What causes the exhaust sound hiccup in TMCC steam locos at low speed?
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Depending on what you are actually hearing, I know that one specific recording used in the separate add on railsounds streamlined tender the recording has a 'hiccup' when the first set of chuffs switches over to the second faster set. I think a couple of locomotives used this generic recording also. It is in the recording, not the triggering. In this instance 'it is what it is'.
I some other instances depending on whether chuff is activated by a hall sensor or a micro-switch, the small capacitor on the micro switch could be bad or a lead disconnected. Check the round magnet on a hall effect sensor axle to see if it hasn't picked up a foreign object and it got caught between the magnet and sensor.
One cause is the chuff switch if you're hearing the chuffs cut out at times.
The hiccup that I am speaking of is at about the 42 sec. mark of this video.
This is not specific to this one engine, but, common across the TMCC range. I have often wondered if Lionel had looked into the cause and a fix?
That's the exact thing I'm talking about, sounds like the RS board missed one chuff trigger. I have heard that a number of times. It's usually the chuff switch.
That is the DCDS and chuff switch getting back in sync. This happens around speed step 8. You just have to live it.
Bill
Bill, why do they get "out of sync"?
John,
The chuff switch controls the smoke fan and starts the first chuff. The DCDS then generates pulses for the chuff. When the engine increases speed, the off time of the switch is of shorter duration and that's when they sync back up, around speed step 8. That is the design of the software.
Bill
So this is just for Odyssey I units?
That's the only one I heard it on. But I don't pay it attention any more
Bill
I tried a non-Odyssey TMCC steamer and didn't have any skip, and the ERR CCM ones don't either.
I have heard that, but I never connected it to the Odyssey installation. I assumed the switch was giving problems, that's usually where I look when they start skipping chuffs. Another little tidbit of info for the hopper, thanks! 
