Fan Cycle

[axe]

Buddy has 09 387 CM871, Has custom tune. Says fan is cycling erratically. Engine running at normal temp, but fan will kick on stay on then cut off. Sometimes every 5 min or so, then sometimes it wont. Full of coolant. Could it be coolant fan switch? Would ac press switch cause it even though not running ac? Any advice on where tom start looking would be appreciated.

[8042]

If he is driving with defrost on the a/c is also on by default, that could be the reason for the fan to cycle.

[Hammerhead]

(12-16-2016 )8042 Wrote:  If he is driving with defrost on the a/c is also on by default, that could be the reason for the fan to cycle.

Yes this, the fan will cycle even more frequently if defrost is on and he's not moving, because the "freon" needs to be cooled to relieve the pressure.
If the fan stays on too long, the A/C Pressure Switch Min Run Time can be adjusted with Insite.
If he is NOT on defrost, check for a faulty pressure switch or wiring of the pressure switch.

[axe]

Defrost not on, not sitting. running down the road. Thanks, I give him the info. Could ac press switch still be causing it even without defrost on?

[hhow55]

(12-16-2016 )axe Wrote:  Defrost not on, not sitting. running down the road. Thanks, I give him the info. Could ac press switch still be causing it even without defrost on?

It could also be low on freon. That was the problem i had one time.

[in2trux]

Check the fan solenoid ,that it's not leaking air and the electrical plug isn't causeing the intermittent on/ off. A bad electrical connection will cause your problem. Default is always on.
I just replaced ours a few months ago, problem solved.

[Rawze]

some possibilities ..

If there is an intermittent connection to the A/C pressure switch, the thermostat sensor, or to the fan solenoid itself, it will cause the fan to come on and off unpredictably. Corrosion in A/C pressure switch is a very common one. As well, if the system is low on refrigerant, and it is right around the minimum of about 50 PSI or so, it can cause the same problem on some trucks.


Lastly ...
You also mentioned "custom tune". If it is MM and you know it was done right, then the re-programming should not be the culprit. If it is not MM, then who knows what someone could have done.


You should be able to find out what is tripping the fan on via ecm or cab control software, etc. as well.

[Nilao]

My engine fan will come on once my air pressure falls under 90 psi. I chased it for weeks trying to figure out why it was cycling every time I pumped my brakes coming down an off ramp or backing into a dock. Come to find out the ecm was closing my air solenoid at 90 psi. The root cause was my air governor was out of adjustment causing the compressor to not kick in till 90 psi. Once I replaced it the issue went away.

[axe]

Yes it's MM . Thanks everyone for info. He says AC pressure switch was corroded up, cleaned it up and all is good. Appreciate the help.