PS-3 F-Unit Couplers

The front coupler malfunction has nothing to do with the tether going to the rear engine, there's no connection between the two.

The PS/1 couplers are not compatible with the PS/2 or PS/3 couplers.  First step will be to obtain and install a proper coupler on the rear unit.

@gunrunnerjohn posted:

The front coupler malfunction has nothing to do with the tether going to the rear engine, there's no connection between the two.

The PS/1 couplers are not compatible with the PS/2 or PS/3 couplers.  First step will be to obtain and install a proper coupler on the rear unit.

Good to know. The effort / price involved in trying to acquire and replace the rear coupler with a PS-3 compatible coupler is probably not worth it. Especially considering shipping & import prices to Canada (Unless I happen to find spare parts at a train show once Covid is done).

As for the front coupler, any idea what the malfunction could be? Loose wire? I always thought the couplers were a fairly fool-proof element of the engine, with not much that can go wrong with them.

First off, check the wiring to the coupler to make sure it's not broken or chaffed/pinched.  If either coupler lead has been grounded, it could have taken out the FET on the PS/3 board that drives the coupler.  Next, unplug the coupler and measure it's resistance, it should be between 2 and 3 ohms.

Took the top off to examine the engine, no wires seemed pinched. Also checked the resistance of the coupler when unplugged. 3.1 Ohms.

I assume it's probably an issue with the board? Unfortunate, but not the end of the world considering every other aspect of the engine is complete functional.

As a final check, you can use a meter to check end to end from the 40 pin connector on the PS/3 board through the coupler.  If it measures end to end the coupler resistance and nothing is grounded, it's most likely the board is bad.  The front coupler goes from pin-3 to PV on pin-12, the rear coupler goes from pin-4 to PV on pin-12.

Click in image to expand graphic

Huh, John you are wrong.  For PS-3 Diesels the front coupler will not work with out a rear coupler sensed or a jumper with a resistor in place.  F units should come with that jumper. 

So you can get a ps-2/3 type coupler and replace the Trail A PS-1 coupler with the PS-2 type and then both will function when you wire it up to the Rear coupler connection on the PS-3 engine.  G

That must not be universal because I just sent one back to the shop that had a broken wire on the front coupler.  After repair I tested it on the bench with just the powered A, and the coupler worked fine.  I didn't have the trailing A unit, so clearly it's coupler wasn't being sensed.

I'm a bit at a loss as to why they'd do something like that.

The PS3 diesel boards need a load on both couplers to prevent the FET on the board from failing.  If only one coupler is connected, it may work for a while, until the FET fails.  That is why newer F units that have protocouplers on both ends  include a resistor load for when a dummy coupler is substituted for close coupling.  This is not a problem with the stacker boards.

Jon, now that's useful information!  I didn't realize this was an issue, and I know I've had these in for repair and the couplers worked stand-alone.  I guess this was included in the MTH ASC course after I took it.

Don't believe me huh:-).   Tech bulletin 07-27-16A

@GGG posted:

Don't believe me huh:-).   Tech bulletin 07-27-16A

Oh, I believe you, and you got back up from Jon, can't ignore both of you!   Besides, now that I cruise through the service notes, I find Jason even provided a diagram.   For some reason, I missed this when it came out, and the issue never actually bit me.  I do note indeed this issue surfaced years after I attended the training classes.

I'm still at a loss as to why the couplers are interlinked, that's very odd.  I have to believe that was some design screw-up, I can't imagine it was intended that way from the beginning.  However, now I know, and I'll strive to remember in the future.

Hadn't checked the forums for a few days, interesting to see there was further development on this topic.

So in the short term, how would I go about making myself one of those resistors to "fool" the engine into thinking a rear coupler is plugged in? Also, where does this resistor go? Does it just plug into the tether port at the back of the engine?

You use the same connector as is on the coupler and a 1/4 watt 4.7K resistor.