DD-15 ('15 Freightliner) - Starter Issue

What might cause a starter to whir (spin) but not engage the flywheel? Or at least that's what it sounds like is going on on key start activation.

Starter was replaced with new thinking (assuming) bad starter, but same thing (some very expensive troubleshooting at emergency after hours). Voltage is good. APU starts great even when cold. No excess voltage draws or drains that can be sensed or detected. All electronics and lights work normally.

No "problems" heard or detected prior to this event. Recent starts all went over as normal.

Stuck in Pasco, WA waiting on Freightliner dealer to open Tuesday. At this point, I can only think maybe the cable from the batteries to the starter is somehow damaged or broken and not delivering enough amperage to the starter? Truck has been downhill-roll and/or pull started three times trying to get to first shop who replaced the starter. Engine was "turned" following new starter install to rule out bad sprocket area on flywheel.

Any ideas?

 

No the local Freightliner dealer was closed for holiday (Saturday). A Cat dealer who does a lot of service work on all makes had the OEM starter and did the work. My shop weekend people gave me their number as an alternate.

If Freightliner was open as they normally are, they would have been the goto people, and maybe they could have troubleshooted more accurately. But who knows.

 

Okay but what does it mean when the starter spins [when it gets the 12 volt signal] but doesn't engage?

 

How could the new replacement starter have the same problem as the original starter from the factory that worked fine until recently?

The hammer tap thing was the first thing tried with the original problem starter.

I can sort of buy that and was wondering that originally, but the mechanic nor I saw anything to indicate a missing or broken jumper wire. The replacement starter has the jumper wire pre-installed, same as the original.

We noticed the starter has a 2 pin plug presumably from the key switch system. Not sure if this is a hot-ground setup or a hot-hot set up meaning it may need to get simultaneous power on both pins to engage correctly? And maybe one of the signal wires is not getting power for whatever reason?