Postwar F3 won't go forward

Scrap the sharp outer edge of the brushes. The brush is misaligned and it is jamming the slot of the facing.

I slightly chamfered the brush edges and adjusted the brush spring tension on one brush as I noticed it was pushing it further out than needed. However, that did not change or fix the forward issue. The e-unit pawl seems to be dropping correctly and the drum rotating, so I wonder if one or more of the contact fingers are out of adjustment.

Peter

I think you're on the right track.  Those bronze fingers sometimes look like they're making proper contact, even when they're not.

Check to see if the tubular bronze axle bearings on the power truck have either worn down to the truck casting on one side or have been pushed into the truck casting. In addition to providing rotational bearing surfaces, these tube also must keep the back of the wheels from rubbing on the truck casting. The axles normally move to one side or the other depending on the direction of travel. It's fairly common for the axle bearing to wear more on the pressured side in the most common direction of travel. Even some experienced repair techs miss this one sometimes. If the back of the wheels contact the casting, the refusal to move can be pretty complete.

Not the only possible cause, but it is common on old engines and is easy to see once you know what to look for.

Switch the two wires to the brushes.  This will confirm or eliminate the e-unit as the issue.

When the e-unit has been eliminated, my guess is that the gear of the worm shaft is  shifting aft under power and binding against the truck frame due to a worn bushing. 

The happens to the rear motors due to being run forward all the time...  the gear binds on the brass truck frame cover on the front motors for the same reason.

Take the e-unit out and look closely at the contacts. Make sure they are not bent out of shape, are not burned through, and are making good contact with the drum. Inspect both the contacts and drum for dirt, grease or oil and clean with a solvent if necessary. Tuner cleaner from Radio Shack works well. If all is well and the motor still does not run properly, then disconnect all wires to the motor to isolate it and power up the motor independently of the e-unit to make sure you do not have a motor issue that may be masking itself as an e-unit problem. Connect a jumper wire from one brush terminal to one field wire. Connect the transformer leads to the other brush terminal and the other field wire. The motor should run properly. Now switch one transformer wire connection with one jumper wire connection and power up the motor again. The motor should run in reverse properly. If it runs properly, you have an e-unit/wiring issue. If the motor does not run properly in both directions, you have a motor problem.

Larry

Just switch the two wires to the brushes.  This will confirm or eliminate the e-unit as the issue.

I had a similar experience and it was the axle bearing.  It was worn down.  When the loco tried to go forward the natural torque on the gears would move the axle into the frame and the wheel would bind up.  Reverse and the gear torque would move the axle the other way freeing up the wheel and away it would go.  It was obvious that the loco was well used and had run in one direction long enough that the bearing was work down.

Thank you for all of the helpful insights. I'll check the e-unit, but I suspect that is either an axle bearing or the worm shaft bearing. Judging from the roller wear, it has been used a lot. Unlike the A-A pairs these A-B combinations are more subject for uneven wear as they would mainly go in one direction.

What does it take to remove the worm shaft? I can see that wheels will have to be pulled to access the axle bearings which is beyond my repair capabilities (don't have the tools for that), so I'll have to give it to my local person to rebuild the truck. I could buy another power truck, but that might still give me the same problem. I assume someone makes or sells replacement bearings?

Peter

Ok, I unsoldered the brush connections and used a set of test leads to "swap" the connections. Applying power, it took off in forward quite strongly, so it appears that I won't need to rebuild the power truck. Now the issue clearly is the e-unit that works fine in two of the three positions but not in the "original" forward.

Can the e-unit be adjusted (cleaned maybe) to fix this issue without taking it apart? reading the response from TrainLarry above seems to indicate that. Is there a good way to clean/adjust test these as I go, so I can just put it back together once?

Peter

Try spraying roller in E unit with electrical contact cleaner. Spray and manually rotate roller so you clean both roller and contacts. After drying it may solve issue.

Yes, It can be adjusted.  Pull it out and remove the bottom plate carefully so you can get a good visual on the fingers.  If they are clean and just one out of adjustment you can bend it carefully to apply pressure on drum.

Sometimes a finger has broken, become burnt, or the drum contact is damaged.  They all can be replaced.  Drums around $4-6, contact assemblies also.  Not to hard to do.  There have been tips on rebuilding E-unit on the forum.  G

Or you could just buy a new electronic E-unit made to mount in the original E-unit mounting holes and so away with the troublesome mechanical device.

It is not necessary to pull the wheels to check the axle bearings. If you cannot see a bit of the bronze tube protruding from the casting behind the wheels and/or if the back of the wheels can touch the truck casting, the axle bearing will either have to be replaced or pushed out a bit from inside the truck. Sometimes that can be done without removing the wheels. You can also invert the entire engine in a cradle, apply power and watch the driven wheels and axles shift slightly from one side to the other as the powered direction changes. Some side movement is normal but the back of the wheels must not touch the truck casting.