Having just spent an entire afternoon trying to find out how to remove loco and tender shells, I thought posting here with a clear heading may make it easier for others to find what is already scattered about on this message board. The exploded view is https://www.bachmanntrains.com/home-usa/dwg/dwgs/HO_260_DCC_sound_.pdf

Loco
1. detach stays from pilot to sides of boiler by pulling them outwards from the boiler
2. remove two screws under rear of cab.
3. optional but recommended - disconnect tender by unscrewing the link and unplugging the wiring harness (can be started by twisting a flat bladed screwdriver so that it pushes up on the lip at the wire-end of the plug and down on the lip of the socket)
4. remove the top of the sand dome (first bump on boiler behind smoke stack) and remove the screw inside.
5. lift the shell off the frame vertically.

Tender
1. remove the front truck to reach the screw on the bottom of the tender just behind the pin to which the loco drawbar connects (easiest if you remove the clamps holding the wire harness and part the wires). Remove this screw
2. gently lift the tender shell vertically upward easing it forward slightly to release the rear.

Jan

PS: I also found it best to remove the loco shell before removing the axle retainer on the bottom of the loco. This retainer incorporates the electrical pickup wipers and is therefore has wires which go up into the area above the socket for the tender wiring harness. By removing the shell first, some of the slack in the wiring above the plug can be fed down to so that there is slack below which simplifies removal of the axle retainer and access to the axles and gearing.

Kevin: My guess is that your problem is where the speaker wires connect to the decoder circuit board in the tender.  I say this because all the connections to the decoder are the same and they recently caused me trouble - flickering loco lamp and intermittent motor. Investigating, I eliminated the loco as a problem and opened up the tender. The way the wires are connected to the decoder circuit board is a failure waiting to happen.  The stripped end of each wires is poked up from beneath through a hole and bent over to make contact with a copper pad on top. It is then held in place with a plastic clip. I found some of the wires were bent in the wrong direction so that the contact was loosened instead of tightened when the clip was pushed on. But regardless, oxidation over time will impair the electrical contact. I recommend soldering these wires to the copper pads.

Jan