If it's releasing only after shutdown, look hard at the reservoir cap when you pressure-test the rest of the system as Tom recommends. Better parts stores will free-rent a tester, and you'll use that to test the system (at the rated relief pressure on the cap), then test the cap itself. Pressure testing with the engine cold allows you to monitor how 'tight' the system is, plus any external leaks will be a lot easier to find as liquid. I happen to favor the OEM or factory caps, as they seem to fit better than most aftermarket caps. If you've had the system open, do a drive with the heater at full hot, and let the coolant flow move any trapped air bubbles to the reservoir.
I don't know the 944 intimately. On the 928, the connection to the reservoir is from the heater circuit return line to the engine. With no flow through that loop, air bubbles never makes it out of the top of the engine. If the 944 is similar, this little exercise will help solve a lot of odd system symptoms, plus it protects the head at the rear from hot-spotting at those air pockets.
Radiator Cooling Fans
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dr bob
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dr bob
1989 928 S4, black with cashmere/black inside
SoCal 928 Group Cofounder
928 Owner's Club Charter Member
Former Ex Bend Yacht Club Commodore Emeritus
Free Advice and Commentary. Use At Your Own Risk!
1989 928 S4, black with cashmere/black inside
SoCal 928 Group Cofounder
928 Owner's Club Charter Member
Former Ex Bend Yacht Club Commodore Emeritus
Free Advice and Commentary. Use At Your Own Risk!
sorry to bring up an old thread. Was your old fan switch an HT brand by chance?mikee30 wrote: Sun Aug 03, 2025 11:12 am 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....
This post reminded me that I have failed to finish adding pictures make it more user friendly. I created this a long time ago and it needs some polishing. Constructive criticism is welcome.
Late 944, Turbo and S2 Cooling Fan Diagnostic Procedure
The fans are controlled by the cooling fan relay which receives inputs from the thermo switch, and the a/c system. The thermo switch has two temperature settings which control the fan speed. The a/c system turns on high speed fan operation whenever the a/c is in operation, or the system is low and at risk of freezing up. Both fans run in tandem and should run at the same speed at the same time, and will cycle between low and high speed until either the a/c is switched off, or the coolant temperature drops below the upper limit. If only one is working, they run continually, or you only get high speed operation, there is a problem.
1. Remove the cooling fan relay from the Central Electric Board (CEB). It is the large rectangular silver relay in the back of the panel. Check the diagram on the fuse panel cover for the exact location. Most of the following checks can be made at the CEB. You will need a bright light to see the markings on the fuse panel.
2. In the Bosch world, a terminal designation of “30” means it is un-switched battery voltage. Check for voltage between ground and positions 30 M1 and 30 M2. You should get 12v + at both positions. If there is no voltage, check the fuses first, then wiring from the battery, the connections to the fuse panel, and the fuse panel itself. If you must remove the CEB, disconnect the battery before lifting the CEB away from the body.
3. Check for continuity between ground and the temp switch positions "TS" (high speed) and "TF" (low speed) on the CEB. If you have continuity, the thermos switch is calling for fan operation. If the car is cold, the thermo switch is bad. To verify this:
a. Disconnect the thermo switch connector at the radiator
b. Jumper connector terminals 2-3, you should have continuity at TS.
c. Jumper connector terminals 1-3, you should have continuity at TF.
4. Fabricate a jumper wire with flat blade connectors on each end, and a 20A fuse in the middle.
5. Connect the jumper from M1 to M1 30, the driver’s fan should run at high speed. If the fan does not run, check for voltage in the connector at the fan motor, if you have voltage but the fan does not run, replace the fan motor.
6. Check the passenger fan by connecting the jumper between M2 and M2 30, the passenger fan should run at high speed. If not, repeat step 5.
7. Next connect the jumper between V1 and M1 30. The driver’s fan should run at low speed. If not the resistor or wiring is bad. Check for voltage at the radiator fan connector and then check the resistance across the resistor. The wiring at the resistors is prone to melted insulation and corrosion.
8. Next connect the jumper between V2 and M2 30, the passenger fan should run at low speed. If it does not run, check the wiring and resistor.
The other positions on the fan relay are:
X = battery power to the logic board in the relay
31 = Ground for the logic board in the relay
AC = AC freeze protection switch
Late 944, Turbo and S2 Cooling Fan Diagnostic Procedure
The fans are controlled by the cooling fan relay which receives inputs from the thermo switch, and the a/c system. The thermo switch has two temperature settings which control the fan speed. The a/c system turns on high speed fan operation whenever the a/c is in operation, or the system is low and at risk of freezing up. Both fans run in tandem and should run at the same speed at the same time, and will cycle between low and high speed until either the a/c is switched off, or the coolant temperature drops below the upper limit. If only one is working, they run continually, or you only get high speed operation, there is a problem.
