Class 101 DMU - jerky starting

[Britannia Builder]

I've just bought an R2697 class 101 3-car DMU on eBay, advertised as new and with DCC decoder fitted, from a model shop seller who has been offering quite a lot of this sort of thing recently (ie brand new non-current models with decoders fitted). It certainly looks new and generally runs very smoothly, but when starting off it often (but not always) gives an annoying jolt and then a momentary pause before accelerating smoothly away. When it stops it sometimes also gives a little jolt after it has come to rest.

 

I've checked the CV values and the decoder manufacturer ID is 151, ESU, and the version is 88, 'Bachmann factory fitted 4 function'. CV 10, the EMF cutoff, is shown as 0 - I tried setting it to 128 but it reset itself to 0, so perhaps it doesn't support EMF feedback.

 

Is this likely to be a decoder issue that might be solved by fitting a different type, or perhaps the motor needs running in, or could it be a faulty motor?

 

Regards, John

[Britannia Builder]

Thanks HFM.  I checked cv49 and it was set to 1 (back emf control on). Cvs 54 and 55 were 32 and 24 and I tried them both on 127 (I found another website that suggested this) but it made no obvious difference. I then set cv49 to 0 to disable back emf and it seems to have completely cured the problem! The DMU pulls away perfectly smoothly and cruises steadily at all speeds. Very strange, since I had the impression that back emf control was a key benefit. Haven't tried it on my gradient yet, so maybe that will need manual throttle adjustment.

 

Any ideas why this should be?

 

Regards, John

[Britannia Builder]

Thanks again HFM, interesting to hear what cvs 54 and 55 do. I'm just happy that I seem to have cured the problem by turning off the back emf control, so I'll leave it at that. I might increase cv 3 to give more realistic acceleration - I used to travel to school on these DMUs in the early 60s and I remember how ponderous they were.

 

Regards, John

[Britannia Builder]

Curiosity got the better of me and I reset cv49 to 1 (back emf on) and reduced cv54 from default 32 to 12, and increased cv55 from 24 to 60, and this also cures the jerkiness while still giving a constant speed on gradients. Apparently as HFM says the default settings are suitable for heavier motors with more momentum. The only downside I've noticed is a slight buzz from the motor for a second or so when starting and stopping.

 

Regards, John

[Britannia Builder]

I've now tried changing cv2 (the starting voltage) from its default 3 to 0, 2 and 4 but they all seem to give the same slight buzz on start/stop. However, it's not a big issue.

 

Regards, John