Atlas O C630 accelerates randomly in command

As I remember my Atlas "O" C630, it has a  TAS electronics package as built. It sounds to me like you should carefully inspect the rpm sensor ring and reader which is  near one of the motors flywheels. Make sure the reader PC board is near but not touching the flywheel tack tape and is aligned with it.

(1.) The mounting of the sensor PC board on the motor can be a problem.

(2.) also the distance between the tack-strip and the sensor head.

(3.) on occasion, because the sensor PC board is mounted, on the motor, the three small wires that power the sensor can break loose, and needs resoldered.

(4.) Some of the upgrade kits, had a paper tack-strip, that was to be glued to the existing flywheel, all so, can be a problem.

(5.)  Have fun, there should be a TAS installation manual in the technical section of the forum. 

Top suspect in my mind is Mike's #3, I've seen a lot of the TAS installations with a broken wire at the sensor.  Next on my list is Mike's #1, the mounting of the sensor and it's spacing from the flywheel strip.

One other thing to check - make sure the two power wires (usually one is yellow and the other is white) to the motor that holds the tach reader are not getting tangled up with the tach reader bracket or the wires to the tach reader. On some installations, manufacturers and individuals will put a rubber band around the motor to try to keep the wires (tach reader and motor power) from moving around and getting tangled up or pulled loose. If the power wires don't have any slack in them, when going around curves they tend to get tangled up with the tach reader bracket and thus cause the head of the tach reader to get too close to the flywheel.

The distance between the tach reader and flywheel should be approximately the thickness of a dime. Too close, the engine runs too fast, too far away, the engine runs too slow. While you have the shell off, be sure to rotate the truck with the tach reader to see if that is causing the tach reader to move around.

Good luck, even getting the shell off and on is a challenge and can get the wires to the motor or tach reader out of place, that is why sometimes there is the rubber band on the motor.

I have gone and checked all the spots you guys suggested. Mounting looked good (reader is also held onto the motor with a zip tie and the plastic bracket); reader distance looked good (exactly the thickness of a dime, minus the lip of the dime); wires going to the sensor also looked good (soldered on well too). Also the tatch lines on the flywheel are painted onto the flywheel and recessed in a bit.

Is it possible it's losing power randomly for a split second and going into conventional mode? I have looked over the bearings and gears and they seem a bit dry, I added some oil to the bearings and some grease to the gears for good measure. While I had the shell off I also took each of the TMCC boards off the main board and reseated them.

Unfortunately I cannot do any more testing of the model at home for the time being, I probably will end up taking it to the museum next weekend again to see if the issue somehow has cleared up

If your club layout features large radius curves and no killer grades. You might try turning the cruise off. It should tell you in the manual the sequence of TMCC commands. It may speed up and slow a bit on it’s run but it  shouldn’t  run away at speed as the sensor is now out of the equation.
If you can run with the shell off. There should be a red LED that blinks each time a stripe passes the sensor. You can spin the flywheel by hand to test it. Once running at speed it stays on steady. But if it takes off running the light should go out.

I ran the engine today for 4 hours and did not come across the issue again. I did also set it in the Legacy remote to operate in Cab1 mode, as I believe I saw a forum thread saying it needs to be run in that mode. Every previous time I ran it I set it to run in TMCC mode, which might have been the source of my issues all along.

It seems like it might be fixed now, but I will touch base if it comes back again