I have been having some problems with my Legacy Lionmaster Big Boy for a couple years.
First issue was with legacy cruise control. At times, the engine would suddenly speed up, then slow and operate OK for a while then repeat the erratic action. I asked Lionel support for help, but being well out of warranty, they suggested that the "tach cage" may have cracked or become loose. I found no problem there. I tried re-seating the R4LC. All seemed OK for a while until the situation happened a few more times.
Rather than send to Lionel for repair, I decided to turn off the odyssey cruise to see what happens.
With the odyssey cruise off, the engine seemed to run fine.
After a while though, at random, the engine would start to move at full throttle when powered up, addressed and after advancing the throttle just one click. If powered down, it would go back to normal and move and then responded to both throttle advances and slowing down normally.
Finally, a couple of weeks ago, after reading a post on this forum that this behavior could be a bad R4LC, I decided to order a replacement from Lionel. The part was still available on the Lionel website.
After the part arrived, I installed it (still with legacy cruise off) and the engine now responds and operates normally.
I got brave today and turned the legacy cruise back on and what a pleasure to operate this nice engine once again with cruise control! I ran it for about 15 minutes or so and so far so good!
Looks like a happy ending to an annoying issue!
Clearly this engine does not want to become a shelf queen!!
