# Loco Speed Suddenly Increases and Decreases



## MtRR75 (Nov 27, 2013)

I got a new loco – a Mehano 4-6-2 – new old stock. It does not run smoothly. My system is DC and HO.

At first, when I ran the loco at regular speeds (close to prototypical – with the transformer at about 1/3 power), it stopped frequently and needed to be nudged to get it going. I examined the loco and noticed that one of the six drive wheels was slightly corroded – uniformly – all of the way around. The other 5 are perfectly shiny. I polished the wheel using my hard-rubber track cleaner. The wheel looks much better, but this only seemed to help a little – or maybe the loco was just getting better on its own, since it had not been run for a very long time.

Then, in an effort to keep the loco going, I turned up the transformer – to about 2/3 power. At first the loco ran more consistently, but with lots of jerks and still some stops. Then suddenly, it took off at high speed. Now what happens is that it runs at high speed (about what I would expect for 2/3 power, but suddenly drops down to a very slow speed, then jumps back up to a high speed. The “low-speed” time periods last from a fraction of a second up to about 5 seconds. If I turn the transformer down, the loco alternates between normal speed and stopping.

This is not a track problem. I have several other steamers all of which run smoothly on this track – except for an occasional jerk when a drive wheel shorts out at the frog.

There is some pattern to where on the track the loco speed drops down. It is more likely to happen just after crossing a turnout, but not always, and it can happen anywhere on the track.

The tender has pickups on its wheels, but all that they do is power the light on the back of the tender – there is no electrical connection between the loco and the tender. When the loco speed drops down, the light on the tender dims at the same time – as does the headlight on the loco.

It appears that something in the loco is partially, but not completely, shorting out the circuit. Any other time that I have had anything short out a circuit, the loco in use stops cold, and after a couple of seconds the transformer (a Tech II) shuts off the power to the circuit (and the overload light comes on).

I should also point out that this loco had a smoke generator in it. After running it at high speeds, the loco felt warm to the touch and smelled slightly of a burning electrical smell. The warm touch could just be from running it fast – since I don’t normally run locos that fast, I am not used to feeling them getting warm. Could the slight smell be the smoke generator? Or are these symptoms of what is wrong with the loco.

So…

(1) What is wrong with this loco, and 

(2) How do I fix it?


----------



## rkenney (Aug 10, 2013)

If the speed dropped and the headlight didn't dim you would have a mechanical problem.

That doesn't happen, you say the headlight dims, so you have a voltage drop!

Why would you have a voltage drop on one section of track and not another?

Most likely cause for voltage drops is always the track. The first test now is simply to supply temporary voltage feeders directly to the errant section of track and see if that helps.:stroke:


----------



## DonR (Oct 18, 2012)

Sounds like shell off time.

As you note, there is something inside the loco 'loading'
the track voltage as indicated by the dimming of lights
when it slows.

Since it's pure DC, tracing the wires should be easy. Somewhere
you may see a bare wire that might be touching the frame or
another wire.

Or also as you mentioned, the smoke unit could be the
culprit. You might disconnect it for a trial run with the shell
off and see if that has any effect.

Since your other locos run smoothly track problems
can most likely be ruled out.

Don


----------



## MtRR75 (Nov 27, 2013)

DonR said:


> Sounds like shell off time.


Thanks for the response. However, I have never removed a steam loco shell -- and only one diesel shell a year or two ago (can't remember how I did that).

So, are there screws to be removed first on this Mehano 4-6-2?
Are there tabs that the shell snaps onto?
Where is the best place to pry the shell off without damaging the loco?
What do you pry with? a small screwdriver?


----------



## golfermd (Apr 19, 2013)

I have no experience with this loco but some pictures might be helpful (pictures of the bottom, etc...).


----------



## MtRR75 (Nov 27, 2013)

An update on this problem....

I got the shell off -- one screw on the rear (facing the tender) and one under the front truck.

All of the wiring looked fine. I ran it one more time -- for just a couple of feet -- same problem -- started way too slow for the power applied, then suddenly took off. I shut it down, but noticed smoke from the rear (the motor area). The motor burned up and froze. I think it was defective from the start -- it must have spent too much time on the shelf doing nothing.

I got lucky and found a replacement motor on ebay. When I tested it before installation, it ran kind of rough and noisy -- but not the radically different speeds of the old one. I got the new motor installed and now the loco runs pretty well -- but kind of rough and noisy. And yes, I lubed the motor spindles and all of the gears and wheels before reassembling. I guess the was also old stock that sat on the shelf too long. But at least for now, I have a running loco.


----------



## ggnlars (Aug 6, 2013)

What is the status of this? The trick to find out where the problem may be is to connect the transformer directly to the motor. This will tell you if the problem is electrical between the track and the motor. A more likely source is the drive rods binding. These will get bent with rough handling. Some time they are totally bound and everything stops. Other times, they will rub intermittently causing operation similar to what you describe. 

You can tune these by loosening the motor. Pulling the motor gear away from the driver gear. This will allow you to push the chassis on the track. You will feel and see the bind, and be able to tweak bend the rods as necessary. When things are running smoothly, grease and oil the gears and bearings, including the rods. Then reassemble and verify all is well.
Good luck,
Larry
Check out my blog: www.llxlocomotives.com


----------



## MtRR75 (Nov 27, 2013)

ggnlars said:


> What is the status of this?


Not much has changed. I'm working mostly on track and wiring right now -- and not much of that either -- with a full-time job and family issues. Most of my locos are in storage right now. I do take them out and run them for 10-15 minutes every few months -- usually as part of track testing. It still starts a little too suddenly and sounds too noisy, but it is running smoother.

I know the sound is in the motor, as I tested it before installation. I may be just spoiled. Most of my other locomotives are better quality. This one just sounds and runs like it is cheap 00 which it is, But I bought this one because it was the style (i.e. a Pacific) and road name that I wanted. I have a nice set of passenger cars for it to pull, but they are all still unbuilt kits. Much to do, but little time...


----------

