Cooling fan cuts in when not required

Are you running your defroster? Or floor/defrost mix? If so, it spins the A/C compressor and will cause the fan to run sometimes.

Could also be a bad pressure switch.

 

Easy.
The default setting of the engine fan is to stay on when there is an issue. E.G. a circuit is broken, no air... or whichever. The MBE are similar to the DDs.

You have a fan control solenoid that should be mounted on the firewall. Try looking next to the air manifold on the firewall outside of the truck with the hood open.

Couple of potential causes:
*Not enough air reaching the solenoid causing it to go on
*Circuit 439L(+) is faulty at the solenoid
*AC binary switch is going open or that circuit is intermittently going open

Take a picture of what you find and post it. Shouldn't be too difficult.

 

I've got a question for you. 2016 4900EX DD16. When the ambient temp drops below freezing the fan will intermittently come on, will stay on for 5-10 minutes. Defrost and AC will both be off. I've asked about this at a couple of Detroit shops and they don't have a clue. They want to plug into the truck to see whats causing it. One shop did and of course it wouldn't do it, he said everything looked fine and to not worry about it. It drives me nuts, and shouldn't be doing this.

 

You check for moisture in the airlines? Just a thought.

To the OP, these newer engines all have a delay feature in their programming to prevent the fan from cycling too often. Combined with the stuck on symptom, I'd guess you're problem is either the solenoid or the wiring to it.

 

No, not sure how that would cause it.

 

It is programmed to do this when the ambient air temperature gets below the freezing point.
They had a lot of problems with DEF lines freezing with people that used winter fronts when not required. ie: anytime temperature is warmer than -30 they are not recommended.
The outside air temperature probe is mounted near the bumper. When you are parked it would get much warmer readings from the engine and radiator, or with the winter front on. Causing the DEF heater lines to shut off.
So their ingenious solution is to run the engine fan, the computer calculates the outside temperature by monitoring the temperature drop of the boosted air going through the charge air cooler.

 

Thanks, I wondered if it had something to do with that.

 

There can be multiple reasons why the engine fan will come on. One I can think of in particularly cold weather is when a winter front might be installed. This causes the CAC to not function correctly causing hot intake temps. This is pointed out in PSL in 14TS11.

"
Use of a winter front on a DD platform engine, particularly those that are fully closed, will cause performance issues and is not recommended on DD Platform engines. Winter fronts can result in the following:
• Excessive fan run time due to higher Charge Air Cooler (CAC) outlet temperatures resulting from low air flow through the CAC
• Increased fuel consumption
• Failure of the DEF system heaters to turn on when needed due to incorrect temperature calculations resulting in fault codes, poor performance of the Aftertreatment System, and power reduction
• Failure of critical emission equipment that will result in vehicle speed inducement to a maximum of 5 mph
"

There's multiple reasons why the engine fan would come on--during cold weather especially. I would suspect when doing calculations for the CAC temps and other various things. The DEF lines being one of them so they don't freeze so that PT knows when to turn on the DEF line heaters and how long to leave them on.

 

I don't use a winter front. Engine has no issues maintaining temp in temps down to -15f.