Odyssey 2 Speed problems

I would think the most likely problem is the tach reader.

However, just guessing and throwing parts at the problem is always a dicey proposition. Someone else can make some suggestions as to what tests to make.

This is an early Legacy model.  If the tach reader is faulty, the engine will take off and go a few feet and then stop.  If the DCDS is the problem, it will take off and never stop.  The indications of the tach reader can also be the DCDS, but I tend to suspect the cheapest component first.

Check for wiring issues between the tach reader and the DCDS if it stops after a few feet.

Thanks John. That’s what I was mainly concerned with is after check the wiring how to know if it was the encoder ring reader or the DCDS.

The encoder ring isn't like the old TMCC Odyssey, so it's not nearly as likely to fail.  I've had a couple of encoder boards in my parts box for years, never did get the chance to use them.  Usually, this ends up being wiring or the DCDS.

Well it was pretty simple, wiring was fine. But as you can suspect if the flywheel doesn't turn with the motor kinda hard to get a speed reading . Both set screws were loose kinda weird but I like simple, might need to go back with some thread locker, they are so small don't want to over torque them and didn't feel as tight on the shaft as I would've liked but it works

That will certainly make it less likely to control the speed!

I am going to "tach" on to this thread.  I have a Lionel SD70Ace 6-28279 that has some of the same characteristics in this thread.

First power up, it get's some jitters to go backwards and sometimes full speed for about 6 feet.

From a stop it accelerates perfectly in forward direction.  Calm cool and collected.

From a stop going in reverse, just one bump of the dial and it's full speed reverse for about 6 feet after I nudge the speed wheel back  counterclockwise.

I noticed in high speed reverse the ditch lights are on so that is telling me the loco knows it's being demanded slow or it thinks it's going slow. 

I have ohmed out both blue wires (Speed #1 and #2) from the tach encoder board to the DCDS and they are fine. 

Can the tach reader be faulty in one direction only?  Since it stops after 6 feet does that indicate the DCDS is fine?

Open to any suggestion.

@Steims posted:

Can the tach reader be faulty in one direction only?  Since it stops after 6 feet does that indicate the DCDS is fine?

Check the end play on the motor armature.  If there is too much end play, the worm will push the flywheel up in one direction, if the gap gets too large, it'll lose the tach signal and take off.

@gunrunnerjohn posted:

Check the end play on the motor armature.  If there is too much end play, the worm will push the flywheel up in one direction, if the gap gets too large, it'll lose the tach signal and take off.

Thanks John for the suggestion.  End play isn't bad at < 1/16".   So in trying to determine if the problem is on the DCDS or the Motor Encoder I did a little test.  I swapped the blue "velocity" wires on the schematic below.  I made it so Velocity 1 went to DCDS-J2-2 and Velocity 2 went to DCDS-J1-1.  I expected to see the locomotive now take off in the Forward direction but I was wrong.  It exhibited the same problem with instant acceleration in Reverse.   

I was curious to see if Encoder terminals 2 and 4 were shunted so I removed the harness and found they are not.

Since I don't know how it's supposed to operate I am going to struggle to troubleshoot.

More suggestions and ideas please. 

This is where I drop in a spare DCDS to see if it's that or the encoder/flywheel.

I don't have a spare laying around but I will dig into a working SD70ACe and start the swapping.  I will report back the results.

Reporting back my problem as promised.  I swapped the suspect DCDS board with a known good locomotive and the problem followed the board.  The behavior is instant shot backwards ~6' on powerup.  It will also start creeping in reverse after prolonged idling.

I see that @amos had success with his AC6000 by replacing just FETs (OGR AC6000 Reversing).  Will that same strategy work on my DCDS board?  If so, I could use a little help tracing down these FETs.

Here are the numbers from the FETs:

IRF5210  IR 608P  KC 7B

IRF540N  IR 719P  5Y AZ (two of these)

IRF5210  IR 736P       LI

The FET's are a likely cause, they're the first thing I'd check.  None of the traditional parts houses have stock for all of these, but here you go.  You'll even have some spares.  FYI, the second set of numbers are just date codes, not a significant part of the part number.

IRF540N TO-220 MOSFET

IRF5210 IRF5210PBF 100V 40A TO-220

Thanks @gunrunnerjohn for the parts lead.  My Fluke has test settings for diodes and capacitors but just haven't seen the FET test position yet. 

Found a video on how to test a loose FET but can I check them while still installed on board?

Given the symptoms, just see if any two pins are shorted to each other.  FWIW, given the prices, I typically change them all even if only one or two come up shorted.

Head scratcher here.  All FET pins checked and nothing appeared abnormal.  Most everything in the 3 - 8 Mohm range.  Corresponding MOSFETs read similar to one another.  Since the symptom is not constant could the MOFSETs be good but what fires them bad? 

Obviously, that could be the case, though that's a much more rare failure mode.

@Steims posted:

Head scratcher here.  All FET pins checked and nothing appeared abnormal.  Most everything in the 3 - 8 Mohm range.  Corresponding MOSFETs read similar to one another.  Since the symptom is not constant could the MOFSETs be good but what fires them bad?

Were you able to find out what the problem was, if not the FETs?

@DaveGG posted:

Were you able to find out what the problem was, if not the FETs?

No I didn't.  After the FETs checked out I threw in the towel for a new board.