2008 silverado cooling fans not working

Hi I have this truck 08 silverado fans are working fine if I turn ac on or if I use the scanner and activate them. But the engine temperature goes to high over 220 and the fans never come on.
looking around but nothing temp sensor is fine it is reading normal.
Wonder if my pcm is the problem please help thanks in advance I'm lost

What scanner do you have? The Snap-On has a list for 'Acc/Elec/Theft Data', and it'll have the cooling fan commands listed. This would be the best data to watch during your high temp condition.

Not sure about the engine in this Silverado, so I guessed a 5.3L. This system uses two fans with two separate fan speeds. In low speed, both fans are run in series by the FAN LO relay, so they both end up spinning at half speed. In high speed, each fan gets its own power and ground, so both spin flat out. A circuit fault with one of the fans can cause the other to run at high speed only.

Here's the 5.3L diagram for reference:



What bi-directional options do you have for the fans? See if you can command low and high speed, and which fans do what when commanding both. Let us know what you find!

Sorry for the little info.
It is a 4.8 it looks like the fan system is the same thanks for the diagram.
I'm using a maxidas ds708
It does have the bi-directional options.
Forgot to mention it is giving me a p0118 code etc circuit high voltage
But as I said before temp readings are good also sensor is been replaced already.
About the bi directional mode on low speed the two fans work fine but on high speed only the one at passenger does.

Do the fans come on if you disconnect the coolant temp sensor and/or bridge the connector?

How did you verify the temp sensor readings are acurate, did you actually crossreference temperature to resistance?

This is cooling/HVAC DATA

I post a picture from data I got I like you to check it the weird thing is that the relay 2 and 3 command shows ON all the time of course the fans are off.
Not sure if that's something normal

Your scan data shows the pcm commands the fans' highspeed on .This would be normal with a P0118, as a preventive step to avoid engine damage...

Do the fans come on during the condition your scan data is showing?

Your scan data does not show the ect, what is it reporting.

If the fans do not run in a situation your scan data describes : Check for proper relay function and wiring integrity from relay to fan...

If the P0118 is a hard code you have to attend this first!!

No fans does not react when the sensor been unplugged or when temperature goes above 210
The PCM is not sending the command for some reason

Well it make more sense to me now about the command thanks.
About the ECT readings it is the top one I know is hard to see it I will post a new picture.
And no the fans are not coming ON
At any time and the ground command from PCM is not there

Cooling/HVAC data

The fact that the driver side hi speed fan does not come on when commanded to points to the relay that is called "fan contro relayl".This relay is pcm ground side controlled, together with the "cooling fan 1relay".

The cooling fan one relay controls the passenger side hi speed cooling fan, whichis working correctly as you state.

So probably the fan control relay does not switch properly between 87a and 87, youmight want to check this relay.

Thanks again it was the control relay l replaced it and now fans are working.
But I have a new question I let it heat up it when up to 226 before the low speed fan come on now it stays in betwing 208 and 226 wonder if it is normal

These Chevrolet switch the fans on at really high temperatures in comparison to other vehicles....this has to do with where the sensor is placed.

If memory serves me well I would expect the low speed fan to switch on at 220°F ~103°C and the high speed fan around 108°C ~ 226°F to engage. You may want to change the readings from Fahrenheit to Celsius just to verify and/or echeck the data in generic OBD rather than in manufacturer data.

Is the fault code gone?

Yes the code is gone.
Switched to Celsius and fan comes on at 108.
Not sure if it is right or not
I have a friend with same truck I'll ask him to let me check it some time.
Thanks a lot for your valuable help

SI states:

low speed engagement: high treshold 108deg C = 226deg F
low treshold 103deg C = 217deg F

high speed engagement: high treshold 111deg C = 231deg F
low treshold 108deg C = 226deg F