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Cooling Fans-when should they come on?

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Old 10-12-2009, 08:13 PM
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210007587
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Default Cooling Fans-when should they come on?

I have a 1998 C5: It appears that the passenger side fan comes on only when the AC compressor is on. The drivers side is what I'm not understanding- sometimes comes on when the engine is cold. Can someone tell me when it should come on? What temp? Thanks
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Old 10-12-2009, 10:27 PM
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If you have only one fan running then you have a problem that needs to be diagnosed. When the fans run both run at the same time at either high speed or low speed.

With the AC on the fans (both) come on at low speed at 185 degrees. They both go to high speed at 194 degrees.

With NO AC running the fans come on at low speed when the coolant reaches 226 degrees. The both go to high speed at 235 degrees.

When the vehicle speed reaches 35mph the fans go OFF no matter what the coolant temp is. Air flow through the radiator is better with the fans freewheeling than with them running.

Hopefully ByronHunter will post the wiring diagram that can help you troubleshoot what may be happening. There are 3 relays involved in the fan operation. One or more may have failed or the fan that runs only on occassion may be worn out.
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Old 10-12-2009, 10:49 PM
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Old 10-12-2009, 11:00 PM
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Originally Posted by 8VETTE7 View Post
If you have only one fan running then you have a problem that needs to be diagnosed. When the fans run both run at the same time at either high speed or low speed.
I agree. The fans are wired in a series/parellel configuration. When they're running in parallel, they turn fast. When is series, they turn slow. They always turn together.

I would start from the motors and work back. Both motors seem to work, so maybe one of the relays are shot. Probe the relay sockets and make sure the pcm is trying to energize them, and check that they are actually switching.
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Old 10-13-2009, 04:46 PM
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Great information. Thanks very much.
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Old 10-13-2009, 08:22 PM
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C5 Cooling Fans Operation

The low speed cooling fan is commanded on when the coolant temperature reaches 108C (226F). It is turned off if the coolant temperature lowers to 104C (219F). The high speed cooling fan is commanded on when the coolant temperature reaches 113C (235F). It is turned off if the coolant temperature lowers to 108C (226F). When the A/C is on and the coolant temperature reaches 85C (185F), the low speed cooling fan will be turned on at vehicle speeds less than 56 kPh (35 mph).
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Old 10-14-2009, 12:09 PM
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Originally Posted by Oldvetter View Post
C5 Cooling Fans Operation

The low speed cooling fan is commanded on when the coolant temperature reaches 108C (226F). It is turned off if the coolant temperature lowers to 104C (219F). The high speed cooling fan is commanded on when the coolant temperature reaches 113C (235F). It is turned off if the coolant temperature lowers to 108C (226F). When the A/C is on and the coolant temperature reaches 85C (185F), the low speed cooling fan will be turned on at vehicle speeds less than 56 kPh (35 mph).
Also if your car has been tuned the on/off temps of each or both fans may have been changed by the tuner.
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Old 10-16-2009, 02:30 PM
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I appreciate all the responses. Great information.
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Old 03-04-2010, 06:57 PM
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Interesting that 226 & 235 degrees F are the temps. for low & high fans to start. I just bought a '98 LS1 & seen 221 F with A/C off & thought I had a problem. I suppose this is normal & not to worry. Thanks for great info.
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Old 01-01-2017, 04:36 PM
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Originally Posted by 8VETTE7 View Post
If you have only one fan running then you have a problem that needs to be diagnosed. When the fans run both run at the same time at either high speed or low speed.

With the AC on the fans (both) come on at low speed at 185 degrees. They both go to high speed at 194 degrees.

With NO AC running the fans come on at low speed when the coolant reaches 226 degrees. The both go to high speed at 235 degrees.

When the vehicle speed reaches 35mph the fans go OFF no matter what the coolant temp is. Air flow through the radiator is better with the fans freewheeling than with them running.

Hopefully ByronHunter will post the wiring diagram that can help you troubleshoot what may be happening. There are 3 relays involved in the fan operation. One or more may have failed or the fan that runs only on occassion may be worn out.
Thanks for posting that. I don't recall ever seeing the temp go beyond 220, even in the summer. Now, in the winter, I see the temp go almost 230, then the fan kicks in at idle. Always great to see that gauge trickling back down
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