There are several potential causes. One is that the clutch mechanism that allows the steps to 'pump' the flywheel has suffered an internal failure, possibly a sprag failure; another is that the drive from the clutch to the resistance has broken (I don't recall if that is a chain or belt on the 4000)
Rather than troubleshoot from distant memory, here's a link to the service manual:
https://www.manualslib.com/products/Stairmaster-4000-Pt-Series-3234686.html
SOURCE: stairmaster climber model 4600 pt pedals go all
Unplug dc power connection then remove right side panel check for loose connections on on alternator, check distance of speed sensor to speed sensor disc you should no more than 3 credit card distance between them. Next remove the black wire from the B+ terminal on the alternator, then remove the brown wire from the field terminal on the alternator.Place a short wire with alligator clips between B+ and field terminals start stepping for about 10 to 15 second you should full resistance in that case its the upper console that if not replace alternator
SOURCE: stairmaster pedal slip
Sound s like it maybe the hub assembly they have a one-way bearing in them and that could be cause the problem go to
http://www.gympart.com/ or call them 1-800-496-7278 they will have the parts for your
SOURCE: My Stairmaster 4000 PT pedals
have diode on the alternator checked , check all wiring connections to be tight, is the battery ok (6 volts ) and last check the motor control board for damage and/or electrical burn
SOURCE: Replace the chain on the stairmaster 4000 pt
This video should help guide you.
Spring Pulley Chain Drive Hub Replacement Video for Stairmaster Stepper...
Spring Pulley Chain Drive Hub Replacement Video for...
Do you have Stairmaster Stepper and...
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Thanks all. Was having some trouble getting casing off and could only peek in at first. The drive chain looked ok as it dtill went from cog to cog, but when I got it properly open it was clear that I had snapped the drive chain and only one side was intact lol. So I've ordered a new one. Mine is an early model without the transmission, so I guess it's a really simple repair. I've ordered the new 31 inch chain and should be stepping again really soon. ??
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