Guys I am having also this f44 error which drives me nuts. Can someone please share a wiring diagram for Caterpillar GP18N ET34L series.
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How does one check the vcm-3 already changed DcDc converter and has new batteries and alternator

Did you fix the problem and if so what was the issue

DTC F-44 is a pwm fault
the possible causes can be:
-poor battery charge
-faulty VCM-3
-faulty DC-DC converter
-faulty harness

After you check the battery and dc/dc converter, I would check the wire harness and the connectors at the vcm-3 and make sure none of the wires are broken. They may not look broken but the internal wire may be broken. You can tug on the wires one at a time and if there is a break in it you will see it stretch, it may even break apart. Cat changed the mfg of their harness's and the core wire is less strands and harder so vibration tends to make them break a lot easier then they used to. This has plagued them ever since they introduced these harness's.
The other stuff should also be fairly easy to check too.
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ok sounds like you already did most of this?
are there any relay box's on this unit?
cat also had issues with relays