I dont know what you mean with "older locos", and I also dont know if this controller works with PWM, but I seem to remember that glockenanker/coreless motors does not like that kind of electrical control in the long run...
Does your locos have coreleess motors?
At what frequency does the controller work? (if it is indeed PWM)

Yes, I can confirm the Snail Controller is a PWM design.

According to the instructions, the LED being on constantly indicates an overload. This could be a short circuit, but could also be due to excessive current draw. 5 pole motors draw considerably more current than the modern coreless ones and 3 pole motors more still, so it's conceivable that the load is so heavy that it triggers the overload protection. A loco that's mechanism isn't running freely (for example due to lack of lubrication) will cause the current draw to be higher again.

Are you using the controller battery powered or from a power supply? It's possible that the overload protection becomes more sensitive as the battery voltage drops, whilst a power supply that's providing more voltage than is required could also do the same. Beware unregulated power supplies which usually give out much more than their nominal voltage when loaded lightly - as it would be in this case.

I believe these controllers are widely used with AZL locomotives, which all have coreless motors so I guess there isn't much risk in that regard. I just checked mine and it's operating at 75Hz which is a good deal lower than I would have expected, though.

Hope this is helpful


Chris

I have used the Snail Controller a lot with Marklin locos, all types of motors, my biggest gripe with that controller is low power output due to it using one 9v battery, and the older 3 and 5 pole motors would stop often and require the hand of god pushing to get them going again. I do not know why that is. But my advice would be to clean the track and maybe the locos.
I also had issues with reliability with Rohuhan controllers and 3/5 pole Marklin motors, wether battery or wall wart powered, when they would stop due to their overcurrent protection feature.
Ultimately I found the older hard wired Marklin controllers to be by far the most reliable with all types of Marklin locos. I have three of them. I do not know about the newer Marklin controllers on sale now. The drawback with these controllers is the rather coarse speed steps, sigh, maybe with Z scale controllers you cannot have it all...
I did buy a modified MRC controller from Z Scale Monster. It appears to work very well but I only use it with my little test loop.