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Discussion Starter · #1 ·
Hi,

construction guy here, definitely not a mechanic.

my gmc 3500 duramax started behaving differently today while towing my empty 18’ flatbed trailer.

the engine began extreme engine braking whenever my foot was off the gas and running at really high rpms when it normally shifts smoothly and quietly.

This is accompanied by what sounds like extreme fan noise.

it sounds like a diesel forklift or dump truck revving up to power hydraulic systems.

it’s making this engine noise even at lower rpms like 1200 and at slow speeds and highway speed.

I was travelling on my normal commute and regularly haul a trailer.

I’ve read the manual and can’t find any settings that change the behaviour and tow/haul mode doesn’t change the behaviour either.

started on the morning commute and was the same on the way home

anyone have any thoughts on this?
 

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Not sure iv heard of it happing before but kinda sounds like the electro viscous fan clutch is stuck locked on.


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Discussion Starter · #3 ·
Not sure iv heard of it happing before but kinda sounds like the electro viscous fan clutch is stuck locked on.


Sent from my iPhone using Tapatalk
Hi thanks for the feedback I really appreciate it!

I watched a video on the viscous fan clutch and how it works. This seems to make sense but it’s not always on, it comes on sometimes when I’m accelerating and sometimes while slowing down but in both cases I think it does seem to be ‘stuck on’ until the transmission finally shifts and then it backs off. It seems like it’s happening more than 50 percent of the time at this point.

Any ideas on what might cause this or how serious it is? We have a second work truck and I’m thinking about parking this one until I can take it to the dealership on Monday.
 

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If it really is the fan, it doesn’t hurt anything. I have heard of a few l5p’s with bad fan clutches, just most fail off.
Maybe posting a video would help


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