Update: 

Bachmann Forum friends, when I flipped the locomotive over and examined the blind driver set, I found one of the screws under the wheel center cap had backed out of the end of the axle.  So the only thing holding the wheel on was the side drive rods which have a huge amount of side play slack in them (like about 3/16" +/-).  So that was why the center blind drivers were not staying on the rails.  After I reset the wheel on the axle and re-installed the axle end screw (now with green loctite applied to both sides), I still noticed there was enough side play in the center driver assembly as to allow the wheels to drop off the rails. So after determining the diameter of the axle, I acquired some slick thin nylon washers from the hardware store (appr. 3/64" thick - a little less than 1/16" thick - each washer) with the correct center hole diameter, and installed one washer on each end of the axle between the valve gear eccentrics and the side frames.  I did the trick of cutting out about a 1/8" piece out of the washers (creating sort of like a piston ring gap), then I could pop them onto the axle with needle nose pliers without removing the drivers from the frame.  This took about half of the side play out of the center driver set.  When I laid a piece of 8' diameter curve on top of the end drivers with flanges, and played with the side play of the center driver.  I could see that there was still a likelihood of the center driver dropping off one rail on the 8' radius curve (it would track the 20' diameter curves fine).  So I added one more nylon washer on each side and it took out the rest of the side play.  Another test with the 8' dia. curved track and this time the center driver stayed on top of both rails.  So I lubricated everything per Bachmann lube recommendations and lubed the new nylon washer spacers as well and test run the locomotive.  So far it appears this has remedied the problem.  I will know more after making more lengthy runs on the outdoor layout, but so far, so good.  Thank again to all for your consult on this little problem.
Mark   

Thanks for the input and ideas to resolve this issue guys.  I have not had a chance to try adding spacers/washers to the center driver yet, but hope to get to it tomorrow or sometime this week.  I will update you all on how it goes.

Mark

Forum Members and Bach-man.  Does Bachmann have a recommendation for alleviating a problem I am having with my Spectrum 2-6-0 Mogul center flangeless driver dropping off the rails creating a wobble which I am sure creates extra stress on the drive line.  I had to remove the guide pin assembly from the frame between the center driver because the locomotive would hang up and stop when going through diverging paths on switches and would also jump going through the straight path of the switches (switches are USAT solid brass).  So now removing the pin assembly has created the situation where the flangeless center driver drops off the inside rail going around curves.  (even 20' diameter curves). 

Would fabricating and installing a plate the same size as the original plate removed - but without the two pins, resolve this issue, or do I need to shim the axles with nylon washers to take up the slack, or is there a better solution to this problem?  Other than this center driver issue, the locomotive is tracking and running great on the USAT track and switches (8' to 20' dia switches and curves). Any advise is appreciated.  Thanks!
Mark 

Thanks Joe.  I greatly appreciate your reply.  This is my first crack at converting a Bachmann Spectrum 2-6-0 locomotive to battery power and wireless DCC control, so I feel like I am flying blind.  But I am learning a lot in the process and making progress.

I have a follow up question for you.  Does the wattage of the resistor matter (i.e. quarter watt, half watt, 1 watt, etc.)?  That ohm value you recommended sounds about right as I was experimenting with a 1K ohm resister (1 watt) last night, and when I put it in series with the headlight and firebox/ashpan LED's, the headlight barely lit while the firebox/ashpan LED's seemed to work okay with flicker.  As I previously noted, there is a small resistor under shrink wrap in one of the leads to the headlight, so it makes sense the 1K ohm resistor I added dropped the voltage too low for the headlight.  I did not see any resistor on the little LED board for the ash pan light LED's.  I could access it when I removed the motor cover under the chassis to access the motor.  I cannot get to the firebox LED's without disassembling the boiler from the chassis (which I am trying to avoid at this point).  The schematic in the Bachmann loco manual does show the resistor in line with the headlight only, but does not show a resistor in line with the firebox/ashpan LED's.  But all lights will be powered from the same G3 decoder terminal, so I just have to determine the right resistor value to place in the line so all lights work correctly. 

FYI, I am placing the Airwire G3 decoder in the tender (rather than in the locomotive boiler) with the battery and Phoenix sound card.  So I wanted to use the existing four wire plug between the loco and tender to bring power from the G3 board terminals for the smoke generator and one of the separate light driver terminals.  I was going to power the lights direct from the battery, but decided to go through the G3 board after all, so I can turn the lights off when running in broad daylight (to reduce battery draw) when the lighting effects would be minimal anyway.

