Reviewing the parts breakdown of that engine, yes it has the typical cherry switch that can and do fail
https://www.lionelsupport.com/...-Steam-Switcher-7805
@Andrews-s-c posted:Bell and whistle function properly so I don't believe the IR connection is a problem.
I actually think that is a false assumption. Since this is a conventional engine, then the sound in the tender is in conventional mode- so the tender and trainsounds board that is specifically IR drawbar enabled I'm 99% sure that is for chuff ONLY. The board is seeing the DC offset for bell or whistle from the track power and activates those functions natively- not across the IR tether.
Which then complicates this slightly in my mind. I say that because at the engine, this is also a 3 wire IR transmitter in the drawbar (sometimes they use tether wording) Unlike a command control engine and railsounds where digital information might be passed by the drawbar IR- this appears to be chuff only and by that, I mean a triggered pulse = chuff.
So if 5V power for the transmit side was pulsing (again, special to the 3 wire style IR transmitter), or if the cherry switch was bad, or some other reason, - say stray light hitting the tender sensor side of the tether, you might also get the machine gun chuff effect. Again, it appears any light pulse = rapid chuff.
I would test the tender alone by itself first. See what it does.
Then you almost would want an IR sensitive camera (most have IR filter that blocks it) to see what the engine is outputing for IR signal.
Again, granted, this is a unique setup I have not seen cross my workbench but the parts breakdowns and lists let me know what components are at play and again, this is unique in what is being sent across the IR drawbar connection
