Engine fan does not turn on until 235F

2005 CHEVROLET SUBURBAN
70,000 MILES
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JENNYINCOLO
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Our SUV is not turning on the engine cooling fans until well above 202 degrees, which is where the low speed fan turns on. We have an OBD2 reader (scan gauge) which tells us the engine temps. At low speeds, especially towing our pop up camper, the temp climbs up to 220F, and the fans do not turn on. The dashboard temperature gauge (analog) shows under 210, even when the scangauge shows 220. The first time we noticed the temperature above 220, we tried to turn on the A/C, but the A/C would not turn on (it just blinked on the button).

When we catch it and turn on the A/C, the fans will turn on and the engine temperature will come down. We tested the breakers, they all came up working. We pulled the temperature sensor while the engine was running, and the fans immediately kicked up to high. Frankly, my husband and I are stumped,I can't find good answers online (hard to search through all the forums), and we're not sure what to try next. Our transmission guy (we just had a transmission cooler installed to help with towing in the mountains) was the one who told us the fans should come on at 202 - he was stumped and suggested a memory wipe of the on board computer (about $150.00). we're hoping to find a less expensive solution. Would a new sensor help?

Thanks for any help or ideas.
Sep 24, 2012 at 7:00 PM
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HMAC300
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The PCM commands the high speed fans "on" under the following conditions:

Engine coolant temperature exceeds approximately 104.25°C (220°F).
A/C refrigerant pressure exceeds approximately 1824 kPa (265 psi).
When certain DTCs set.

At idle and very low vehicle speeds the cooling fans are only allowed to increase in speed if required. This insures idle stability by preventing the fans from cycling between high and low speed.
a tech 2 really needs to be hooked up to t his to see if the gauge is out of calibration. there are three relays sas well you might try cleaning the ground for this as well as having the coolant temperature sender checked, it may have high ohms and not working right.
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Sep 24, 2012 at 7:20 PM
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JENNYINCOLO
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Thanks. I'll take a look at these things and try them out.
Nov 6, 2012 at 8:16 PM
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JENNYINCOLO
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We've done some checking and here is what we found:

1. The gauge on the dash is not out of calibration. We are using a Scangauge tool (OBD2 reader) to determine the engine temperature, and it matches up with the analog gauge on the dash.
2. Do not think the grounds need cleaning because when we unplug the coolant temperature sender, the engine automatically kicks all the fans up to high.
3. We manually checked all the relays - they respond when we hook up current.
4. We think the computer is not turning on the fans at the right time. For example, we were driving home after a weekend camping and got stuck in bumper to bumper traffic downtown after a football game. We noticed that the engine temp was 220, so we turned on the A/C. The fans turned on, then immediately kicked up to high (they were not on at all while we were idling in traffic) and the temperature came down. The engine somehow did not "know" to turn on the fans in this situation, even though the temp was high.

Our transmission guy suggested that we get the computer reprogrammed at a dealer. Is there any other solution we can try before shelling out $150.00 to have that done?

Thanks for the diagram - very helpful!

Nov 10, 2012 at 4:14 PM
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HMAC300
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the only thing i can think of to do would be to change the coolant sensor, maybe it's screwed up or they have comeout with an updated one. I really can't think of anything else to help out. It seems from your description everything works like relays, etc except not letting pcm turn on at the correct time. teh only other thing i could think of would be to follow the pcm and see if any wires like the blue andgreen as you have the diagram may be hitting something and not letting relays connect to turn fan on...
Nov 10, 2012 at 4:39 PM
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JENNYINCOLO
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Thanks again for the feedback. we will look at the coolant temperature sensor and then head to the dealer for a computer check (if that doesn't work). Appreciate the feedback - looks like we're at the end of the line for things we can try here at home.
Nov 10, 2012 at 5:04 PM
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HMAC300
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well, make sure that you look at the electrical lines if they just partially ground out it will tell the computer that everything is okay. that includes the wires to the coolant sensor.
Nov 10, 2012 at 5:06 PM
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JOSE8198
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Mine is doing the same thing! overheats while in idle. As soon as I turn the A/C on the temperature comes crashing down. Ever find the problem?
May 21, 2020 at 7:55 AM
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ASEMASTER6371
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Good afternoon,

The previous was solved with a coolant sensor.

https://www.2carpros.com/articles/how-a-coolant-temperature-sensor-works

The temperature for the fan to come on is 208 degrees.

Your cooling fans should come at that temperature. If you remove the connector for the coolant sensor, the fans should kick on immediately.

https://www.2carpros.com/articles/how-an-electric-cooling-fan-works

If you give me the vehicle information, I can look up a diagram for you.

Roy



May 21, 2020 at 1:22 PM