if the command has a read preference setting even if they are all up.

Attached sccc.js that demonstrates this issue.

The problem appears to be coming from the _lockType helper:

https://github.com/mongodb/mongo/blob/r3.1.9/src/mongo/client/syncclusterconnection.cpp#L621

where it tries to determine if the command is a write-type command. It does this by getting the name of the command and appending the help: 1 field.

The problem comes in when there is a read preference setting and triggers the ServerSelectionMetadata::downconvert code here:

https://github.com/mongodb/mongo/blob/r3.1.9/src/mongo/client/syncclusterconnection.cpp#L253

This will result in the command being wrapped around the 'query' field; causing the _lockType helper to run the "query" command and fail. The command will fail on all 3 config servers and will ultimately make mongos return the error "all servers down/unreachable when querying".