I've got one TCS AMD4 I bought several years back in one of a set of Atlas GP38s I bought. I recently bought a second one to put in another one of the GP38s, with the intent of consisting them together. A couple interesting things turned up, so I thought I'd post for the record.
Versions: the original AMD4 is version 45 (CV7 value). The new one is version 88.
There are a couple minor physical differences in the decoder, most noticeable is the LEDs.
The software differences are the interesting part.
With BEMF off (CV61 = 32), the old one goes considerably faster in both 28 and 128 speed step mode at lower speed steps. It's possible that's related to mechanical issues, the only way to be sure would have been to swap the two decoders between the locos and I wasn't interested enough in the answer to do that.
With BEMF on (CV61 = 33), they are both perfectly speed matches through the speed range, IF you are in 128 speed step mode. But if I put the throttle in 28 speed step mode, the old one goes faster in speed step 1 than the new one does. There are differences on other speed steps too. The new one seems to have a more linear speed curve in 28 step mode, so I'd guess that's what the code change was to fix.
I also checked to see if a bug I'd noticed on the old was one fixed. With BEMF on, if you set CV5 to 90 to lower the top speed considerably, turn off all momentum so it's easy to see the speed steps, and step through the speeds one at a time slowly in 128 speed step mode, every 3rd or 4th step the loco slows down a little instead of speeding up. I haven't checked to see if it happens with BEMF off. Turns out the new one has the identical bug - the locos stay perfectly in sync throughout their sawtooth speed curve.
The bottom line is if you are going to consist old and new AMD4s together it can work very well, but only with BEMF on and in 128 speed step mode.
