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Discussion Starter #1 (Edited)
Need a little help trying to avoid going to dealership. I have a 2017 3500 extended with 98,915 miles. Just recently replaced my radiator cooling fans with a unit from Rockauto. After replacing it, I tested it by bringing the engine temp up without A/C and the fan kicked on at about 220 degree F and temp came down to around 204 degrees and fan turned off. Then turned A/C on and fan turned on as it should and all was good. Now the problem is that the fan only comes on with the A/C on and "short cycles" at the high speed. The fan doesn't come on without the A/C. Checked the two 60 AMP fuses for the 1st speed and 2nd speed and they are good. Not sure if it could be a relay problem since both fans come on even though they are "short cycling". Anyone have any ideas what I need to look at next?

I replaced the original fan because it started to shake due to the fan blades being loose (most likely worn bearings right?). It was the upper fan of the two.
 

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I did the same thing. Started to wobble so I replaced with one from Rock Auto. Don't know if it doesn't come in without the AC on. But I do know both fans run really loud a lot now. Not just when it was really hot outside like before with the factory fans.

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Discussion Starter #3
Yes, you're cooling fans are suppose to come on when the A/C compressor comes on. When the A/C compressor comes on, should be low speed to my understanding, I could be wrong. The dealer told me today that the low speed relay is bad along with the resistor. I'm not confident with the resistor diagnosis, but I can see the relay being bad. Now I just need to find the correct relays to purchase so I can install it. Dealer wants $600 to replace relay and resistor!!
 

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Discussion Starter #4
Can anyone with a strong electrical background tell me how common it is for resistors to fail, especially a new one. I would think that if anything is gonna go is the fuse since they protect the electrical circuit instead of the resistor. If the low speed resistor did fail causing the low speed relay to fail, it would have to be a defective resistor. Anyone have any thoughts?
 

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Discussion Starter #9
You should be able to bench test the resistor.

I could. I have quality multi meters, just don't know how to test the resistors that are on these fans, it's not lilke a resistor on a circuit board. I'm not an electrician, so I don't know how to test these.
 

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Discussion Starter #10
If you figure this out please post here. Could it be that my low speed fan relay is bad so that's why they are always running on high?



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I tested the old original relays and there's nothing wrong with them. I'm gonna take it back to the dealer Monday morning :/
 

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Discussion Starter #11
Update: It was the resistors that failed, both of them. Put a meter on them and there was zero continuity on either of them. I used the resistors from my old factory fan assembly (which I replaced to begin with due to a wobbling fan blades, but something told me to hold on to it!) and the fans seem to be operating properly. The fans came on when the temperature reached 219 degress and brought the temp down to about 204 degrees and turned off. Then I turned the A/C on and the fans turned on with the A/C, all at the lower fan speed, not the high speed. The dealer did make the proper diagnosis but they wanted to charge me $600 to replace the resistors. I will include pictures of the failed resistors that came with the new fan assembly from RockAuto.


 

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Interesting, thanks for sharing. Seems these cooling fan resistors use the same concept as a HVAC blower motor resistor. Typically if the fan works on high speed only, that's an indication the resistor is bad.

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Discussion Starter #15 (Edited)
Interesting, thanks for sharing. Seems these cooling fan resistors use the same concept as a HVAC blower motor resistor. Typically if the fan works on high speed only, that's an indication the resistor is bad.

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You're welcome. I had no idea that the fans runnning like that is a symptom of a bad resistors. I would think that if the resistors failed, there would be no flow of electricity and the fans wouldn't run at all. Wish I was much stronger in the electrical side of things. I do find strange that my PM is now running a bit hotter than it use to. On the freeway the temp has been running around 210-212 degrees F. Used to run lower, around 201-206 degrees F. Maybe the new fan doesn't move as much air? Or is there something else going on...who knows.
 

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The fans running on high probably reduces air flow through the radiators at HWY speed when they shouldn't be running. Do you know if you can buy the fan resistors seperately?
 
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