Z-axis problem Prusa i3 rework!! Home stretch!!

I started this as my first build thinking it would be finished quickly..
I believe what I have is a bad a4988 driver for the z-axis but the problem inherently begins to intensify the smaller the motor is used.
I've really started to pry out the information on these drivers but can't seem to find an easy answer as to why they don't pick a direction when running the marlin firmware my z speed is 2 mm/sec.

The printer is made with the bigtree-tech.com ramps 1.4 and a mega2560. The A4988 stepper drivers are the stock pololu drivers. All drivers but 1 out of 5 seemed to work just fine (RUNNING THE WIKI STEPPER PROGRAM ON THE BREAD BOARD).
As you can see x & y work just fine but have been trying to calibrate the z-axis motors. I've tested the 47 N.cm and the 59 N.cm stepper motors from stepperonline through eBay, seems although the 47's should have enough torque and run fine on the bread board they've been the toughest things to calibrate on the printer.

All of the drivers are setup with microstepping, and z-axis motors are in parallel. This the full range on the potentiometer from 2A to 0A. Even though they are in parallel and should only have to receive between ~ 0.6v-0.8v or there's no sweet spot. The voltage on the direction pin reads 0.1v-0.35v.

If you know about the pololu a4988 boards that I don't please leave comments!!

The videos I have are linked here

[youtu.be]
(NON-WORKING Z AXIS)

[youtu.be]
(X&Y AXIS RUNNING AT OPTIMAL PERFORMANCE)

I'M HOPING IT'S NOT A POLOLU BOARD..

The way they're moving, they're wired wrong or there is a bad connection. Check with a multi-meter on the steppers to find the correct wiring.

Check your DEFAULT_MAX_FEEDRATE in Marlin, the default for the Z is 5 which is too high, try 2

#define DEFAULT_MAX_FEEDRATE {500, 500, 2, 25}

Or from Pronterface send the printer an M203 command:
M203 Z2

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My updated Instructable on our Prusa i3 Build
[www.instructables.com]

Looking at your videos I'd agree with the wiring hypothesis. But rule our driver boards by swapping a known working one onto the z stepper position. If it is a bad pololu don't worry they are only a couple of dollars.

Check your wiring with a multimeter, the pair of wires at one side of the motor connector should have resistance, and so should the other pair but there should be none between the two pairs. This check will also ensure you don't have a mismatch between the two motors which are wired in parallel, you can't always be sure of the wire colours unless the motors are identical.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

@stephenrc I did try putting new connection on the wire, for the x & y motors this what the problem was. Even with the new tighter connectors the problem persists.

@steveroy I actually do have my federate down to 2, at one point before i made this post the motors moved briefly in one direction but it didn't seem to keep operating that way. I ordered the pololu black board that you can solder yourself in hopes that this will work right.

@DJDemonD that's something I didn't think about, I will do an ohm reading when I get out of work tonight, I have 2 sets of motors that could ultimately be used for the z-axis but preferably I'd like to use the smaller 47Ncm's in parallel and the use the 59's in a bigger build.

Using smaller motors series until I can get them to work properly then I will solder them into the parallel arrangement. All 4 motors I'm testing have new connections because the factory connectionsettings were loose from probably being tested before they sent them out. The Black a4988 pololu board has yet to come in. I will bring updates when I have it all hooked up.

That did it, I got the driver in today and both 47N.cm z motors run perfectly at .75 volts wired in parallel. Don't know why but I think pololu just sent me duds.