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My engine fan will not turn on. Either with the manual

Customer Question
My engine fan will...

My engine fan will not turn on. Either with the manual switch or just letting the ecm do it by water temp. I've replaced the fan air solenoid and it still will not engage. Unless I cut the wires to the fan solenoid and make it run constantly. This is a cat c15 in a 2004 peterbilt 379

Mechanic's Assistant: What's the make/model/year of your truck? Engine type?

Peterbilt 2004 379 cat c15

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The problem started as intermittent, but now seems to be continuous once the truck has been drivien and warmed up.

Submitted: 3 years ago.Category: Medium and Heavy Trucks
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Answered in 9 hours by:
5/14/2019
Technician: Cat Man, Cat Engine Expert replied 3 years ago
Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 10,344
Experience: I am ASE Master Truck Certified.
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Thanks for asking your question. I will try and help you out.
To turn the fan on. The ecm removes power from pin 11. I would check the wire coming from pin 11 and see if it has power.
Also, some fan take power to engage. If this is the case on your truck. There will be a normally closed relay the wire from pin 11 will connect to.Please reply back if you need more help. I am happy to assist.
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Customer reply replied 3 years ago
I am not getting any power whatsoever at pin 11 on the ecm. There is continuity there, but no power.
Technician: Cat Man, Cat Engine Expert replied 3 years ago
If there is no power when the engine is running. The ecm wants the fan on. If it’s not on. Your problem is with the fan hub, control valve, or the wiring beteeen the ecm and fan.
Sometimes the ac pressure switch, manual override, and also a relay in there can cause your problem.
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Customer reply replied 3 years ago
I've tested the override switch, it seems to be fine. I've bypassed the a/c pressure switch and the fan still will not work. The only way I can get the fan to come on is if I unplug the fan solenoid. I have already replaced the solenoid. Where is the fan relay located?
Technician: Cat Man, Cat Engine Expert replied 3 years ago
It’s between the ecm and solenoid. Not sure exactly where. Sorry.
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Customer reply replied 3 years ago
Ok, thank you for your help. I'll limp her home and do an in depth search.
Technician: Cat Man, Cat Engine Expert replied 3 years ago
You are very welcome. I am happy I could help.
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