1. Remove the cooling fan relay from the Central Electric Board (CEB). It is the large rectangular silver relay in the back of the panel. Check the diagram on the fuse panel cover for the exact location. Most of the following checks can be made at the CEB. You will need a bright light to see the markings on the fuse panel.
2. In the Bosch world, a terminal designation of “30” means it is un-switched battery voltage. Check for voltage between ground and positions 30 M1 and 30 M2. You should get 12v + at both positions. If there is no voltage, check the fuses first, then wiring from the battery, the connections to the fuse panel, and the fuse panel itself. If you must remove the CEB, disconnect the battery before lifting the CEB away from the body.
3. Check for continuity between ground and the temp switch positions "TS" (high speed) and "TF" (low speed) on the CEB. If you have continuity, the thermos switch is calling for fan operation. If the car is cold, the thermo switch is bad. To verify this:
a. Disconnect the thermo switch connector at the radiator
b. Jumper connector terminals 2-3, you should have continuity at TS.
c. Jumper connector terminals 1-3, you should have continuity at TF.
4. Fabricate a jumper wire with flat blade connectors on each end, and a 20A fuse in the middle.
5. Connect the jumper from M1 to M1 30, the driver’s fan should run at high speed. If the fan does not run, check for voltage in the connector at the fan motor, if you have voltage but the fan does not run, replace the fan motor.
6. Check the passenger fan by connecting the jumper between M2 and M2 30, the passenger fan should run at high speed. If not, repeat step 5.
7. Next connect the jumper between V1 and M1 30. The driver’s fan should run at low speed. If not the resistor or wiring is bad. Check for voltage at the radiator fan connector and then check the resistance across the resistor. The wiring at the resistors is prone to melted insulation and corrosion.
8. Next connect the jumper between V2 and M2 30, the passenger fan should run at low speed. If it does not run, check the wiring and resistor.
The other positions on the fan relay are:
X = battery power to the logic board in the relay
31 = Ground for the logic board in the relay
AC = AC freeze protection switch
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Tom
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@fasterfaster post your issues here on carpokes and we can help pin it down.Cruise98 wrote: Mon Sep 22, 2025 12:33 pm This post reminded me that I have failed to finish adding pictures make it more user friendly. I created this a long time ago and it needs some polishing. Constructive criticism is welcome.
Late 944, Turbo and S2 Cooling Fan Diagnostic Procedure
The fans are controlled by the cooling fan relay which receives inputs from the thermo switch, and the a/c system. The thermo switch has two temperature settings which control the fan speed. The a/c system turns on high speed fan operation whenever the a/c is in operation, or the system is low and at risk of freezing up. Both fans run in tandem and should run at the same speed at the same time, and will cycle between low and high speed until either the a/c is switched off, or the coolant temperature drops below the upper limit. If only one is working, they run continually, or you only get high speed operation, there is a problem.
1. Remove the cooling fan relay from the Central Electric Board (CEB). It is the large rectangular silver relay in the back of the panel. Check the diagram on the fuse panel cover for the exact location. Most of the following checks can be made at the CEB. You will need a bright light to see the markings on the fuse panel.
2. In the Bosch world, a terminal designation of “30” means it is un-switched battery voltage. Check for voltage between ground and positions 30 M1 and 30 M2. You should get 12v + at both positions. If there is no voltage, check the fuses first, then wiring from the battery, the connections to the fuse panel, and the fuse panel itself. If you must remove the CEB, disconnect the battery before lifting the CEB away from the body.
3. Check for continuity between ground and the temp switch positions "TS" (high speed) and "TF" (low speed) on the CEB. If you have continuity, the thermos switch is calling for fan operation. If the car is cold, the thermo switch is bad. To verify this:
a. Disconnect the thermo switch connector at the radiator
b. Jumper connector terminals 2-3, you should have continuity at TS.
c. Jumper connector terminals 1-3, you should have continuity at TF.
4. Fabricate a jumper wire with flat blade connectors on each end, and a 20A fuse in the middle.
5. Connect the jumper from M1 to M1 30, the driver’s fan should run at high speed. If the fan does not run, check for voltage in the connector at the fan motor, if you have voltage but the fan does not run, replace the fan motor.
