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This is a RailKing 30-1394 (4-8-2 L3 Mohawk with 3V PS2). The engine will run smooth as silk as long as "smoke" isn't on. As soon as I press "smoke" on the DCS remote the engine takes off at least +20 SMPH. If I have smoke on and run it at slow speed the engine will lunge with every chuff. First the chuff, then the lunge, so the operation is very jerky until reaching a faster speed. Turn the smoke off while engine is at fast speed I immediately hear the brake squeal sound as the engine quickly slows to minus 20 SMPH (speed references are approximate). The smoke unit does not produce smoke even set at "Max" with 20 volts going to the track (I generally run my trains between 12 and 15V). I took the smoke unit apart, and the wick is properly pinched between the two elements and plenty of fluid. I re-seated every connector on the engine and tender but no luck. The amps don't go above .03/.04. Any ideas as to what's going on?

Rich

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OK, did some further testing. The fan doesn't spin but it isn't seized, it turns freely by hand. When I disconnected the fan motor, the engine takes off at 2 SMPH, and there is no chuffing, only the usual sounds that play while the engine is stopped/idle. With the fan motor disconnected, there is a 1.4 amp draw, and pressing smoke button on or off has no effect on the speed or the amp draw. When I plugged the motor back in, the chuffing came back, and the engine reverts back to the behavior as described in my original post. Disconnecting the smoke elements has no affect on the speed, sounds, or amp draw. I could check if there is power going to the heat elements but need to know if that current is ac or dc.

Nope.  A disconnected smoke unit should not stop chuffing.  I believe you have a broken wire to the tach unit, probably the ground line.  When the fan gets pulse it feeds back enough voltage to the tach to get a signal to generate a chuff since they share the 5V line.

Replace the tether. BC-0000217 since this is an easy fix with the detachable external tether.

If that fan does not run it may be bad also so replace it too.  BUT Does the motor spin when power applied (disconnected from board)  May just be a loose impeller on the shaft.  Dot of superglue will fix that.  BE-0000041 is fan motor.  G

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