PS2 Upgrade Challenge

Quite an interesting issue.  How does the engine react in conventional mode?  Inspect the engine wiring.  Motor leads, and smoke leads.  Any stray wire strands on the PCB?

When you say smoke cycles on and off, it should if the engine is moving.  Chuff and puff syncronized.

The head light dimming may be a drop in voltage on the PV circuit.  Same that feed motor and smoke element.  Of course a short is a short, and turning smoke off shouldn't be the issue, unless the smoke elements are the cause of the short.  Did you modify the smoke unit properly?   G

FWIW, on several upgrades, I've had to relocate those diodes as they don't fit in the recess for the tether connector.  Even after grinding it away significantly, I still had to relocate the diodes.  It's easy just to put them in-line in the locomotive harness.

I just spoke to a MTH service tech and it appears I somehow shorted one of the diodes in the engine harness.  There is one for each motor direction and they share power with the light & smoke unit.  Everything he stated fits the symptoms.  I will be testing tonight.

I'm just thankful I did not short the board.  Been there, done that!

Sounds right, I noticed they are a tight fit for many engines.

They're common 1N4003 diodes, available almost anywhere.  Like I said, relocating them may be the right option here.

Putting them inline makes alot of sense.  I will give it a try.

Thanks!

Diodes replaced (1N4004) - everything works perfectly!  One more thing to be careful of when performing an upgrade.

Good Job Scott.  Those Diodes do feed the PV for the engine.  Makes sense that it was shorted vice open.  Or, was it just grounded to the frame?  Did you test the diode once removed?  G

I never lost one of those, but I suspect if you short them to the frame and power on, they might go, considering they're connected to the motor.

The diodes are pretty interesting, I guess they ran out of wires and that was a way to get the common over to the locomotive for lights and smoke.

I tested the original diodes before I removed them as well as afterward.  The response on both was almost identical - .56 on one direction, infinity on the other.  However, one of the diodes read .53.  I also looked to see if the leads from either diode could touch the frame but saw no indication.  Despite this, I changed them anyway since they cost $1.50 and I was tired of fighting the problem.  I measured the new diodes after installation and they read .50. 

For good measure, I covered the exposed surfaces with electrical tape (following this, I assembled/disassembled the unit and looked for indentations in the tape - there were none).

Now the engine works perfectly.  I don't know if the diodes were in fact bad or if there was a short on one of the diode leads. I am assuming if there was a short then I would be experiencing bigger problems.

Scott,  actually those diodes are testing Sat with voltage drop one way and not the other.  if the diode was shorted or damaged you would have a voltage drop both ways, or no voltage drop for an open diode.

I imagine that the diode from the foward lead was touching chassis ground.  You had positive voltage at the motor terminal for reverse, and a completed path to ground probably through the diode.  This allowed the motor to have some current flow and that is why it could be pushed and roll.

Nicks in the motor leads that touch the chassis or stray wires at the terminal that touch the motor can, are common problems when an engine starts up and immediately starts to move.  A chassis ground on the motor side is more forgiving because of more robust FET controlling motor current and thicker board traces at the power supply.

When this grounding occurs on the lighting or coupler side, it usually results in thin board traces burning up or low current FETs being damaged.

I always examine where the engine PCB has to go and if there is clearance with the chassis frame, and also the shell when placed on top.  When in doubt I insulate with electrical tape on both sides, or move the diodes as mentioned.  G

The recess where the engine tether connector goes is frequently a problem area.  The PS/1 tether didn't need as much clearance, and I've had to grind away metal on a number of them, and I've moved the diodes into the harness on two because they simply would never fit in the connector area.

Thank you for the clarification.  I have upgraded a number of locomotives and will probably upgrade a dozen more.  I will save this discussion for reference.