6. Check the passenger fan by connecting the jumper between M2 and M2 30, the passenger fan should run at high speed. If not, repeat step 5.
7. Next connect the jumper between V1 and M1 30. The driver’s fan should run at low speed. If not the resistor or wiring is bad. Check for voltage at the radiator fan connector and then check the resistance across the resistor. The wiring at the resistors is prone to melted insulation and corrosion.
8. Next connect the jumper between V2 and M2 30, the passenger fan should run at low speed. If it does not run, check the wiring and resistor.
The other positions on the fan relay are:
X = battery power to the logic board in the relay
31 = Ground for the logic board in the relay
AC = AC freeze protection switch
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fasterfaster
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Thanks for the nudge! That troubleshooting guide from @Cruise98 is gold.Tom wrote: Sat Dec 06, 2025 7:33 am @fasterfaster post your issues here on carpokes and we can help pin it down.
I think this thread may have already highlighted my bozo mistake - I didn't realize fan-high only works with ignition on. Will double check that today and revert back here.
Just in case, here's my challenge: my fan won’t run in high mode. because it runs in low, i assumed it was a bad relay and took the opportunity to upgrade to an F9T solid state relay with diagnostics. Just because i was already in there replacing my alternator, I replaced the thermoswitch. no difference. The fan still won’t run on high, even when the “high” test button on the F9T relay is used. i don’t think it’s a bad motor because the fan runs on the low circuit. I’m guessing the high circuit must be shorted somewhere, but hoping I don’t have to trace the wiring.
Marc
88.5 951 M030 Red on Black
88.5 951 M030 Red on Black
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fasterfaster
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Turns out I am indeed a bozo, lol.
Ignition needs to be on for fan-high. Everything working fine now.
Ignition needs to be on for fan-high. Everything working fine now.
Marc
88.5 951 M030 Red on Black
88.5 951 M030 Red on Black
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Tom
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I was just coming back to the computer to suggest that. I just installed my own F9 relay and pushed the buttons to see the slow work but not the fast. I know mine work, so after a minute or two I realized it's no different than the thermoswitch -- ignition has to be on for the fast speed but not for the slow speed.fasterfaster wrote: Sat Dec 06, 2025 4:01 pm Turns out I am indeed a bozo, lol.
Ignition needs to be on for fan-high. Everything working fine now.
Hi all, thought I'd post how this concluded so it can maybe help someone one day...
The mysterious coolant leak that occurred after shut down was simply caused by the radiator temp sensor not being torqued down hard enough. Ironically, even my "homemade" pressure test via the coolant reservoir cap wasn't enough psi to reproduce the leak.
I've seen a few folks post a similar issue...I was only able to figure it out by removing all the plastic panels under the car. What was happening was the coolant would drip out of the switch after shutdown and fall onto the plastic - then slide towards the passenger side where it would then escape. This made it seem like the leak was on the passenger side.
Thanks all!
The mysterious coolant leak that occurred after shut down was simply caused by the radiator temp sensor not being torqued down hard enough. Ironically, even my "homemade" pressure test via the coolant reservoir cap wasn't enough psi to reproduce the leak.
I've seen a few folks post a similar issue...I was only able to figure it out by removing all the plastic panels under the car. What was happening was the coolant would drip out of the switch after shutdown and fall onto the plastic - then slide towards the passenger side where it would then escape. This made it seem like the leak was on the passenger side.
Thanks all!
Great conclusion.
For what it’s worth, that seal doesn’t use the threads to seal. Its sealing surface is the plastic to the base of the sensor/crush washer.
Porsche OEM washer is now alluminum. Wahler ship with a fiber gasket. Don’t treat it like a normal crush washer - this is a low torque situation. BMW’s of the same era don’t even have a washer there. It’s sensor straight to the plastic radiator.
I put a bit of a smear of hylomar/curil t2 on both sides of the washer.
The sensor should be torqued just tight enough not to leak and not any further.
For what it’s worth, that seal doesn’t use the threads to seal. Its sealing surface is the plastic to the base of the sensor/crush washer.
Porsche OEM washer is now alluminum. Wahler ship with a fiber gasket. Don’t treat it like a normal crush washer - this is a low torque situation. BMW’s of the same era don’t even have a washer there. It’s sensor straight to the plastic radiator.
I put a bit of a smear of hylomar/curil t2 on both sides of the washer.
The sensor should be torqued just tight enough not to leak and not any further.