99.9% chance it's a tether issue. With the locomotive running slowly wiggle the tether. If it speeds up and the chuffing stops it's a tether issue. Either the tether has a loose wire or one of the pins on the receptacle in the locomotive has a loose pin on one of the tach reader wires. Fix is either to resolder the pins on the receptacle or replace the tether, depending on which one is causing the issue.

@Knight007 posted:

I've noticed an odd issue with my PS2 upgraded Big Boy in that it seems to run fine except for intermittently jumping in speed for a second at random places around my track.  The jump in speed is brief but very abrupt and risks breaking couplers on longer consists or possibly derailing the front wheels.  This seems to happen more after first running the locomotive after a few weeks of sitting on the shelf and appears to work itself out after several minutes of running.  I've cleaned the pickup rollers and oiled all the rods but it didn't seem to make much of a difference.  Any insights would be welcome.  Thanks!

Typically, if the lurch is a speed increase, also combined with loss of chuff sound and lack of puffing smoke at that time- then we suspect something, somewhere in the tachometer feedback loop. It does not mean the tach is "bad" or the sensor is bad, it means that the tach in the engine on the motor and the board is in the tender (most likely) connected via wires in the tether.

The fact it happens intermittently could be a problem in the tether wiring (it can and does fail from flexing) but also could be a cracked solder joint on the engine side mounting PCB

or other failure. It could be the tach tape or some other issue with optical sensor mounting.

Thus lies a minor problem- the upgrade PS2 kits as far as I know came with one piece tether/wiring harness in steam kits. In other words, actual PS2 Steam engines delivered from MTH often use a 2 part tether with the portion that goes between the engine and tender being a plug in at both ends (right angle one end, straight on the other- both 10 pin straight through wiring).

Again, the problem is, now, given what is in stock at the moment- you might have to buy the current stocked "universal version" of the 10 pin tether, and splice in all 10 wires to your existing harness at the board end in the tender.

Similar topic and problem.

https://ogrforum.ogaugerr.com/...0#171907990373318750

All that said, if PS3 steam upgrade kits (which use the same steam PS2 harness) are supposed to be coming in stock, it's unknown if we might see harnesses- specifically the typical steam upgrade harness for PS2 3V (also PS3/2 "steam upgrade harness) come in.

Protosound 3 Steam Upgrade Kits arriving in March!

https://ogrforum.ogaugerr.com/...4#172893497837113794

@Vernon Barry posted:

All that said, if PS3 steam upgrade kits (which use the same steam PS2 harness) are supposed to be coming in stock, it's unknown if we might see harnesses- specifically the typical steam upgrade harness for PS2 3V (also PS3/2 "steam upgrade harness) come in.

I'll bet money and given you odds that they'll ship with the exact same harness as they used to, and the harness that also shipped with the PS/2 upgrade kit.  I can't imagine them changing the kit contents, they can barely keep their heads above water over at MTH!

Well, I opened the tender and indeed the tether is completely hardwired to the boards, there is no simple plug at the other end.  After fiddling with the tether and plug that connections to the engine while it was running, I found that there may be a wonky wire within the tether cable.  I rotated it a bit at the cusp of the tender and "locked it down" and it seemed to remedy the surging, but it's obviously not a completely permanent fix as there appears to be a wire breaking inside the bundle in the cord.  Not sure what to do now as I don't have the soldering skills to replace all those board connections with a new tether. Perhaps I'll just have to be careful with the position of the tether as it goes between the tender and engine and adjust it again if it starts acting up.

It's a ~30 minute job to change the tether. Just cut the old one off with some slack and splice in one of these:

Just match the wire colors, solder, heat shrink, done.

@Knight007 posted:

, I opened the tender and indeed the tether is completely hardwired to the boards, there is no simple plug at the other end.  After fiddling with the tether and plug that connections to the engine while it was running, I found that there may be a wonky wire within the tether cable.  I rotated it a bit at the cusp of the tender and "locked it down" and it seemed to remedy the surging, but it's obviously not a completely permanent fix as there appears to be a wire breaking inside the bundle in the cord.  Not sure what to do now as I don't have the soldering skills to replace all those board connections with a new tether.                                                     Perhaps I'll just have to be careful with the position of the tether as it goes between the tender and engine and adjust it again if it starts acting up.                                                           

                     Might work for awhile  ?????    Hopefully a second wire's  insulation doesn't rub off against the first wire and turn the board into an expensive piece of toast.

Sir Knight , do you have a good model train repair shop nearby or a favorite brother -law who repairs electronic equipment ?         Of course there are competent people on the forum that might be able to get this new tether attached to the wires at the board for you at a reasonable fee .

@Dallas Joseph posted:

Of course there are competent people on the forum that might be able to get this new tether attached to the wires at the board for you at a reasonable fee .

He's pretty close to @GGG, a very capable MTH ASC.  This should be child's play for George.

Yeah, I just took it to GGG to replace the tether.  He does good work.  $1000+ engine isn't deserving of being in less than 100% condition.

PERFECT  SOLUTION  ......You are absolutely right Sir Knight .  No sense in trashing that Big Boy.

GGG fixed the Big Boy no problem... apparently there was a broken / breaking wire leading to the tach.  For future reference, it's a blue wire and should be the first place anyone else with this sort of speed surging problem should look.