Engine fan wont turn off

[JimT]

I must apologize for one thing, I mistakenly confused your engine, an 870, with a different post about a cm2250 and pointed to the SCR can as being an issue when it doesn't exist on your truck. I updated my post, however it doesn't change any of the other information.

Bad ECM tuning can manifest itself in many ways, and that includes indirectly causing your fan issue. There could be a dozen other faults that have been suppressed in the ECM and you will never know about them, not even with Insite or other 3rd party software. As noted in other threads on this forum, the ECM on 870's is difficult to tune properly.

--------------

As for the fan, if the problem isn't air pressure and you've eliminated every wire that can be tested with a multimeter, then the only thing left that doesn't involve throwing parts at it would be to hook up Insite and monitor the Engine Fan State. The result should help narrow down the source of the problem.

If you haven't already (I didn't see this mentioned), you can set up a free account with Cummins Quickserve and look up lots of fault diagnostics, troubleshooting info, parts and maintenance info and wiring diagrams. The troubleshooting steps usually include instructions on how to test different wires/connections for correct electrical functionality.