Cooling fans not working unless I disconnect the power to the ETC?

Tiny
RICH TRINCA
  • MEMBER
  • 2005 PONTIAC GRAND PRIX
  • 3.8L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 306,000 MILES
My cooling fans are not working even with the A/C on. When I disconnect the yellow wire on the temperature sensor the fans work. The cooling fans only come on when I disconnect then temp sensor power wire. I tried turning on the A/C, but the fans did not come on.

I replaced the fan1 relay and it still didn't work. I checked the ground wire on the ETC for continuity with the ground on the battery and frame and that has continuity, and I checked the power wire on the ETC for continuity to the positive terminal on the battery and that has continuity as well. It reads the temperature correctly but will not energize the fans. I'm thinking I need to replace the Temperature Sensor (ETC).
Sunday, January 26th, 2025 AT 2:38 PM

7 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 50,162 POSTS
The sensor is bypassed when the A/C is on. It sounds like the PCM might be bad, but we should check the g100 ground near the battery. Here are the wiring diagrams of the fan system so you can see how the fans work. There are two other relays you need to check as well. Here are the locations for those as well:

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

If the A/C is low on charge the fans will not come on FYI. Also, if the engine temperature gauge is working normal the sensor is probably good. Here are the locations of the Check out the images (below). Please let us know what happens.
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Monday, January 27th, 2025 AT 10:40 AM
Tiny
RICH TRINCA
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Can a cooling temperature sensor read the right temperature but still be bad so that it doesn't energize the fan?
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Tuesday, January 28th, 2025 AT 1:39 PM
Tiny
KEN L
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Not really, it is more like the PCM is bad. But let's connect a scanner to see what the PCM sees the engine temperature as, this will help us. Please go over this guide which will help:

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Please go over this guide and get back to us.
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Wednesday, January 29th, 2025 AT 8:34 AM
Tiny
RICH TRINCA
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The engine temperature is reading correctly on the dash. There are no codes and no check engine light.
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Wednesday, January 29th, 2025 AT 9:18 AM
Tiny
KEN L
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As you can see by the cooling fan wiring daigrams above you have 3 relays and two fuses that run the system I would make sure the fuses are good and swap out the relays, if they are okay I would get a rebuilt PCM. You can get a preprogrammed unit by searching Google or Ebay. Here is the location and instructions below. Please let us know what happens.
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Thursday, January 30th, 2025 AT 11:22 AM
Tiny
RICH TRINCA
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Can a cooling temperature sensor read the wrong temperature, so that it doesn't energize the fan? I checked the resistance between the old sensor and a new one and they get different readings. The new one was around 3,000 ohms and the old one around 2000 ohms.
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Saturday, February 1st, 2025 AT 6:12 PM
Tiny
KEN L
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Usually the temperature gauge will reflect the reading, but I would try swapping the sensor out to see what happens.
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Sunday, February 2nd, 2025 AT 8:55 AM

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