I have pulled the main board with the smoke generator attached to it, out the front of the smokebox.  Bachmann kindly put in enough lead length that I can pull it all the way out and access it). I have traced the leads from the four wire plug at the rear of the locomotive to the locos internal main board and figured out which two pins send power to all the lights (headlight, firebox/ashpan LED's), and which two pins send power to the loco's internal board circuit for the smoke generator.  So I believe I will be able to use the four pin connector between the loco and tender to supply power to both lights and smoke gen. separately from the appropriate decoder terminals on the G3 board. I have added a third two wire lead w/connector between the tender and loco to bring the battery power through the Airwire G3 card motor driver terminals direct to the motor.  I actually removed the track power leads from the locomotive driver frames motor, and disconnected the tender track power pickups so that all locomotive functions are powered only by the Li-ion battery. The motor seems to be working perfectly wired direct to the G3 motor terminals. I can use the Airwire throttle to control the motor speed and direction.

So I am down to just figuring out what will be the best resistor values to use in the power leads from the battery through the G3 board, to the lights and also the smoke generator.  The 14.8 V Li-ion battery is actually putting out 16.2 V.  I think I have the smoke gen figured out as noted below.  So I just need to get the light circuit fine tuned with the right resistor in line so I don't burn out the lighting LED's but send through enough voltage so they all work correctly. 

For the smoke generator, the Bachmann manual that came with the locomotive recommends not to exceed 16 V input to the smoke generator circuit.  So of course my battery is running at 16.2 V.  With the 1K ohm resistor (measured 975 ohms) added in line to the smoke gen, it wont make smoke and has less than 1 V (.88 V) measured at the generator.  With a 1 ohm resistor added in line it makes smokes like crazy with a reading of 11.8 V at the smoke generator, so I think that will work.  I experimented with hooking up a track power MRC Tech II power pack rated at 10.6 V output to the smoke generator, and it produced "moderate" smoke at wide open throttle max output.  So the 1 volt increase from 10.6 to 11.8 V greatly enhances the smoke produced, and is well below the Bachmann recommended safe max voltage of 16 V.

So that is where I am at.  Thanks again for any help you all can provide on the last piece of this electrical puzzle and any other advice you can share on this overall conversion to battery power and wireless control.

Mark

Thanks for the reply Bill. There is nothing on the ash pan small board except the led and soldered connections, no other components, so they must be on the main board. I have been able to pull the main board out the front of the smoke box, so I should be able to trace circuits. I printed a schematic of the Loco wiring for DC,  but it doesn't help much.  I just don't want to fry the LED'S by putting too much voltage to them. Track voltage is in the 10-11 v range, while the battery is rated at 14.8 but actually puts out 16+ volts.
I'll try the large scale forum. Thanks for the lead.

Sure would be nice if someone from Bachmann tech support at least
would reply to this question.

Friends:
Looking for some advice on powering lights on a G scale Spectrum 2-6-0 Mogul #81498 (DCC ready).  I am adding an Airwire G3 DCC board, Phoenix P8 sound and 14.8v Li-ion battery power supply to the locomotive.  I am installing the Airwire and Phoenix boards, speaker and the battery all in the tender (yes it all fits).  The loco and tender have two connections between them.  A two pin to the chuff trigger which will be connected to the P8 board.  The four pin connector I plan to use to run power to the lights on the locomotive and the smoke generator (one pair for the lights and one pair for the smoke generator).  Finally I am adding a third two wire connection from the Airwire board direct to the motor on the locomotive.  I have also disabled/removed all track power pickups on the loco and tender, so only the battery can provide power.

My question is this:  It appears there is a resistor in the lead to the headlight, but I cannot find any resistor in line to the LED's that light the ashpan and firebox.  With the 14.8 V battery being used, will I need to add a resistor in the line from the battery to the ashpan/firebox LED's, and what size resistor should I use.  I am going to go direct from the battery to the lights, so that when I switch on the battery, the lights will light immediately which will show I have power.  Since steam loco headlights were typically lit once and left on, and the ash pan/fireboxes would always have light when the loco is under steam, I figure these can be on all the time the battery is switched on.  But I need to know if I need to protect the LED's in the ashpan/firebox with a resistor using the 14.8 v battery.  Thanks in advance for the feedback.  Mark