The judgment creditor can try to garnish your income, however, they must get court approval to do so. If you are making payments and you cannot afford to pay anymore more, then the court will probably not permit the judgment creditor to collect the full monthly payment from you. Additionally, some types of income (i.e. social security) are exempt from garnishment. You should contact an attorney in your area to discuss this. Many offer free consultations. Good luck.
The above answer is for general information purposes and does not establish an attorney-client relationship.
No one or company can garnish anyone's wages without first obtaining a judgment and then issuing and execution to the sheriff. You would have received notice of a lawsuit first, either by certified mail (if permitted in your jurisdiction) or by personal service. You would have an opportunity to respond, at which time, you should hire a lawyer to help you. Do not hire any lawyer; get one that is familiar with consumer law and debt defense. I suggest looking at NACA.NET.
Under Washington law the judgment creditor has the right to issue garnishments, unless you already have an agreed payment plan in place. If you are unilaterally sending money to the creditor, that does not have the legal effect of preventing them from issuing a garnishment against you. I'm sorry for that bad news, but that is leverage that the creditor does have against you.
Do they know where you work? If so, consider how much money they would get from you through a wage garnishment (25% of your take home) and consider that when trying to agree with them on an agreeable payment plan.
Best of luck,
Tim L. Eblen