I now have three whistle tenders and a diesel horn running (not all at the same time) on my O27 test layout, using a ZWC and a single 180 watt brick. The 3 tenders all whistle superbly on my old PW ZW. Two of them will not whistle at all on my test layout with the ZWC, while the third tender will gave the lamest of whistles (low rpm on whistle motor) every now and then, maybe 1% or 2% of the time. The diesel horn works flawlessly on both the ZW and the ZWC.
Thoughts on a fix?
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No takers????? 
The ZWC does not have the compensating winding, so increase the voltage by 5-7 volts and try again. Purely my opinion, but the available DC offset seems "weaker" versus a PW ZW as well, so the relay might have trouble latching.
That's my thought too, not the right wave shape or it's voltage off is a bit low.
I have a modern loco that refuses to whistle via a disk, and two PW that won't whistle via "diode transformer". (however, one tender will work if you use one heavy diode shunting the 2 rail power legs for activation (rather than in series inline on one of the two legs as usual).
They will sometimes work if you use two diodes in series. All work with 3 diodes in series , but something ran funny with them so I haven't "played" since then.It was "glitchy" on a 1033 with 6a GP diodes. Going to 10a cleaned it up (& cooled the button box down..... use a real project box 
Tin boxes get HOT
Yes, where is the throttle position when you press the whistle button?
Does the diesel have a battery?
What stationmeister said...
OMYGOSH, time slipped away on me, and I forgot about this thread!
So, I took one of the offending tenders (2671W) that would only operate while on the ZW and checked it by disconnecting the relay's hot wire and jumpering an ordinary "D" cell to the coil - worked great, so I was confident the relay contacts/armature might just need tweaking.
So I bent and tweaked stuff on the relay until it finally worked. Of course the relay then wanted to stick in the "on" position even after letting go of the whistle button, but a small piece of painter's masking tape between the armature and the coil cured that. It now works OK on either the ZW or the ZWC. It also works on my homemade whistle controller, but barely - I didn't string quite enough diodes in that controller, so it's anemic.
The lesson learned (again) is that some relays work better than others - but why? I think Bob Hannon nailed it in his book "J.L. Cowen's Postwar Lionel Trains" when he said, "The bad news is that the underlying cardboard bobbin (in the relay coil) and the thickness of the insulating paper between the layers varied. When combined with being wound a slightly different tensions (sic), the result was that coils were produced that had slightly different resistance values for the same turn and layer count, and with varying overall lengths."
Indeed, my coil measured 11.8 ohms while Bob lists the values between 13.2 and 14.4 ohms, testifying to the variability of these coils, I presume.
So anywho, my intention is to next put together an electronic relay as described in more than a couple articles on OGR, best one being IMO here and seeing how that compares - it would be nice to have the lower switching resistance from an SSR than what 70 year old relay contacts can provide!
George
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