Inconsistent Y homing behaviour RAMPS/MARLIN/COREXY

Hi all

I would appreciate some ideas to troubleshoot a weird issue. I have built multiple 3D printers of different types and this is my own design of CoreXY, of which I have built several examples, so I am surprised to be having this issue, and it's either going to be something ridiculously simple I forgot, or something tricky to troubleshoot. I need to try to fix this by Monday...!

Environment: 2 Printers in one chassis, each is running:

Arduino Mega 2560 Clone
RAMPS 1.4, Diode D2 removed.
ATX PSU
Latest Marlin.
Pronterface

Marlin has been heavily tweaked for optimal behaviour on a CoreXY.

On one of the 2 3D printers, it behaves correctly. It homes first -X to a min X opto endstop, then positive Y to a Y max opto endstop, then negative Z to a Z min inductive sensor.

On the other 3D printer, wired identically, with the same components, same firmware installed one after the other, I get a different behaviour: It homes first -X to a min X opto endstop, then negative Z to a Z min inductive sensor. Y homing is skipped altogether.

I tested both machines having already homed manually each axis, then powered off, unplugged USBs and closed and re-opened Pronterface.

The weird thing that is throwing my troubleshooting, is that on the misbehaving printer, I can home the Y axis perfectly, every time, and the endstop always reports correct status.
I have already tried swapping out the Y endstop and wire for a new one that works perfectly on the other printer.

What on earth could be causing this different behaviour in homing, on one of the two identically built printers running the same firmware?

Small additional piece of information - on the printer that is misbehaving, if I have already homed X, but not Y or Z, I get a different behaviour:

X backs off 5mm, then returns to home (min endstop), then Y homes to max endstop as it should! Then Z homes correctly.

So the incorrect behaviour is narrowed down to on one of 2 identical machines, if X is not already homed, G28 will home X, skip Y, and home Z.

Ideas anyone please?
Thanks, Alex

That is very strange. Have you tried swapping the boards to see if that's the issue?

Off-topic (kinda): do you mind posting picture of the printer? I am intrigued by the "2 printers in 1 chassis". Also, what tweaks did you do to optimize Marlin for a CoreXY?

Edited 1 time(s). Last edit at 03/18/2016 04:52PM by gmh39.

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Photo attached as requested. Will replace both the RAMPS and Arduino Mega with new ones as my second shot this evening - the first will be to re-seat every connector in case that was the reason...

Replaced both RAMPS and Arduino Mega. Same issue still occuring.

It seems to be intermittent, and may depend upon where I have pushed the carriage to before homing...

Test Method:

1) Home X-Y-Z via Pronterface
2) Note behaviour A
3) Motors off
4) Push carriage to rough middle of build bed
5) Home X-Y-Z via Pronterface
6) Note behaviour B

Test 1:
1) Homed x, then y.
2) Sent Y -100, X +100
3) Motors Off
4) Home all - behaved correctly X-Y-Z
Repeated steps 2-4 4 times - all perfectly.

Test 2:
1) Homed x, then y.
2) Sent Y -200, X +200
3) Motors Off
4) Home all - behaved correctly X-Y-Z
Repeated steps 2-4 4 times - all perfectly.

Test 3:
1) Homed x, then y.
2) Sent Y -200, X +300
3) Motors Off
4) Home all - behaved correctly X-Y-Z
Repeated steps 2-4 4 times - all perfectly.

Test 4:
1) Homed x, then y.
2) z +20
3) Motors Off
4) Manually pushed X carriage to about Y -100, X +100
5) Home all - behaved correctly X-Y-Z

Repeated steps 2-5 4 times - all perfectly.