due to some limitation in the cooling capacity (e.g., old crusty radiator) and think that a lower temp t-stat will lower the operating temp. In that case, as dr bob says, it doesn't really matter if the t-stat is opening at 83C or 33C -- either way, it's already open and the operating temp is limited by the (compromised) cooling capacity of the system...
And another thing... a lot of places try to pass off a VW t-stat for the 944 since it seems to fit. Only problem is it doesn't flow as much as a 944 t-stat, so it reduces the cooling capacity of the system. Sure, it might open at 80C, which sounds promising, but it restricts coolant flow and makes the engine run hotter than it should...
Based on your symptoms, the thermo-fan switch is the most likely cause, as it’s a common failure point causing fans to run continuously.
1. To confirm the thermo-fan switch is the issue;
Disconnect the electrical connector from the thermo-fan switch (it has three pins on a 1986-1991 951). You may need to remove the upper radiator hose or airbox for access. Set your multimeter to continuity or resistance..
Terminals 2 and 3 (TS circuit, low-speed): Should show no continuity (open circuit) if the coolant is cold.
Terminals 1 and 3 (TF circuit, high-speed): Should also show no continuity.
With the ignition off, jumper terminals 2 and 3 on the connector (not the switch itself). The fans should run at low speed.
Jumper terminals 1 and 3. If the ignition is on, the fans should run at high speed.
If the fans respond correctly, the wiring and relay are functional, and the switch is likely the issue.
If either pair shows continuity with the engine cold, the switch is likely stuck closed and faulty. If the switch tests fine, the issue is likely in the wiring or relay circuit.
NOTE: Car should be cold (coolant temp well below 92°C).
2. To Determine the temperature settings of your current switch, Check the Part Number by removing the switch, look for a part number or date stamp on the housing. The stock switch should have a production date post-February 1988, indicating the improved version with 92/102°C settings.
3. To test the wiring of the switch, Check for Shorts at the Fuse Panel.
Yes, test the radiator fan switch first it’s a likely culprit. Also, check for a stuck relay or a short in the wiring, especially if the relay swap didn’t help. If the switch is faulty, replace it. If not, dig deeper into the wiring or ECU.
Hi all, thanks for all the info and background. It turns out it was the switch. I followed the procedure on pelicanparts to remove the switch from the top of the car. If it helps anyone, you can change it without removing the cross over pipe as the article suggested, but it was quite annoying working with it in the way. I think next time I would tackle the job from underneath the car.
Everything is working great with the fans now, but I keep having an issue that if I drive the car to operating temperature, it will release coolant a few minutes after I park. I haven't been able to pinpoint where the coolant is coming from since I need to remove the undertray but I did check all the hoses I removed + my rad hose...everything shows no signs of a leak. I'm not sure if maybe the system has air in it causing the coolant to be released from the coolant reservoir overflow....
The overflow hose is loose and flexible. You can pull it up and put it into a clear plastic water bottle to confirm that's where the coolant is coming from. Chances are very good that's the source, and that it is happening because the system needs to be vented more. It often takes 2 or 3 tries to fully vent the system. If venting doesn't help and/or the coolant is not coming from the overflow, then I'd suggest getting a cooling system pressure tester -- no easier way to pinpoint a leak -- but my money is on coolant coming out the overflow tube due to air that needs to be vented out...
), but swapping it didn't solve the problem. I'm suspecting the issue is with the radiator fan switch.
