2007 379 fan runs all the time

[hct5]

2007 379 the fan runs none stop once up to temp, the coolant temp is around 200-210 all the time even empty running down the highway with the A/C off. all the parameters in insite look good. coolant has been flushed and filled with new and the radiator has been blown out. oil temp sits around 220. pulled the thermostate and everything looks good there as well and is the updated part. Any thouths of what to check next? it has a cummins 870.

[MPDiagnostics]

Check fan drive state in Insite. What’s it say when is on? Air Conditioning? Or Coolant Temp? Or anything else?

Also even though A/c is off - the fan switches on the A/C system still can cause the fan to sometimes come on if there is a fault with them. It could be wiring or switch itself. But you have to confirm the fan state first which will tell you the reason why the fan was turned on.

Maybe even bad fan solenoid failing at higher temps around the engine?

Just some ideas

[Rawze]

On my own truck, the engine fan will come on and just stay on perpetually if the A/C refrigerant pressure is below 50 psi.

[mikkhh]

(07-17-2025 )Rawze Wrote:  On my own truck, the engine fan will come on and just stay on perpetually if the A/C refrigerant pressure is below 50 psi.

There is clear reason for that - if A/C has no cooling then the pressure is going to rise fast causing relief valve on AC compressor to release pressure OR something else will explode leaving behind green cloud of oil and refrigerant, some cars may have relief valve that is one time blow only.

[Rawze]

(07-17-2025 )mikkhh Wrote:  

(07-17-2025 )Rawze Wrote:  On my own truck, the engine fan will come on and just stay on perpetually if the A/C refrigerant pressure is below 50 psi.

There is clear reason for that - if A/C has no cooling then the pressure is going to rise fast causing relief valve on AC compressor to release pressure OR something else will explode leaving behind green cloud of oil and refrigerant, some cars may have relief valve that is one time blow only.

I sure hope that some idiot in some engine company did not have those same idiotic thoughts ...

Sorry,.. but that is the dumbest excuse to turn on the ENGINE FAN that I have ever heard wen the pressure is low. If the pressure is too low, then simply do not run the A/C compressor, and give an A/C pressure fault, and not switch on the engine fan like an idiot!.

Personally, I think they do tit to punish the truck owner,.,.. and force them to fix the A/C system, which is a pure money maker for the OEM when it acts up due to it being designed like s$it on purpose.

[hhow55]

Problem maybe a bad Manual Fan switch, or an issue with the air supply line for fan clutch. Could also be a corrosion issue.

[mikkhh]

Well newer car AC pumps are constantly running, they have solenoid inside to regulate plate that is going to vary stroke of the pistons. There is no magnetic clutch anymore.

Bluetec Merc E class 350cdi around 2014 year was the car that had cooling fan running at 100%, there was fault code for low AC pressure. After fitting another pressure sensor fan runs like it should.

Without fan coming on and pump running the relief valve is going to go pop in matter of minutes. Has happened on similar Mercs to me.

After filling AC I check with gauges the pressures and when fan kicks on. On some older Sprinter vana it takes 25bar for fan to kick on.

[hct5]

a/c system is working properly, went for a road test with a loaded trailer. once it hits 210 fan comes on and the coolant temp drops to 200 fan shuts off , within 20 seconds or so the temp is back at 210 and the fan turns on agian.