'17 3500 Rad cooling fan does not come on with hot engine. Bad fan relay?

[cricha]

I had another thread about changing a thermostat that got off topic a bit, but same problem.
Im having an ongoing problem with my truck overheating that Im having trouble getting diagnosed.
The problem is the fan does not come on when the coolant temps get hot above 225. the fan does come on however, when the a/c cycles, but it is just low speed. I took it to a shop and the mechanic could not get the fan to cycle full on, but said the fan clutch was working because the fan would cycle with a/c. here's my question:
I found two relays, one for low speed rad fan, one for high speed rad fan. I dont know how these fit into the operating logic, but wondering if the high speed fan relay isnt working, could that be my problem?
Also, what is the fuse RAD FAN PWM (F01) for? THANKS for any input on this!!!

 

[Wiredawg]

I have a 2017 Ram 5500, Here is the electrical diagram for the cooling system.

Looking for more info.

cheers, Ron

 

[Wiredawg]

And Hope these help as I look more:

Looks like the ECT signals the PCM voltage inputs that the PCM controls operation of fan etc...

Cheers, Ron

 

[AH64ID]

I also posted in your other thread, but the fan relays are for
the gas motors with electric fans.

Our fans are engine driven and use pulse width modulation to control the rpm, that’s likely what the PWM fuse is for.

 

[cricha]

Thanks for the abundance of info Ron! steep learning curve here.
interesting, the PWM fuse is missing in my fuse panel. the plot thickens...

 

[Wiredawg]

And here is a trouble shooting chart:

Not seeing too much more on cooling. Hope this helps.

Cheers, Ron

 

[gsbrockman]

There’s a high probability that your fan clutch is defective.

 

[cricha]

I can rule out almost everything on the troubleshooting list Ron posted. My thoughts would be temp sensor or fan clutch partial malfunction. temp sensor seems to be functioning ok, im getting a temp number on my screen that seems acurate. so maybe fan clutch works at low rpm, but not high rpm. I can replace the fan clutch and the temp sensor fairly easily. still curious that there is not a fuse in place on F01 RAD FAN PWM. if anyone has a '17 3500 and wouldnt mind checking the fuse panel to see if there should be a fuse in that spot, i would appreciate it. THANKS!!

 

[Wiredawg]

Looks like fan voltage provided by PCM and according to diagram I posted above: fuse 78 feeds voltage to the run/start relay, fused ignition switch output to PCM... looks like PCM breaks out battery to several circuits including fan clutch

 

[gsbrockman]

Additionally....I have recalled some that experienced water pump failures that not only caused the serpentine belt to come off, but also took out the serpentine belt tensioner. Amongst these somewhat more catastrophic failures, a few have also had damage to the fan clutch wiring. Has any of this happened during your ownership @cricha ?

 

[cricha]

checked all the wiring and all looks pretty good. havent had any other failures that would have damaged anything. all the plugs are seated well.

 

[AH64ID]

That fuse is not installed on my ‘18 3500, so much for that theory.

It sounds like a bad fan clutch.

 

[cricha]

looks like time to order a fan clutch. Genos should have an oem one.

 

[Wiredawg]

That fuse is not installed on my ‘18 3500, so much for that theory.

It sounds like a bad fan clutch.

fuse #78 (10 amp) not installed on your truck? I haven't gone outside to check mine, this is for a C&C, but would expect it to be the same.

 

[cricha]

fuse #78 not installed on your truck? I haven't gone outside to check mine, this is for a C&C, but would expect it to be the same.

It was fuse F 01 on the legend is says RAD FAN PWM. 80 amp fuse

 

[Wiredawg]

It was fuse F 01 on the legend is says RAD FAN PWM. 80 amp fuse

OK, I'm reading schematic I posted. above... no other fuses listed. You may have better info.

 

[AH64ID]

OK, I'm reading schematic I posted. above... no other fuses listed. You may have better info.

It's just a fuse listed on the fuse cover, but it has an asterix next to it for *if equipped.

I didn't realize it was an 80A fuse until I went and looked at my truck, no way the fan on these trucks needs 80A.

 

[Wiredawg]

It's just a fuse listed on the fuse cover, but it has an asterix next to it for *if equipped.

I didn't realize it was an 80A fuse until I went and looked at my truck, no way the fan on these trucks needs 80A.

John, that's my point, the schematic I posted appears to show #78, 10 amp, feeding PCM, which feeds the fan and some sensors. I wouldn't think the fan clutch would pull much current.

Not saying it's not the fan, just saying the fan is controlled based upon inputs from sensors via the PCM.

Ron

 

[Wiredawg]

Here is the Rad-Fan connector:

If you're comfortable testing, apply 12VDC fused bat to pin B to see if you can get the fan to engage full speed. You may want to install an inline switch so you can power it after starting the truck and quickly disconnect it after it engages fully. If it engages, it's likely good and possibly a sensor or sensor wiring. Manual doesn't show any details on the fan internals so no help there.

Anyhoo, that's pretty much all I can do from here.

Please keep us posted what you do and how it turns out.

Cheers and good luck, Ron

 

[Ozymandias]

Here is the Rad-Fan connector:
View attachment 126217
View attachment 126218

If you're comfortable testing, apply 12VDC fused bat to pin B to see if you can get the fan to engage full speed. You may want to install an inline switch so you can power it after starting the truck and quickly disconnect it after it engages fully. If it engages, it's likely good and possibly a sensor or sensor wiring. Manual doesn't show any details on the fan internals so no help there.

Anyhoo, that's pretty much all I can do from here.

Please keep us posted what you do and how it turns out.

Cheers and good luck, Ron

I don't think that this will work out because then the command and return Fan speed will be out of spec and the truck will go to limp mode.