Why would my sst18 john deere zero turn steer in

There are routine maintenance procedures for the steering actuators and filters. See the operators manual.

Manuals are available for download free of charge from the John Deere website.

https://techpubs.deere.com/en-US/Search/Equipment#

This may not be the problem but is a good place to start.

Sorry we have no parts locator services here.

Don't worry I'm sure you will get the solution of your problem, just check the link I attach.
John Deere Spin Steer SST15 SST16 SST18 Service Repair Manual

Greetings, I read your question and I think Reliable-Store can help you in this. Here's the site link.
John Deere Spin Steer SST15 SST16 SST18 Service Repair Manual

Russell if you truly want to figure out this problem you need to look at the wires and ware they connect .And what they connect to then drew your own wiring diagram its not that difficult if you break down and do a few tests with an OHM meter to see what is connected and what happens when you through a switch or run a jumper wire to a ground .
If you think about its not really that complex .

I copied this right out of the operators manual for this machine, and it is a zero turn machine as well

" NOTE: Operating the mower while backing up is strongly discouraged. The Reverse Implement Option should be used only when operating another attachment or when the operator deems it necessary to reposition the machine with the mower engaged. "

an educated guess says they deactivated the steering in reverse
also I hope you have a supply of air filters for your steering valves for this model as ive replaced a few already for folks that have these

front wheels turn?>

I’m happy to assist further over the phone at https://www.6ya.com/expert/james_1cb31da1bc8c6102

Some more details would help. Your tractor has a combined electronic and vacuum controlled steering mechanism. Some people have reported your issue just because of a low voltage issue, and replacing the battery fixed it. In other cases there are two micro controller switches under the fender deck on the right side. These switches should both be able to be depressed and "click" and retract with a healthy solid feel. If not replace the switch. There are also 2 vacuum 'pots' and a vacuum system associated with your steering control. A vacuum leak may also be a culprit (although in my head that might be more constant). Does the steering work in reverse?

on the inside underneath the mower by the handles there is a bolt , this bolt allows you to push the mower when it is not running, so take brake off and see if you can push the mower if you can then the bolts need tighten. The blades are engaged by the PTO..... wait a minute sounds like you have a belt problem. check the belts make sure they are on all the pulleys. Hope This helps Tim