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Discussion Starter · #1 ·
Hi guys I have got myself a new temp sensor the one that goes on the lower radiator hose. I see it's only a 2 minute job, just wanted to know on a cold engine someone said I have to drain all the coolant out first is this correct? Was kind of hoping I could just pop the old one out and replace it. Any help is appreciated thanks alot.
 

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E30M3 Race F10 535 R1150Rt M Coupe
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It's easy:

Open up the radiator (expansion tank) cap half way. Squeeze the upper hose with all you've got. At the same time tighten the cap. Now your cooling system is under slight vacuum.
Have everything at the READY.

Pop the sensor out and have the new one slightly lubed. Make sure the old O Ring comes out, have a pick ready if not. Pop the new sensor in and make sure it clicks. Give it a tug to be sure it's ALL the way in. Reconnect the electrical connector.
Open up the cap and top off.

Loss should be minimal.
 

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Discussion Starter · #4 ·
It's easy:

Open up the radiator (expansion tank) cap half way. Squeeze the upper hose with all you've got. At the same time tighten the cap. Now your cooling system is under slight vacuum.
Have everything at the READY.

Pop the sensor out and have the new one slightly lubed. Make sure the old O Ring comes out, have a pick ready if not. Pop the new sensor in and make sure it clicks. Give it a tug to be sure it's ALL the way in. Reconnect the electrical connector.
Open up the cap and top off.

Loss should be minimal.
Great. Your a star mate. Didn't fancy draining the whole cooling system. Thanks alot really appreciate it 👍

Oh can I just drive straight away don't have to bleed the system or anything
 

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E30M3 Race F10 535 R1150Rt M Coupe
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If you've only lost a very small amount (above) a top off will be fine.
Folks run into issues when the coolant level drops below the minimum mark.
 

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Discussion Starter · #7 ·
Sorry to be a pain my car is an automatic and has been trying to overheat in heavy traffic soon as I'm moving again it's fine I've got no coolant leaks etc. My electric fan isn't coming on and I did have a sensor fault code so I've just replaced it. My air con isn't gassed at the minute it's empty, does my air con need to be filled for it to activate the electric fan or should the fan come on regardless once I press the AC button.
 

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Sorry to be a pain my car is an automatic and has been trying to overheat in heavy traffic soon as I'm moving again it's fine I've got no coolant leaks etc. My electric fan isn't coming on and I did have a sensor fault code so I've just replaced it. My air con isn't gassed at the minute it's empty, does my air con need to be filled for it to activate the electric fan or should the fan come on regardless once I press the AC button.
 

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Discussion Starter · #9 ·
Sorry to be a pain my car is an automatic and has been trying to overheat in heavy traffic soon as I'm moving again it's fine I've got no coolant leaks etc. My electric fan isn't coming on and I did have a sensor fault code so I've just replaced it. My air con isn't gassed at the minute it's empty, does my air con need to be filled for it to activate the electric fan or should the fan come on regardless once I press the AC button.
Thank you some told me when you press the AC button the electric fan comes on straight away which is not the case. Thanks again for all your help. It's a good feeling not to have to worry about it overheating
 

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E30M3 Race F10 535 R1150Rt M Coupe
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Sorry to be a pain my car is an automatic and has been trying to overheat in heavy traffic soon as I'm moving again it's fine I've got no coolant leaks etc. My electric fan isn't coming on and I did have a sensor fault code so I've just replaced it. My air con isn't gassed at the minute it's empty, does my air con need to be filled for it to activate the electric fan or should the fan come on regardless once I press the AC button.
Given that your car is an automatic AND the coolant temps are fine when motoring (air moving through radiator) this is a classic sign of a worn out fan clutch.
The fan clutch is supposed to start locking when there's NOT enough air flowing through the radiator; you sitting in traffic. That's what it's there for.
On a automatic, the mechanical fan IS THE PRIMARY cooling device.
 

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Discussion Starter · #13 ·
Given that your car is an automatic AND the coolant temps are fine when motoring (air moving through radiator) this is a classic sign of a worn out fan clutch.
The fan clutch is supposed to start locking when there's NOT enough air flowing through the radiator; you sitting in traffic. That's what it's there for.
On a automatic, the mechanical fan IS THE PRIMARY cooling device.
My mechanical was spinning all the time as it's supposed to from the minute I turned the engine on until I switched it off. My electric fan wasn't kicking in at all.
 

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Spinning yes, but with verve? Perform a rolled up newspaper test the second the needle starts heading north of straight up.
BE READY to turn the heat on, as a supplemental cooling device.
 

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Discussion Starter · #16 ·
I did just change the temp sensor and left it idling for half an hour and it stayed perfectly at 12 o'clock. And the mechanical fan seems to spin fast, I do get the odd squeak sounds like a belt but normally when it's wet for some reason. I've had a brand new water pump done and belts were all changed around 6k miles ago.
 

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Discussion Starter · #17 ·
Did the aux fan come on at all while idling? The cooling outlet temp sensor is what the aux electric fan runs off of.

A year or two ago, I replaced an aftermarket outlet temp sensor with a used genuine BMW one I had laying around and suddenly the aux fan became much more active at idle. The fan comes on more frequently than with the aftermarket sensor. I do have functional A/C which is always on, but the quality of sensor did make a difference in frequency and length of aux fan runtime.

As for the ocassional squeaks, might be good to check the condition of the pulleys, especially the idler pulley on the top alternator bolt. If they spin freely and make any noise, its a good idea to replace them before they decide to seize up and trash the new belts.
I did that newspaper test on my mechanical fan and it stopped easily :( guess I need a new fan clutch I was certain it was trying to overheat as my aux fan wasn't coming on so. I'm going to drive it today and test it in traffic heaters at the ready just see how I get on.
 

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Discussion Starter · #18 ·
I have drove my car for 3 hours this morning normal and traffic driving no overheating and gauge perfect at 12 o'clock. My A/C is empty but when I press the AC button the electric fan now spins for around 30 seconds then goes off, before I changed the temp switch it didn't run at all even with the AC on. My question is the reason it's turning back off with the AC on because it's not cold enough hence not needing to stay on? I have my m.o.t on Friday so I am going to ask the garage to re gas my ac and see if it stays on.
 

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This means it sucked air into the system when the upper hose expanded. Depending on how much air was sucked in (how much the upper hose was compressed), it might need to be bled the air out.
You don't need to compress the upper hose at all if this is done on the cool morning as the system is slightly under vacuum pressure -- just don't touch the tank cap.
I've NEVER had air in the above method. In fact you can see the liquid right there in the cavity.

If it did suck in enough air to warrant a re bleed, there would be coolant on the ground. After it chugged out, tantamount to an upside down gas can
 

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Discussion Starter · #20 ·
I've NEVER had air in the above method. In fact you can see the liquid right there in the cavity.

If it did suck in enough air to warrant a re bleed, there would be coolant on the ground. After it chugged out, tantamount to an upside down gas can
What he told me to do worked perfectly. I didn't loose a drop of coolant and it didn't need bleeding. Great little trick considering a mechanic told me to drain the whole system for a one minute job.
 
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