I'll readily admit this is a strange one. Here are the facts:

1) This loco has been a reliable workhorse until relatively recently. Struggles to make it up one section of track, to the point of stalling, that none of my other mainline locos have problems tackling. Therefore, doesn't seem to be a voltage drop issue with that section of track.
2) Gave it a maintenance service which included lubrication, cleaning the rotor, and replacing the brushes. No improvement.
3) Sent it to a well-known reputable service center for a checkup. They found no issues, gave it a clean bill of health, and sent it back. Still has the same problem.
4) Same behavior whether pulling a load of cars or by itself. My two shunting locos pull the same load of cars up the same hill, with the same throttle setting, no problem.
5) I wondered if one of the pickup shoes wasn't working, so put it on its back on my test bench and used my 6501 test bench trafo to independently apply power to each shoe. Loc seems to run really strong with either shoe connected. Reversing is not an issue.
6) Goes like the proverbial "scalded ape" on the downhill section without any changes to throttle. In other words, will barely pull the hill if given enough throttle and then dramatically picks up speed on the downhill.
7) Telex coupling and reversal unit, if it matters.

Here is where it gets *really* weird:
1) Behavior doesn't seem to manifest itself if I turn the loco around so that it goes up what is normally the downhill section of that loop and then down the normally uphill section.
2) Behavior is not nearly as bad if I put the loco in reverse and then go backwards up the hill section that is problematic.

It behaves as if the motor has greatly reduced torque going in the forward direction, but only on the one section of track Any troubleshooting ideas or suggestions greatly appreciated. I'll try to post a YouTube video to demonstrate what I'm talking about. TIA.

- MR

Edited by user 02 May 2024 18:12:47(UTC)  | Reason: Not specified

This may sound crazy, but try running the loco in reverse pulling the same train, and see if it stalls or slows at the same places on your layout.

Cheers…..

Mike

Thanks for the great ideas, gents. I will check all of them.

To add some detail that I omitted from my original post:

I did clean the commutator with my usual method - the rubber eraser end of a pencil and rubbing alcohol. I will give fine-grained sandpaper a try, as suggested. Good point about cleaning the gaps.

I did consider trying to pull the load going backwards, but I wasn't sure how well that would work since there's no proper coupling on the front of the loc. Especially, since there's a curve involved. I'll give that a shot and report back.

Excellent idea about the reversing unit contacts. The service center brought up a possibly bad reversing unit when I told them I still had the same problem. Will definitely take another look at that.

I also wondered about a shorted winding in the rotor or armature, but somewhat discounted that idea because I didn't think that would be direction dependent.

Thanks again. Keep the ideas coming!!

- MR

I would think seriously about the possibility of the commutator turning on the armature shaft. This could certainly affect the forward/reverse running characteristics.

I don't know off hand how the commutator should be oriented with the armature magnet poles, but comparing it with another armature would help.

If the commutator has moved on the armature axle this could also break a wire on one of the armature windings which would also affect pulling power.