replace the motor
replace broken parts with 3D printed parts
I love the AHM C628. Over the years I've obtained about 10 working models and a few more that are in very rough shape, to include one that has a terribly deformed shell from sunlight.
Pretty much everything I say about the C628 will also apply to the GP18, C424 and FP45. They all have the same mechanism. Also, these models were sold as Model Power and Bachmann.
If you got any of these AHM or IHC or Model Power or Bachmann locomotives as a kid or on ebay or at at train show or anywhere else and find that they have broken parts or don't work at all then I have some very good news for you.
There is almost nothing too broken to fix on any of these models. Did you totally crack off and break the motor clips or the couplers?
So I got this C628 and it had all kinds of broken and missing parts. One of the motor clips was gone, the gear towers were missing the small clips that hold the two parts together (those always break off). The couplers were broken off and the side frames wouldn't stay on.
First, sideframes. Don't cut the couple pockets all the way off, leave a little bit so the sideframes stay on. But, since you probably know that after the fact now, you'll need to drill a tiny hole for a 2-56 screw to keep them on. Problem solved.
You've got a huge hole to plug in the front and if you do it the old fashioned way of cutting a pilot out of styrene, which is a lot of work, you still need a platform to screw in the couplers. The first thing I do is take my digital calipers and measure the open pilot and then make an insert on my 3d printer thick enough to screw in couplers. I'm going to go into making pilots in an upcoming post. Quickly, what you do is make a box the size of the opening and thick enough to screw in the couplers and make a 2nd box where you subtract the opening for the couplers. If you've done any design, that will make total sense, if not, believe me when I say that is a 2 move 3d construction that is as simple as they come.
The motor can be replaced easily because it has a standard shaft size meaning that the couplings on the shaft will fit a new motor with no modification. The motor is a 2mm shaft, so take a look on ebay and find the one you like. I personally like the 24 volt motors because I know they won't burn out. Remember that you can go higher voltage than 12 volts, but not lower. 24 is a good number because power packs that have pulse or older high power models can often crank out 20 volts or more which is very hard on 12 volt motors. A 24 volt motor can take the abuse, the only downside is that it will be a bit slower than the 200mph original version you're used to. Something with around 20,000 rpm at 24v will give you about half that much on your layout and will be a little slower than and Athearn Blue Box locomotive.
I like to shave the handrails off these locomotives and replace them with brass wire that I solder in place. Just eyeball the size when you you're at the hobby store. If you're not sure, .032" wire is best to use the first time before you go smaller. Smaller wire is a lot less forgiving.
Shortly there will be a video on youtube that shows each of these fixes. Don't give up on these locomotives no matter what you see under the hood. Even the gears can be replaced if you know what to look for, but gear replacement is a whole other topic.