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Tune Up for HO Athearn Drives How I have solved problems I have found with poorly performing (Athearn drives and similar) motive power:
....in no particular order:

Gunk on wheels:   Reduced with extra weight. GPs should weigh in at over 14 oz, SDs even more. SWs and other switchers as much weight as fits in the unit.  My heavy F units hardly ever require wheel cleaning, and they are running on highly polished stock sintered iron wheels!

Bad electrical contact between frame and truck bolster, or between frame and bottom motor clip:  Even if it looks great, sometimes they are just unexplainably cantankerous.  OR bad contact with stock Athearn motor clip assembly due to rust, bent wrong or any number of other problems. These are all fixed by using the "Five-Wire System" to assure positive contact from truck sideframes to both sides of motor.  Note:  Operation of some Proto 2000/1000, Bachmann Plus and Spectrum, as well as some Kato locomotives can be improved by adding wires to bypass truck-to-frame wipers.  Most Atlas and Athearn Genesis units already have a version of the five-wire system factory installed, and most DCC-ready or DCC-equipped locomotives have wires on both sides of the trucks.

Carbon buildup between motor commutator segments:    This should be checked especially if otherwise the motor started running ok, but gradually deteriorated just about when you thought it should be broken in.  These particles start to short out the motor & gradually sap its strength.  This can be cleared by CAREFULLY cleaning with a very slim instrument such as the smallest jewelers screwdrivers. Make sure that there are no burrs on the edges of the commutator segments.

Weak brush springs:   can increase contact resistance between motor frame & commutator.   The brushes should be replaced, or the spirng slightly expanded to get better contact force.  This should particulary be suspected if the motor has overheated due to high current drain, which can anneal the brush spring wire sometimes.

Unit goes bang bang bang bang, or one truck doesn't run smooth or at all:  Most likely a split plastic axle gear; if so, one set of wheels will also turn easily & not stay in guage.  The axle gear will need to be replaced & the wheelset re-guaged.

Any binding anywhere:  truck towers, idler gears, u-joints or slip shafts that don't slip.  Including flash on the internal idler gears.  Find it & correct as needed.

Or too much slop in the motor shaft., or in the worm shaft:  This requires adding shim washers. In the case of the motor, this requires disassembly of the motor to correct.  These problems should be suspected particularly if the unit runs noticeably better in one direction and worse in the other.

Check also for the half axles working over to bind against the bearing blocks in the new Athearn trucks, or binding in the sideframe bushing in the old ones:  In the old ones, this can often be fixed by adding shims & just a drp of lube oil. Not a drop, just a drp.

Or the insides of the wheels can be rubbing against the internal metal sideframes: (new trucks). I sometimes have to bend the internal sideframes a whole lot to keep them away from the backsides of the wheels.

Hope this helps.