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GM Tech (Cam)
GM Tech (Cam), Chevy Technician
Category: Chevy
Satisfied Customers: 15181
Experience:  GM Grand Master Technician 2007. 14 years experience.
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97 chevy venture. fans wont turn on unless i take harness off

Customer Question

97 chevy venture. fans wont turn on unless i take harness off of coolant temp sensor, if i plug back in, fans stop and it will overheat. when i take off harness to sensor they just run all the time. someone told me it was the computer, but i changed also and still no help.
Submitted: 7 years ago.
Category: Chevy
Expert:  Kevin Bourcier replied 7 years ago.

Can you look on the radiator and see if you have a " fan " module .

 

 

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Customer: replied 7 years ago.
Yes it does
Expert:  Kevin Bourcier replied 7 years ago.
Try unplugging that . Do the fans stop ?
Customer: replied 7 years ago.
Isn't that actually the coolant level sensor?
Expert:  Kevin Bourcier replied 7 years ago.

Yes it is . But for some reason in the diagram it shows it tied in to the relay . It doesn't make sense to me . Have a look at this diagram for the pressure switch . Also have a look at the relay diagram . Try unplugging the relays 1 at a time so we can determine which one is being commanded " on "

 

 

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Customer: replied 7 years ago.
Ok, relays #10,9,12 turn off either one of the fans. #9 and 12 shut off the fan closer to radiator cap and the other one shuts off the opposite fan, I can't seem to find the fan switch. I disconnected the level sensor and the fans keep running(bare in mind I have disconnected the coolant temp sensor in order to get fans running to do these tests)
Expert:  Kevin Bourcier replied 7 years ago.

I am sorry . I read your original post wrong . I thought it was staying on all the time . Have you had your radiator checked ? Are you sure that you have filled the cooling system fully and bleed the air out of it ?

Customer: replied 7 years ago.
Radiator is definitely circulating, I also have a slight leak somewhere underneat on backside of engine. I can't really say my 1/2 a** mechanic blead the system correctly. How could I make sure? Are these things that would make the fans not turn on when plugged into coolant temp sensor? The coolant level is good. That's why a 3rd opinion thought it might be the computer not sending signal to fans
Expert:  Kevin Bourcier replied 7 years ago.

The fact that the fans turn on tells you the ECM circuit is working . Take the ECT out of the engine , plug it in get a really boiling hot pot of water and lower the sensor in to see if it turns the fans on . ( make sure the key is on )

Customer: replied 7 years ago.
ill have to try that in the morning, wharts the best way to get that sensor off without breaking it?
Expert:  Kevin Bourcier replied 7 years ago.
The Engine Coolant temperature sensor on the motor .?
Customer: replied 7 years ago.
yes. nevermind i think i have the socket, when i got the new ect it had a small tab for the harness i guess, didnt really want to let the socket go on smoothly.
Expert:  Kevin Bourcier replied 7 years ago.
Oh . I see what you mean .
Customer: replied 7 years ago.
ok after i do the boiling water test, whats next? if the fans turn on then what? if they dont? how do i properly bleed the system myself?
Expert:  Kevin Bourcier replied 7 years ago.

Fill the cooling system and open the bleed valve run the engine until there are no bubbles coming out ( straight coolant ) .

 

 

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Edited by Kevin Bourcier on 2/27/2010 at 9:53 PM EST